Moved chapters each to their own file.

Real changes:
(a) added section on special class methods.
(b) corrected typos spotted by Stoffel Erasmus.

1 files changed, 9 insertions, 2737 deletions

diff --git a/Doc/ref.tex b/Doc/ref.tex
index 875659e..ff4e75b 100644
--- a/Doc/ref.tex
+++ b/Doc/ref.tex
@@ -57,2742 +57,14 @@ the {\em Python Tutorial}.
 
 \pagenumbering{arabic}
 
-\chapter{Introduction}
-
-This reference manual describes the Python programming language.
-It is not intended as a tutorial.
-
-While I am trying to be as precise as possible, I chose to use English
-rather than formal specifications for everything except syntax and
-lexical analysis.  This should make the document better understandable
-to the average reader, but will leave room for ambiguities.
-Consequently, if you were coming from Mars and tried to re-implement
-Python from this document alone, you might have to guess things and in
-fact you would probably end up implementing quite a different language.
-On the other hand, if you are using
-Python and wonder what the precise rules about a particular area of
-the language are, you should definitely be able to find them here.
-
-It is dangerous to add too many implementation details to a language
-reference document --- the implementation may change, and other
-implementations of the same language may work differently.  On the
-other hand, there is currently only one Python implementation, and
-its particular quirks are sometimes worth being mentioned, especially
-where the implementation imposes additional limitations.  Therefore,
-you'll find short ``implementation notes'' sprinkled throughout the
-text.
-
-Every Python implementation comes with a number of built-in and
-standard modules.  These are not documented here, but in the separate
-{\em Python Library Reference} document.  A few built-in modules are
-mentioned when they interact in a significant way with the language
-definition.
-
-\section{Notation}
-
-The descriptions of lexical analysis and syntax use a modified BNF
-grammar notation.  This uses the following style of definition:
-\index{BNF}
-\index{grammar}
-\index{syntax}
-\index{notation}
-
-\begin{verbatim}
-name:           lc_letter (lc_letter | "_")*
-lc_letter:      "a"..."z"
-\end{verbatim}
-
-The first line says that a \verb\name\ is an \verb\lc_letter\ followed by
-a sequence of zero or more \verb\lc_letter\s and underscores.  An
-\verb\lc_letter\ in turn is any of the single characters `a' through `z'.
-(This rule is actually adhered to for the names defined in lexical and
-grammar rules in this document.)
-
-Each rule begins with a name (which is the name defined by the rule)
-and a colon.  A vertical bar
-(\verb\|\) is used to separate alternatives; it is the least binding
-operator in this notation.  A star (\verb\*\) means zero or more
-repetitions of the preceding item; likewise, a plus (\verb\+\) means
-one or more repetitions, and a question mark (\verb\?\) zero or one
-(in other words, the preceding item is optional).  These three
-operators bind as tightly as possible; parentheses are used for
-grouping.  Literal strings are enclosed in double quotes.  White space
-is only meaningful to separate tokens.  Rules are normally contained
-on a single line; rules with many alternatives may be formatted
-alternatively with each line after the first beginning with a
-vertical bar.
-
-In lexical definitions (as the example above), two more conventions
-are used: Two literal characters separated by three dots mean a choice
-of any single character in the given (inclusive) range of ASCII
-characters.  A phrase between angular brackets (\verb\<...>\) gives an
-informal description of the symbol defined; e.g. this could be used
-to describe the notion of `control character' if needed.
-\index{lexical definitions}
-\index{ASCII}
-
-Even though the notation used is almost the same, there is a big
-difference between the meaning of lexical and syntactic definitions:
-a lexical definition operates on the individual characters of the
-input source, while a syntax definition operates on the stream of
-tokens generated by the lexical analysis.  All uses of BNF in the next
-chapter (``Lexical Analysis'') are lexical definitions; uses in
-subsequenc chapter are syntactic definitions.
-
-\chapter{Lexical analysis}
-
-A Python program is read by a {\em parser}.  Input to the parser is a
-stream of {\em tokens}, generated by the {\em lexical analyzer}.  This
-chapter describes how the lexical analyzer breaks a file into tokens.
-\index{lexical analysis}
-\index{parser}
-\index{token}
-
-\section{Line structure}
-
-A Python program is divided in a number of logical lines.  The end of
-a logical line is represented by the token NEWLINE.  Statements cannot
-cross logical line boundaries except where NEWLINE is allowed by the
-syntax (e.g. between statements in compound statements).
-\index{line structure}
-\index{logical line}
-\index{NEWLINE token}
-
-\subsection{Comments}
-
-A comment starts with a hash character (\verb\#\) that is not part of
-a string literal, and ends at the end of the physical line.  A comment
-always signifies the end of the logical line.  Comments are ignored by
-the syntax.
-\index{comment}
-\index{logical line}
-\index{physical line}
-\index{hash character}
-
-\subsection{Line joining}
-
-Two or more physical lines may be joined into logical lines using
-backslash characters (\verb/\/), as follows: when a physical line ends
-in a backslash that is not part of a string literal or comment, it is
-joined with the following forming a single logical line, deleting the
-backslash and the following end-of-line character.  For example:
-\index{physical line}
-\index{line joining}
-\index{backslash character}
-%
-\begin{verbatim}
-moth_names = ['Januari', 'Februari', 'Maart',     \
-              'April',   'Mei',      'Juni',      \
-              'Juli',    'Augustus', 'September', \
-              'Oktober', 'November', 'December']
-\end{verbatim}
-
-\subsection{Blank lines}
-
-A logical line that contains only spaces, tabs, and possibly a
-comment, is ignored (i.e., no NEWLINE token is generated), except that
-during interactive input of statements, an entirely blank logical line
-terminates a multi-line statement.
-\index{blank line}
-
-\subsection{Indentation}
-
-Leading whitespace (spaces and tabs) at the beginning of a logical
-line is used to compute the indentation level of the line, which in
-turn is used to determine the grouping of statements.
-\index{indentation}
-\index{whitespace}
-\index{leading whitespace}
-\index{space}
-\index{tab}
-\index{grouping}
-\index{statement grouping}
-
-First, tabs are replaced (from left to right) by one to eight spaces
-such that the total number of characters up to there is a multiple of
-eight (this is intended to be the same rule as used by {\UNIX}).  The
-total number of spaces preceding the first non-blank character then
-determines the line's indentation.  Indentation cannot be split over
-multiple physical lines using backslashes.
-
-The indentation levels of consecutive lines are used to generate
-INDENT and DEDENT tokens, using a stack, as follows.
-\index{INDENT token}
-\index{DEDENT token}
-
-Before the first line of the file is read, a single zero is pushed on
-the stack; this will never be popped off again.  The numbers pushed on
-the stack will always be strictly increasing from bottom to top.  At
-the beginning of each logical line, the line's indentation level is
-compared to the top of the stack.  If it is equal, nothing happens.
-If it larger, it is pushed on the stack, and one INDENT token is
-generated.  If it is smaller, it {\em must} be one of the numbers
-occurring on the stack; all numbers on the stack that are larger are
-popped off, and for each number popped off a DEDENT token is
-generated.  At the end of the file, a DEDENT token is generated for
-each number remaining on the stack that is larger than zero.
-
-Here is an example of a correctly (though confusingly) indented piece
-of Python code:
-
-\begin{verbatim}
-def perm(l):
-        # Compute the list of all permutations of l
-
-    if len(l) <= 1:
-                  return [l]
-    r = []
-    for i in range(len(l)):
-             s = l[:i] + l[i+1:]
-             p = perm(s)
-             for x in p:
-              r.append(l[i:i+1] + x)
-    return r
-\end{verbatim}
-
-The following example shows various indentation errors:
-
-\begin{verbatim}
-    def perm(l):                        # error: first line indented
-    for i in range(len(l)):             # error: not indented
-        s = l[:i] + l[i+1:]
-            p = perm(l[:i] + l[i+1:])   # error: unexpected indent
-            for x in p:
-                    r.append(l[i:i+1] + x)
-                return r                # error: inconsistent dedent
-\end{verbatim}
-
-(Actually, the first three errors are detected by the parser; only the
-last error is found by the lexical analyzer --- the indentation of
-\verb\return r\ does not match a level popped off the stack.)
-
-\section{Other tokens}
-
-Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
-exist: identifiers, keywords, literals, operators, and delimiters.
-Spaces and tabs are not tokens, but serve to delimit tokens.  Where
-ambiguity exists, a token comprises the longest possible string that
-forms a legal token, when read from left to right.
-
-\section{Identifiers}
-
-Identifiers (also referred to as names) are described by the following
-lexical definitions:
-\index{identifier}
-\index{name}
-
-\begin{verbatim}
-identifier:     (letter|"_") (letter|digit|"_")*
-letter:         lowercase | uppercase
-lowercase:      "a"..."z"
-uppercase:      "A"..."Z"
-digit:          "0"..."9"
-\end{verbatim}
-
-Identifiers are unlimited in length.  Case is significant.
-
-\subsection{Keywords}
-
-The following identifiers are used as reserved words, or {\em
-keywords} of the language, and cannot be used as ordinary
-identifiers.  They must be spelled exactly as written here:
-\index{keyword}
-\index{reserved word}
-
-\begin{verbatim}
-and        del        for        in         print
-break      elif       from       is         raise
-class      else       global     not        return
-continue   except     if         or         try
-def        finally    import     pass       while
-\end{verbatim}
-
-%	# This Python program sorts and formats the above table
-%	import string
-%	l = []
-%	try:
-%		while 1:
-%			l = l + string.split(raw_input())
-%	except EOFError:
-%		pass
-%	l.sort()
-%	for i in range((len(l)+4)/5):
-%		for j in range(i, len(l), 5):
-%			print string.ljust(l[j], 10),
-%		print
-
-\section{Literals} \label{literals}
-
-Literals are notations for constant values of some built-in types.
-\index{literal}
-\index{constant}
-
-\subsection{String literals}
-
-String literals are described by the following lexical definitions:
-\index{string literal}
-
-\begin{verbatim}
-stringliteral:  "'" stringitem* "'"
-stringitem:     stringchar | escapeseq
-stringchar:     <any ASCII character except newline or "\" or "'">
-escapeseq:      "'" <any ASCII character except newline>
-\end{verbatim}
-\index{ASCII}
-
-String literals cannot span physical line boundaries.  Escape
-sequences in strings are actually interpreted according to rules
-simular to those used by Standard C.  The recognized escape sequences
-are:
-\index{physical line}
-\index{escape sequence}
-\index{Standard C}
-\index{C}
-
-\begin{center}
-\begin{tabular}{|l|l|}
-\hline
-\verb/\\/	& Backslash (\verb/\/) \\
-\verb/\'/	& Single quote (\verb/'/) \\
-\verb/\a/	& ASCII Bell (BEL) \\
-\verb/\b/	& ASCII Backspace (BS) \\
-%\verb/\E/	& ASCII Escape (ESC) \\
-\verb/\f/	& ASCII Formfeed (FF) \\
-\verb/\n/	& ASCII Linefeed (LF) \\
-\verb/\r/	& ASCII Carriage Return (CR) \\
-\verb/\t/	& ASCII Horizontal Tab (TAB) \\
-\verb/\v/	& ASCII Vertical Tab (VT) \\
-\verb/\/{\em ooo}	& ASCII character with octal value {\em ooo} \\
-\verb/\x/{\em xx...}	& ASCII character with hex value {\em xx...} \\
-\hline
-\end{tabular}
-\end{center}
-\index{ASCII}
-
-In strict compatibility with in Standard C, up to three octal digits are
-accepted, but an unlimited number of hex digits is taken to be part of
-the hex escape (and then the lower 8 bits of the resulting hex number
-are used in all current implementations...).
-
-All unrecognized escape sequences are left in the string unchanged,
-i.e., {\em the backslash is left in the string.}  (This behavior is
-useful when debugging: if an escape sequence is mistyped, the
-resulting output is more easily recognized as broken.  It also helps a
-great deal for string literals used as regular expressions or
-otherwise passed to other modules that do their own escape handling.)
-\index{unrecognized escape sequence}
-
-\subsection{Numeric literals}
-
-There are three types of numeric literals: plain integers, long
-integers, and floating point numbers.
-\index{number}
-\index{numeric literal}
-\index{integer literal}
-\index{plain integer literal}
-\index{long integer literal}
-\index{floating point literal}
-\index{hexadecimal literal}
-\index{octal literal}
-\index{decimal literal}
-
-Integer and long integer literals are described by the following
-lexical definitions:
-
-\begin{verbatim}
-longinteger:    integer ("l"|"L")
-integer:        decimalinteger | octinteger | hexinteger
-decimalinteger: nonzerodigit digit* | "0"
-octinteger:     "0" octdigit+
-hexinteger:     "0" ("x"|"X") hexdigit+
-
-nonzerodigit:   "1"..."9"
-octdigit:       "0"..."7"
-hexdigit:        digit|"a"..."f"|"A"..."F"
-\end{verbatim}
-
-Although both lower case `l' and upper case `L' are allowed as suffix
-for long integers, it is strongly recommended to always use `L', since
-the letter `l' looks too much like the digit `1'.
-
-Plain integer decimal literals must be at most $2^{31} - 1$ (i.e., the
-largest positive integer, assuming 32-bit arithmetic).  Plain octal and
-hexadecimal literals may be as large as $2^{32} - 1$, but values
-larger than $2^{31} - 1$ are converted to a negative value by
-subtracting $2^{32}$.  There is no limit for long integer literals.
-
-Some examples of plain and long integer literals:
-
-\begin{verbatim}
-7     2147483647                        0177    0x80000000
-3L    79228162514264337593543950336L    0377L   0x100000000L
-\end{verbatim}
-
-Floating point literals are described by the following lexical
-definitions:
-
-\begin{verbatim}
-floatnumber:    pointfloat | exponentfloat
-pointfloat:     [intpart] fraction | intpart "."
-exponentfloat:  (intpart | pointfloat) exponent
-intpart:        digit+
-fraction:       "." digit+
-exponent:       ("e"|"E") ["+"|"-"] digit+
-\end{verbatim}
-
-The allowed range of floating point literals is
-implementation-dependent.
-
-Some examples of floating point literals:
-
-\begin{verbatim}
-3.14    10.    .001    1e100    3.14e-10
-\end{verbatim}
-
-Note that numeric literals do not include a sign; a phrase like
-\verb\-1\ is actually an expression composed of the operator
-\verb\-\ and the literal \verb\1\.
-
-\section{Operators}
-
-The following tokens are operators:
-\index{operators}
-
-\begin{verbatim}
-+       -       *       /       %
-<<      >>      &       |       ^       ~
-<       ==      >       <=      <>      !=      >=
-\end{verbatim}
-
-The comparison operators \verb\<>\ and \verb\!=\ are alternate
-spellings of the same operator.
-
-\section{Delimiters}
-
-The following tokens serve as delimiters or otherwise have a special
-meaning:
-\index{delimiters}
-
-\begin{verbatim}
-(       )       [       ]       {       }
-;       ,       :       .       `       =
-\end{verbatim}
-
-The following printing ASCII characters are not used in Python.  Their
-occurrence outside string literals and comments is an unconditional
-error:
-\index{ASCII}
-
-\begin{verbatim}
-!       @       $       "       ?
-\end{verbatim}
-
-They may be used by future versions of the language though!
-
-\chapter{Data model}
-
-\section{Objects, values and types}
-
-{\em Objects} are Python's abstraction for data.  All data in a Python
-program is represented by objects or by relations between objects.
-(In a sense, and in conformance to Von Neumann's model of a
-``stored program computer'', code is also represented by objects.)
-\index{object}
-\index{data}
-
-Every object has an identity, a type and a value.  An object's {\em
-identity} never changes once it has been created; you may think of it
-as the object's address in memory.  An object's {\em type} is also
-unchangeable.  It determines the operations that an object supports
-(e.g. ``does it have a length?'') and also defines the possible
-values for objects of that type.  The {\em value} of some objects can
-change.  Objects whose value can change are said to be {\em mutable};
-objects whose value is unchangeable once they are created are called
-{\em immutable}.  The type determines an object's (im)mutability.
-\index{identity of an object}
-\index{value of an object}
-\index{type of an object}
-\index{mutable object}
-\index{immutable object}
-
-Objects are never explicitly destroyed; however, when they become
-unreachable they may be garbage-collected.  An implementation is
-allowed to delay garbage collection or omit it altogether --- it is a
-matter of implementation quality how garbage collection is
-implemented, as long as no objects are collected that are still
-reachable.  (Implementation note: the current implementation uses a
-reference-counting scheme which collects most objects as soon as they
-become unreachable, but never collects garbage containing circular
-references.)
-\index{garbage collection}
-\index{reference counting}
-\index{unreachable object}
-
-Note that the use of the implementation's tracing or debugging
-facilities may keep objects alive that would normally be collectable.
-
-Some objects contain references to ``external'' resources such as open
-files or windows.  It is understood that these resources are freed
-when the object is garbage-collected, but since garbage collection is
-not guaranteed to happen, such objects also provide an explicit way to
-release the external resource, usually a \verb\close\ method.
-Programs are strongly recommended to always explicitly close such
-objects.
-
-Some objects contain references to other objects; these are called
-{\em containers}.  Examples of containers are tuples, lists and
-dictionaries.  The references are part of a container's value.  In
-most cases, when we talk about the value of a container, we imply the
-values, not the identities of the contained objects; however, when we
-talk about the (im)mutability of a container, only the identities of
-the immediately contained objects are implied.  (So, if an immutable
-container contains a reference to a mutable object, its value changes
-if that mutable object is changed.)
-\index{container}
-
-Types affect almost all aspects of objects' lives.  Even the meaning
-of object identity is affected in some sense: for immutable types,
-operations that compute new values may actually return a reference to
-any existing object with the same type and value, while for mutable
-objects this is not allowed.  E.g. after
-
-\begin{verbatim}
-a = 1; b = 1; c = []; d = []
-\end{verbatim}
-
-\verb\a\ and \verb\b\ may or may not refer to the same object with the
-value one, depending on the implementation, but \verb\c\ and \verb\d\
-are guaranteed to refer to two different, unique, newly created empty
-lists.
-
-\section{The standard type hierarchy} \label{types}
-
-Below is a list of the types that are built into Python.  Extension
-modules written in C can define additional types.  Future versions of
-Python may add types to the type hierarchy (e.g. rational or complex
-numbers, efficiently stored arrays of integers, etc.).
-\index{type}
-\indexii{data}{type}
-\indexii{type}{hierarchy}
-\indexii{extension}{module}
-\index{C}
-
-Some of the type descriptions below contain a paragraph listing
-`special attributes'.  These are attributes that provide access to the
-implementation and are not intended for general use.  Their definition
-may change in the future.  There are also some `generic' special
-attributes, not listed with the individual objects: \verb\__methods__\
-is a list of the method names of a built-in object, if it has any;
-\verb\__members__\ is a list of the data attribute names of a built-in
-object, if it has any.
-\index{attribute}
-\indexii{special}{attribute}
-\indexiii{generic}{special}{attribute}
-\ttindex{__methods__}
-\ttindex{__members__}
-
-\begin{description}
-
-\item[None]
-This type has a single value.  There is a single object with this value.
-This object is accessed through the built-in name \verb\None\.
-It is returned from functions that don't explicitly return an object.
-\ttindex{None}
-\obindex{None@{\tt None}}
-
-\item[Numbers]
-These are created by numeric literals and returned as results by
-arithmetic operators and arithmetic built-in functions.  Numeric
-objects are immutable; once created their value never changes.  Python
-numbers are of course strongly related to mathematical numbers, but
-subject to the limitations of numerical representation in computers.
-\obindex{number}
-\obindex{numeric}
-
-Python distinguishes between integers and floating point numbers:
-
-\begin{description}
-\item[Integers]
-These represent elements from the mathematical set of whole numbers.
-\obindex{integer}
-
-There are two types of integers:
-
-\begin{description}
-
-\item[Plain integers]
-These represent numbers in the range $-2^{31}$ through $2^{31}-1$.
-(The range may be larger on machines with a larger natural word
-size, but not smaller.)
-When the result of an operation falls outside this range, the
-exception \verb\OverflowError\ is raised.
-For the purpose of shift and mask operations, integers are assumed to
-have a binary, 2's complement notation using 32 or more bits, and
-hiding no bits from the user (i.e., all $2^{32}$ different bit
-patterns correspond to different values).
-\obindex{plain integer}
-
-\item[Long integers]
-These represent numbers in an unlimited range, subject to avaiable
-(virtual) memory only.  For the purpose of shift and mask operations,
-a binary representation is assumed, and negative numbers are
-represented in a variant of 2's complement which gives the illusion of
-an infinite string of sign bits extending to the left.
-\obindex{long integer}
-
-\end{description} % Integers
-
-The rules for integer representation are intended to give the most
-meaningful interpretation of shift and mask operations involving
-negative integers and the least surprises when switching between the
-plain and long integer domains.  For any operation except left shift,
-if it yields a result in the plain integer domain without causing
-overflow, it will yield the same result in the long integer domain or
-when using mixed operands.
-\indexii{integer}{representation}
-
-\item[Floating point numbers]
-These represent machine-level double precision floating point numbers.  
-You are at the mercy of the underlying machine architecture and
-C implementation for the accepted range and handling of overflow.
-\obindex{floating point}
-\indexii{floating point}{number}
-\index{C}
-
-\end{description} % Numbers
-
-\item[Sequences]
-These represent finite ordered sets indexed by natural numbers.
-The built-in function \verb\len()\ returns the number of elements
-of a sequence.  When this number is $n$, the index set contains
-the numbers $0, 1, \ldots, n-1$.  Element \verb\i\ of sequence
-\verb\a\ is selected by \verb\a[i]\.
-\obindex{seqence}
-\bifuncindex{len}
-\index{index operation}
-\index{item selection}
-\index{subscription}
-
-Sequences also support slicing: \verb\a[i:j]\ selects all elements
-with index $k$ such that $i <= k < j$.  When used as an expression,
-a slice is a sequence of the same type --- this implies that the
-index set is renumbered so that it starts at 0 again.
-\index{slicing}
-
-Sequences are distinguished according to their mutability:
-
-\begin{description}
-%
-\item[Immutable sequences]
-An object of an immutable sequence type cannot change once it is
-created.  (If the object contains references to other objects,
-these other objects may be mutable and may be changed; however
-the collection of objects directly referenced by an immutable object
-cannot change.)
-\obindex{immutable sequence}
-\obindex{immutable}
-
-The following types are immutable sequences:
-
-\begin{description}
-
-\item[Strings]
-The elements of a string are characters.  There is no separate
-character type; a character is represented by a string of one element.
-Characters represent (at least) 8-bit bytes.  The built-in
-functions \verb\chr()\ and \verb\ord()\ convert between characters
-and nonnegative integers representing the byte values.
-Bytes with the values 0-127 represent the corresponding ASCII values.
-The string data type is also used to represent arrays of bytes, e.g.
-to hold data read from a file.
-\obindex{string}
-\index{character}
-\index{byte}
-\index{ASCII}
-\bifuncindex{chr}
-\bifuncindex{ord}
-
-(On systems whose native character set is not ASCII, strings may use
-EBCDIC in their internal representation, provided the functions
-\verb\chr()\ and \verb\ord()\ implement a mapping between ASCII and
-EBCDIC, and string comparison preserves the ASCII order.
-Or perhaps someone can propose a better rule?)
-\index{ASCII}
-\index{EBCDIC}
-\index{character set}
-\indexii{string}{comparison}
-\bifuncindex{chr}
-\bifuncindex{ord}
-
-\item[Tuples]
-The elements of a tuple are arbitrary Python objects.
-Tuples of two or more elements are formed by comma-separated lists
-of expressions.  A tuple of one element (a `singleton') can be formed
-by affixing a comma to an expression (an expression by itself does
-not create a tuple, since parentheses must be usable for grouping of
-expressions).  An empty tuple can be formed by enclosing `nothing' in
-parentheses.
-\obindex{tuple}
-\indexii{singleton}{tuple}
-\indexii{empty}{tuple}
-
-\end{description} % Immutable sequences
-
-\item[Mutable sequences]
-Mutable sequences can be changed after they are created.  The
-subscription and slicing notations can be used as the target of
-assignment and \verb\del\ (delete) statements.
-\obindex{mutable sequece}
-\obindex{mutable}
-\indexii{assignment}{statement}
-\index{delete}
-\stindex{del}
-\index{subscription}
-\index{slicing}
-
-There is currently a single mutable sequence type:
-
-\begin{description}
-
-\item[Lists]
-The elements of a list are arbitrary Python objects.  Lists are formed
-by placing a comma-separated list of expressions in square brackets.
-(Note that there are no special cases needed to form lists of length 0
-or 1.)
-\obindex{list}
-
-\end{description} % Mutable sequences
-
-\end{description} % Sequences
-
-\item[Mapping types]
-These represent finite sets of objects indexed by arbitrary index sets.
-The subscript notation \verb\a[k]\ selects the element indexed
-by \verb\k\ from the mapping \verb\a\; this can be used in
-expressions and as the target of assignments or \verb\del\ statements.
-The built-in function \verb\len()\ returns the number of elements
-in a mapping.
-\bifuncindex{len}
-\index{subscription}
-\obindex{mapping}
-
-There is currently a single mapping type:
-
-\begin{description}
-
-\item[Dictionaries]
-These represent finite sets of objects indexed by strings.
-Dictionaries are mutable; they are created by the \verb\{...}\
-notation (see section \ref{dict}).  (Implementation note: the strings
-used for indexing must not contain null bytes.)
-\obindex{dictionary}
-\obindex{mutable}
-
-\end{description} % Mapping types
-
-\item[Callable types]
-These are the types to which the function call (invocation) operation,
-written as \verb\function(argument, argument, ...)\, can be applied:
-\indexii{function}{call}
-\index{invocation}
-\indexii{function}{argument}
-\obindex{callable}
-
-\begin{description}
-
-\item[User-defined functions]
-A user-defined function object is created by a function definition
-(see section \ref{function}).  It should be called with an argument
-list containing the same number of items as the function's formal
-parameter list.
-\indexii{user-defined}{function}
-\obindex{function}
-\obindex{user-defined function}
-
-Special read-only attributes: \verb\func_code\ is the code object
-representing the compiled function body, and \verb\func_globals\ is (a
-reference to) the dictionary that holds the function's global
-variables --- it implements the global name space of the module in
-which the function was defined.
-\ttindex{func_code}
-\ttindex{func_globals}
-\indexii{global}{name space}
-
-\item[User-defined methods]
-A user-defined method (a.k.a. {\em object closure}) is a pair of a
-class instance object and a user-defined function.  It should be
-called with an argument list containing one item less than the number
-of items in the function's formal parameter list.  When called, the
-class instance becomes the first argument, and the call arguments are
-shifted one to the right.
-\obindex{method}
-\obindex{user-defined method}
-indexii{user-defined}{method}
-\index{object closure}
-
-Special read-only attributes: \verb\im_self\ is the class instance
-object, \verb\im_func\ is the function object.
-\ttindex{im_func}
-\ttindex{im_self}
-
-\item[Built-in functions]
-A built-in function object is a wrapper around a C function.  Examples
-of built-in functions are \verb\len\ and \verb\math.sin\.  There
-are no special attributes.  The number and type of the arguments are
-determined by the C function.
-\obindex{built-in function}
-\obindex{function}
-\index{C}
-
-\item[Built-in methods]
-This is really a different disguise of a built-in function, this time
-containing an object passed to the C function as an implicit extra
-argument.  An example of a built-in method is \verb\list.append\ if
-\verb\list\ is a list object.
-\obindex{built-in method}
-\obindex{method}
-\indexii{built-in}{method}
-
-\item[Classes]
-Class objects are described below.  When a class object is called as a
-parameterless function, a new class instance (also described below) is
-created and returned.  The class's initialization function is not
-called --- this is the responsibility of the caller.  It is illegal to
-call a class object with one or more arguments.
-\obindex{class}
-\obindex{class instance}
-\obindex{instance}
-\indexii{class object}{call}
-
-\end{description}
-
-\item[Modules]
-Modules are imported by the \verb\import\ statement (see section
-\ref{import}).  A module object is a container for a module's name
-space, which is a dictionary (the same dictionary as referenced by the
-\verb\func_globals\ attribute of functions defined in the module).
-Module attribute references are translated to lookups in this
-dictionary.  A module object does not contain the code object used to
-initialize the module (since it isn't needed once the initialization
-is done).
-\stindex{import}
-\obindex{module}
-
-Attribute assignment update the module's name space dictionary.
-
-Special read-only attributes: \verb\__dict__\ yields the module's name
-space as a dictionary object; \verb\__name__\ yields the module's name
-as a string object.
-\ttindex{__dict__}
-\ttindex{__name__}
-\indexii{module}{name space}
-
-\item[Classes]
-Class objects are created by class definitions (see section
-\ref{class}).  A class is a container for a dictionary containing the
-class's name space.  Class attribute references are translated to
-lookups in this dictionary.  When an attribute name is not found
-there, the attribute search continues in the base classes.  The search
-is depth-first, left-to-right in the order of their occurrence in the
-base class list.
-\obindex{class}
-\obindex{class instance}
-\obindex{instance}
-\indexii{class object}{call}
-\index{container}
-\index{dictionary}
-\indexii{class}{attribute}
-
-Class attribute assignments update the class's dictionary, never the
-dictionary of a base class.
-\indexiii{class}{attribute}{assignment}
-
-A class can be called as a parameterless function to yield a class
-instance (see above).
-\indexii{class object}{call}
-
-Special read-only attributes: \verb\__dict__\ yields te dictionary
-containing the class's name space; \verb\__bases__\ yields a tuple
-(possibly empty or a singleton) containing the base classes, in the
-order of their occurrence in the base class list.
-\ttindex{__dict__}
-\ttindex{__bases__}
-
-\item[Class instances]
-A class instance is created by calling a class object as a
-parameterless function.  A class instance has a dictionary in which
-attribute references are searched.  When an attribute is not found
-there, and the instance's class has an attribute by that name, and
-that class attribute is a user-defined function (and in no other
-cases), the instance attribute reference yields a user-defined method
-object (see above) constructed from the instance and the function.
-\obindex{class instance}
-\obindex{instance}
-\indexii{class}{instance}
-\indexii{class instance}{attribute}
-
-Attribute assignments update the instance's dictionary.
-\indexiii{class instance}{attribute}{assignment}
-
-Special read-only attributes: \verb\__dict__\ yields the attribute
-dictionary; \verb\__class__\ yields the instance's class.
-\ttindex{__dict__}
-\ttindex{__class__}
-
-\item[Files]
-A file object represents an open file.  (It is a wrapper around a C
-{\tt stdio} file pointer.)  File objects are created by the
-\verb\open()\ built-in function, and also by \verb\posix.popen()\ and
-the \verb\makefile\ method of socket objects.  \verb\sys.stdin\,
-\verb\sys.stdout\ and \verb\sys.stderr\ are file objects corresponding
-the the interpreter's standard input, output and error streams.
-See the Python Library Reference for methods of file objects and other
-details.
-\obindex{file}
-\index{C}
-\index{stdio}
-\bifuncindex{open}
-\bifuncindex{popen}
-\bifuncindex{makefile}
-\ttindex{stdin}
-\ttindex{stdout}
-\ttindex{stderr}
-\ttindex{sys.stdin}
-\ttindex{sys.stdout}
-\ttindex{sys.stderr}
-
-\item[Internal types]
-A few types used internally by the interpreter are exposed to the user.
-Their definition may change with future versions of the interpreter,
-but they are mentioned here for completeness.
-\index{internal type}
-
-\begin{description}
-
-\item[Code objects]
-Code objects represent executable code.  The difference between a code
-object and a function object is that the function object contains an
-explicit reference to the function's context (the module in which it
-was defined) which a code object contains no context.  There is no way
-to execute a bare code object.
-\obindex{code}
-
-Special read-only attributes: \verb\co_code\ is a string representing
-the sequence of instructions; \verb\co_consts\ is a list of literals
-used by the code; \verb\co_names\ is a list of names (strings) used by
-the code; \verb\co_filename\ is the filename from which the code was
-compiled.  (To find out the line numbers, you would have to decode the
-instructions; the standard library module \verb\dis\ contains an
-example of how to do this.)
-\ttindex{co_code}
-\ttindex{co_consts}
-\ttindex{co_names}
-\ttindex{co_filename}
-
-\item[Frame objects]
-Frame objects represent execution frames.  They may occur in traceback
-objects (see below).
-\obindex{frame}
-
-Special read-only attributes: \verb\f_back\ is to the previous
-stack frame (towards the caller), or \verb\None\ if this is the bottom
-stack frame; \verb\f_code\ is the code object being executed in this
-frame; \verb\f_globals\ is the dictionary used to look up global
-variables; \verb\f_locals\ is used for local variables;
-\verb\f_lineno\ gives the line number and \verb\f_lasti\ gives the
-precise instruction (this is an index into the instruction string of
-the code object).
-\ttindex{f_back}
-\ttindex{f_code}
-\ttindex{f_globals}
-\ttindex{f_locals}
-\ttindex{f_lineno}
-\ttindex{f_lasti}
-
-\item[Traceback objects]
-Traceback objects represent a stack trace of an exception.  A
-traceback object is created when an exception occurs.  When the search
-for an exception handler unwinds the execution stack, at each unwound
-level a traceback object is inserted in front of the current
-traceback.  When an exception handler is entered, the stack trace is
-made available to the program as \verb\sys.exc_traceback\.  When the
-program contains no suitable handler, the stack trace is written
-(nicely formatted) to the standard error stream; if the interpreter is
-interactive, it is also made available to the user as
-\verb\sys.last_traceback\.
-\obindex{traceback}
-\indexii{stack}{trace}
-\indexii{exception}{handler}
-\indexii{execution}{stack}
-\ttindex{exc_traceback}
-\ttindex{last_traceback}
-\ttindex{sys.exc_traceback}
-\ttindex{sys.last_traceback}
-
-Special read-only attributes: \verb\tb_next\ is the next level in the
-stack trace (towards the frame where the exception occurred), or
-\verb\None\ if there is no next level; \verb\tb_frame\ points to the
-execution frame of the current level; \verb\tb_lineno\ gives the line
-number where the exception occurred; \verb\tb_lasti\ indicates the
-precise instruction.  The line number and last instruction in the
-traceback may differ from the line number of its frame object if the
-exception occurred in a \verb\try\ statement with no matching
-\verb\except\ clause or with a \verb\finally\ clause.
-\ttindex{tb_next}
-\ttindex{tb_frame}
-\ttindex{tb_lineno}
-\ttindex{tb_lasti}
-\stindex{try}
-
-\end{description} % Internal types
-
-\end{description} % Types
-
-\chapter{Execution model}
-\index{execution model}
-
-\section{Code blocks, execution frames, and name spaces} \label{execframes}
-\index{code block}
-\indexii{execution}{frame}
-\index{name space}
-
-A {\em code block} is a piece of Python program text that can be
-executed as a unit, such as a module, a class definition or a function
-body.  Some code blocks (like modules) are executed only once, others
-(like function bodies) may be executed many times.  Code block may
-textually contain other code blocks.  Code blocks may invoke other
-code blocks (that may or may not be textually contained in them) as
-part of their execution, e.g. by invoking (calling) a function.
-\index{code block}
-\indexii{code}{block}
-
-The following are code blocks:  A module is a code block.  A function
-body is a code block.  A class definition is a code block.  Each
-command typed interactively is a separate code block; a script file is
-a code block.  The string argument passed to the built-in functions
-\verb\eval\ and \verb\exec\ are code blocks.  And finally, the
-expression read and evaluated by the built-in function \verb\input\ is
-a code block.
-
-A code block is executed in an execution frame.  An {\em execution
-frame} contains some administrative information (used for debugging),
-determines where and how execution continues after the code block's
-execution has completed, and (perhaps most importantly) defines two
-name spaces, the local and the global name space, that affect
-execution of the code block.
-\indexii{execution}{frame}
-
-A {\em name space} is a mapping from names (identifiers) to objects.
-A particular name space may be referenced by more than one execution
-frame, and from other places as well.  Adding a name to a name space
-is called {\em binding} a name (to an object); changing the mapping of
-a name is called {\em rebinding}; removing a name is {\em unbinding}.
-Name spaces are functionally equivalent to dictionaries.
-\index{name space}
-\indexii{binding}{name}
-\indexii{rebinding}{name}
-\indexii{unbinding}{name}
-
-The {\em local name space} of an execution frame determines the default
-place where names are defined and searched.  The {\em global name
-space} determines the place where names listed in \verb\global\
-statements are defined and searched, and where names that are not
-explicitly bound in the current code block are searched.
-\indexii{local}{name space}
-\indexii{global}{name space}
-\stindex{global}
-
-Whether a name is local or global in a code block is determined by
-static inspection of the source text for the code block: in the
-absence of \verb\global\ statements, a name that is bound anywhere in
-the code block is local in the entire code block; all other names are
-considered global.  The \verb\global\ statement forces global
-interpretation of selected names throughout the code block.  The
-following constructs bind names: formal parameters, \verb\import\
-statements, class and function definitions (these bind the class or
-function name), and targets that are identifiers if occurring in an
-assignment, \verb\for\ loop header, or \verb\except\ clause header.
-(A target occurring in a \verb\del\ statement does not bind a name.)
-
-When a global name is not found in the global name space, it is
-searched in the list of ``built-in'' names (which is actually the
-global name space of the module \verb\builtin\).  When a name is not
-found at all, the \verb\NameError\ exception is raised.
-
-The following table lists the meaning of the local and global name
-space for various types of code blocks.  The name space for a
-particular module is automatically created when the module is first
-referenced.
-
-\begin{center}
-\begin{tabular}{|l|l|l|l|}
-\hline
-Code block type & Global name space & Local name space & Notes \\
-\hline
-Module & n.s. for this module & same as global & \\
-Script & n.s. for \verb\__main__\ & same as global & \\
-Interactive command & n.s. for \verb\__main__\ & same as global & \\
-Class definition & global n.s. of containing block & new n.s. & \\
-Function body & global n.s. of containing block & new n.s. & \\
-String passed to \verb\exec\ or \verb\eval\
-	& global n.s. of caller & local n.s. of caller & (1) \\
-File read by \verb\execfile\
-	& global n.s. of caller & local n.s. of caller & (1) \\
-Expression read by \verb\input\
-	& global n.s. of caller & local n.s. of caller & \\
-\hline
-\end{tabular}
-\end{center}
-
-Notes:
-
-\begin{description}
-
-\item[n.s.] means {\em name space}
-
-\item[(1)] The global and local name space for these functions can be
-overridden with optional extra arguments.
-
-\end{description}
-
-\section{Exceptions}
-
-Exceptions are a means of breaking out of the normal flow of control
-of a code block in order to handle errors or other exceptional
-conditions.  An exception is {\em raised} at the point where the error
-is detected; it may be {\em handled} by the surrounding code block or
-by any code block that directly or indirectly invoked the code block
-where the error occurred.
-\index{exception}
-\index{raise an exception}
-\index{handle an exception}
-\index{exception handler}
-\index{errors}
-\index{error handling}
-
-The Python interpreter raises an exception when it detects an run-time
-error (such as division by zero).  A Python program can also
-explicitly raise an exception with the \verb\raise\ statement.
-Exception handlers are specified with the \verb\try...except\
-statement.
-
-Python uses the ``termination'' model of error handling: an exception
-handler can find out what happened and continue execution at an outer
-level, but it cannot repair the cause of the error and retry the
-failing operation (except by re-entering the the offending piece of
-code from the top).
-
-When an exception is not handled at all, the interpreter terminates
-execution of the program, or returns to its interactive main loop.
-
-Exceptions are identified by string objects.  Two different string
-objects with the same value identify different exceptions.
-
-When an exception is raised, an object (maybe \verb\None\) is passed
-as the exception's ``parameter''; this object does not affect the
-selection of an exception handler, but is passed to the selected
-exception handler as additional information.
-
-See also the description of the \verb\try\ and \verb\raise\
-statements.
-
-\chapter{Expressions and conditions}
-\index{expression}
-\index{condition}
-
-{\bf Note:} In this and the following chapters, extended BNF notation
-will be used to describe syntax, not lexical analysis.
-\index{BNF}
-
-This chapter explains the meaning of the elements of expressions and
-conditions.  Conditions are a superset of expressions, and a condition
-may be used wherever an expression is required by enclosing it in
-parentheses.  The only places where expressions are used in the syntax
-instead of conditions is in expression statements and on the
-right-hand side of assignment statements; this catches some nasty bugs
-like accedentally writing \verb\x == 1\ instead of \verb\x = 1\.
-\indexii{assignment}{statement}
-
-The comma plays several roles in Python's syntax.  It is usually an
-operator with a lower precedence than all others, but occasionally
-serves other purposes as well; e.g. it separates function arguments,
-is used in list and dictionary constructors, and has special semantics
-in \verb\print\ statements.
-\index{comma}
-
-When (one alternative of) a syntax rule has the form
-
-\begin{verbatim}
-name:           othername
-\end{verbatim}
-
-and no semantics are given, the semantics of this form of \verb\name\
-are the same as for \verb\othername\.
-\index{syntax}
-
-\section{Arithmetic conversions}
-\indexii{arithmetic}{conversion}
-
-When a description of an arithmetic operator below uses the phrase
-``the numeric arguments are converted to a common type'',
-this both means that if either argument is not a number, a
-\verb\TypeError\ exception is raised, and that otherwise
-the following conversions are applied:
-\exindex{TypeError}
-\indexii{floating point}{number}
-\indexii{long}{integer}
-\indexii{plain}{integer}
-
-\begin{itemize}
-\item	first, if either argument is a floating point number,
-	the other is converted to floating point;
-\item	else, if either argument is a long integer,
-	the other is converted to long integer;
-\item	otherwise, both must be plain integers and no conversion
-	is necessary.
-\end{itemize}
-
-\section{Atoms}
-\index{atom}
-
-Atoms are the most basic elements of expressions.  Forms enclosed in
-reverse quotes or in parentheses, brackets or braces are also
-categorized syntactically as atoms.  The syntax for atoms is:
-
-\begin{verbatim}
-atom:           identifier | literal | enclosure
-enclosure:      parenth_form | list_display | dict_display | string_conversion
-\end{verbatim}
-
-\subsection{Identifiers (Names)}
-\index{name}
-\index{identifier}
-
-An identifier occurring as an atom is a reference to a local, global
-or built-in name binding.  If a name can be assigned to anywhere in a
-code block, and is not mentioned in a \verb\global\ statement in that
-code block, it refers to a local name throughout that code block.
-Otherwise, it refers to a global name if one exists, else to a
-built-in name.
-\indexii{name}{binding}
-\index{code block}
-\stindex{global}
-\indexii{built-in}{name}
-\indexii{global}{name}
-
-When the name is bound to an object, evaluation of the atom yields
-that object.  When a name is not bound, an attempt to evaluate it
-raises a \verb\NameError\ exception.
-\exindex{NameError}
-
-\subsection{Literals}
-\index{literal}
-
-Python knows string and numeric literals:
-
-\begin{verbatim}
-literal:        stringliteral | integer | longinteger | floatnumber
-\end{verbatim}
-
-Evaluation of a literal yields an object of the given type (string,
-integer, long integer, floating point number) with the given value.
-The value may be approximated in the case of floating point literals.
-See section \ref{literals} for details.
-
-All literals correspond to immutable data types, and hence the
-object's identity is less important than its value.  Multiple
-evaluations of literals with the same value (either the same
-occurrence in the program text or a different occurrence) may obtain
-the same object or a different object with the same value.
-\indexiii{immutable}{data}{type}
-
-(In the original implementation, all literals in the same code block
-with the same type and value yield the same object.)
-
-\subsection{Parenthesized forms}
-\index{parenthesized form}
-
-A parenthesized form is an optional condition list enclosed in
-parentheses:
-
-\begin{verbatim}
-parenth_form:      "(" [condition_list] ")"
-\end{verbatim}
-
-A parenthesized condition list yields whatever that condition list
-yields.
-
-An empty pair of parentheses yields an empty tuple object.  Since
-tuples are immutable, the rules for literals apply here.
-\indexii{empty}{tuple}
-
-(Note that tuples are not formed by the parentheses, but rather by use
-of the comma operator.  The exception is the empty tuple, for which
-parentheses {\em are} required --- allowing unparenthesized ``nothing''
-in expressions would causes ambiguities and allow common typos to
-pass uncaught.)
-\index{comma}
-\index{tuple}{display}
-
-\subsection{List displays}
-\indexii{list}{display}
-
-A list display is a possibly empty series of conditions enclosed in
-square brackets:
-
-\begin{verbatim}
-list_display:   "[" [condition_list] "]"
-\end{verbatim}
-
-A list display yields a new list object.
-\obindex{list}
-
-If it has no condition list, the list object has no items.  Otherwise,
-the elements of the condition list are evaluated from left to right
-and inserted in the list object in that order.
-\indexii{empty}{list}
-
-\subsection{Dictionary displays} \label{dict}
-\indexii{dictionary}{display}
-
-A dictionary display is a possibly empty series of key/datum pairs
-enclosed in curly braces:
-\index{key}
-\index{datum}
-\index{key/datum pair}
-
-\begin{verbatim}
-dict_display:   "{" [key_datum_list] "}"
-key_datum_list: [key_datum ("," key_datum)* [","]
-key_datum:      condition ":" condition
-\end{verbatim}
-
-A dictionary display yields a new dictionary object.
-\obindex{dictionary}
-
-The key/datum pairs are evaluated from left to right to define the
-entries of the dictionary: each key object is used as a key into the
-dictionary to store the corresponding datum.
-
-Keys must be strings, otherwise a \verb\TypeError\ exception is
-raised.  Clashes between duplicate keys are not detected; the last
-datum (textually rightmost in the display) stored for a given key
-value prevails.
-\exindex{TypeError}
-
-\subsection{String conversions}
-\indexii{string}{conversion}
-
-A string conversion is a condition list enclosed in reverse (or
-backward) quotes:
-
-\begin{verbatim}
-string_conversion: "`" condition_list "`"
-\end{verbatim}
-
-A string conversion evaluates the contained condition list and
-converts the resulting object into a string according to rules
-specific to its type.
-
-If the object is a string, a number, \verb\None\, or a tuple, list or
-dictionary containing only objects whose type is one of these, the
-resulting string is a valid Python expression which can be passed to
-the built-in function \verb\eval()\ to yield an expression with the
-same value (or an approximation, if floating point numbers are
-involved).
-
-(In particular, converting a string adds quotes around it and converts
-``funny'' characters to escape sequences that are safe to print.)
-
-It is illegal to attempt to convert recursive objects (e.g. lists or
-dictionaries that contain a reference to themselves, directly or
-indirectly.)
-\obindex{recursive}
-
-\section{Primaries} \label{primaries}
-\index{primary}
-
-Primaries represent the most tightly bound operations of the language.
-Their syntax is:
-
-\begin{verbatim}
-primary:        atom | attributeref | subscription | slicing | call
-\end{verbatim}
-
-\subsection{Attribute references}
-\indexii{attribute}{reference}
-
-An attribute reference is a primary followed by a period and a name:
-
-\begin{verbatim}
-attributeref:   primary "." identifier
-\end{verbatim}
-
-The primary must evaluate to an object of a type that supports
-attribute references, e.g. a module or a list.  This object is then
-asked to produce the attribute whose name is the identifier.  If this
-attribute is not available, the exception \verb\AttributeError\ is
-raised.  Otherwise, the type and value of the object produced is
-determined by the object.  Multiple evaluations of the same attribute
-reference may yield different objects.
-\obindex{module}
-\obindex{list}
-
-\subsection{Subscriptions}
-\index{subscription}
-
-A subscription selects an item of a sequence (string, tuple or list)
-or mapping (dictionary) object:
-\obindex{sequence}
-\obindex{mapping}
-\obindex{string}
-\obindex{tuple}
-\obindex{list}
-\obindex{dictionary}
-\indexii{sequence}{item}
-
-\begin{verbatim}
-subscription:   primary "[" condition "]"
-\end{verbatim}
-
-The primary must evaluate to an object of a sequence or mapping type.
-
-If it is a mapping, the condition must evaluate to an object whose
-value is one of the keys of the mapping, and the subscription selects
-the value in the mapping that corresponds to that key.
-
-If it is a sequence, the condition must evaluate to a plain integer.
-If this value is negative, the length of the sequence is added to it
-(so that, e.g. \verb\x[-1]\ selects the last item of \verb\x\.)
-The resulting value must be a nonnegative integer smaller than the
-number of items in the sequence, and the subscription selects the item
-whose index is that value (counting from zero).
-
-A string's items are characters.  A character is not a separate data
-type but a string of exactly one character.
-\index{character}
-\indexii{string}{item}
-
-\subsection{Slicings}
-\index{slicing}
-\index{slice}
-
-A slicing (or slice) selects a range of items in a sequence (string,
-tuple or list) object:
-\obindex{sequence}
-\obindex{string}
-\obindex{tuple}
-\obindex{list}
-
-\begin{verbatim}
-slicing:        primary "[" [condition] ":" [condition] "]"
-\end{verbatim}
-
-The primary must evaluate to a sequence object.  The lower and upper
-bound expressions, if present, must evaluate to plain integers;
-defaults are zero and the sequence's length, respectively.  If either
-bound is negative, the sequence's length is added to it.  The slicing
-now selects all items with index $k$ such that $i <= k < j$ where $i$
-and $j$ are the specified lower and upper bounds.  This may be an
-empty sequence.  It is not an error if $i$ or $j$ lie outside the
-range of valid indexes (such items don't exist so they aren't
-selected).
-
-\subsection{Calls} \label{calls}
-\index{call}
-
-A call calls a callable object (e.g. a function) with a possibly empty
-series of arguments:
-\obindex{callable}
-
-\begin{verbatim}
-call:           primary "(" [condition_list] ")"
-\end{verbatim}
-
-The primary must evaluate to a callable object (user-defined
-functions, built-in functions, methods of built-in objects, class
-objects, and methods of class instances are callable).  If it is a
-class, the argument list must be empty; otherwise, the arguments are
-evaluated.
-
-A call always returns some value, possibly \verb\None\, unless it
-raises an exception.  How this value is computed depends on the type
-of the callable object.  If it is:
-
-\begin{description}
-
-\item[a user-defined function:] the code block for the function is
-executed, passing it the argument list.  The first thing the code
-block will do is bind the formal parameters to the arguments; this is
-described in section \ref{function}.  When the code block executes a
-\verb\return\ statement, this specifies the return value of the
-function call.
-\indexii{function}{call}
-\indexiii{user-defined}{function}{call}
-\obindex{user-defined function}
-\obindex{function}
-
-\item[a built-in function or method:] the result is up to the
-interpreter; see the library reference manual for the descriptions of
-built-in functions and methods.
-\indexii{function}{call}
-\indexii{built-in function}{call}
-\indexii{method}{call}
-\indexii{built-in method}{call}
-\obindex{built-in method}
-\obindex{built-in function}
-\obindex{method}
-\obindex{function}
-
-\item[a class object:] a new instance of that class is returned.
-\obindex{class}
-\indexii{class object}{call}
-
-\item[a class instance method:] the corresponding user-defined
-function is called, with an argument list that is one longer than the
-argument list of the call: the instance becomes the first argument.
-\obindex{class instance}
-\obindex{instance}
-\indexii{instance}{call}
-\indexii{class instance}{call}
-
-\end{description}
-
-\section{Unary arithmetic operations}
-\indexiii{unary}{arithmetic}{operation}
-\indexiii{unary}{bit-wise}{operation}
-
-All unary arithmetic (and bit-wise) operations have the same priority:
-
-\begin{verbatim}
-u_expr:         primary | "-" u_expr | "+" u_expr | "~" u_expr
-\end{verbatim}
-
-The unary \verb\"-"\ (minus) operator yields the negation of its
-numeric argument.
-\index{negation}
-\index{minus}
-
-The unary \verb\"+"\ (plus) operator yields its numeric argument
-unchanged.
-\index{plus}
-
-The unary \verb\"~"\ (invert) operator yields the bit-wise inversion
-of its plain or long integer argument.  The bit-wise inversion of
-\verb\x\ is defined as \verb\-(x+1)\.
-\index{inversion}
-
-In all three cases, if the argument does not have the proper type,
-a \verb\TypeError\ exception is raised.
-\exindex{TypeError}
-
-\section{Binary arithmetic operations}
-\indexiii{binary}{arithmetic}{operation}
-
-The binary arithmetic operations have the conventional priority
-levels.  Note that some of these operations also apply to certain
-non-numeric types.  There is no ``power'' operator, so there are only
-two levels, one for multiplicative operators and one for additive
-operators:
-
-\begin{verbatim}
-m_expr:         u_expr | m_expr "*" u_expr | m_expr "/" u_expr | m_expr "%" u_expr
-a_expr:         m_expr | aexpr "+" m_expr | aexpr "-" m_expr
-\end{verbatim}
-
-The \verb\"*"\ (multiplication) operator yields the product of its
-arguments.  The arguments must either both be numbers, or one argument
-must be a plain integer and the other must be a sequence.  In the
-former case, the numbers are converted to a common type and then
-multiplied together.  In the latter case, sequence repetition is
-performed; a negative repetition factor yields an empty sequence.
-\index{multiplication}
-
-The \verb\"/"\ (division) operator yields the quotient of its
-arguments.  The numeric arguments are first converted to a common
-type.  Plain or long integer division yields an integer of the same
-type; the result is that of mathematical division with the `floor'
-function applied to the result.  Division by zero raises the
-\verb\ZeroDivisionError\ exception.
-\exindex{ZeroDivisionError}
-\index{division}
-
-The \verb\"%"\ (modulo) operator yields the remainder from the
-division of the first argument by the second.  The numeric arguments
-are first converted to a common type.  A zero right argument raises
-the \verb\ZeroDivisionError\ exception.  The arguments may be floating
-point numbers, e.g. \verb\3.14 % 0.7\ equals \verb\0.34\.  The modulo
-operator always yields a result with the same sign as its second
-operand (or zero); the absolute value of the result is strictly
-smaller than the second operand.
-\index{modulo}
-
-The integer division and modulo operators are connected by the
-following identity: \verb\x == (x/y)*y + (x%y)\.  Integer division and
-modulo are also connected with the built-in function \verb\divmod()\:
-\verb\divmod(x, y) == (x/y, x%y)\.  These identities don't hold for
-floating point numbers; there a similar identity holds where
-\verb\x/y\ is replaced by \verb\floor(x/y)\).
-
-The \verb\"+"\ (addition) operator yields the sum of its arguments.
-The arguments must either both be numbers, or both sequences of the
-same type.  In the former case, the numbers are converted to a common
-type and then added together.  In the latter case, the sequences are
-concatenated.
-\index{addition}
-
-The \verb\"-"\ (subtraction) operator yields the difference of its
-arguments.  The numeric arguments are first converted to a common
-type.
-\index{subtraction}
-
-\section{Shifting operations}
-\indexii{shifting}{operation}
-
-The shifting operations have lower priority than the arithmetic
-operations:
-
-\begin{verbatim}
-shift_expr:     a_expr | shift_expr ( "<<" | ">>" ) a_expr
-\end{verbatim}
-
-These operators accept plain or long integers as arguments.  The
-arguments are converted to a common type.  They shift the first
-argument to the left or right by the number of bits given by the
-second argument.
-
-A right shift by $n$ bits is defined as division by $2^n$.  A left
-shift by $n$ bits is defined as multiplication with $2^n$; for plain
-integers there is no overflow check so this drops bits and flip the
-sign if the result is not less than $2^{31}$ in absolute value.
-
-Negative shift counts raise a \verb\ValueError\ exception.
-\exindex{ValueError}
-
-\section{Binary bit-wise operations}
-\indexiii{binary}{bit-wise}{operation}
-
-Each of the three bitwise operations has a different priority level:
-
-\begin{verbatim}
-and_expr:       shift_expr | and_expr "&" shift_expr
-xor_expr:       and_expr | xor_expr "^" and_expr
-or_expr:       xor_expr | or_expr "|" xor_expr
-\end{verbatim}
-
-The \verb\"&"\ operator yields the bitwise AND of its arguments, which
-must be plain or long integers.  The arguments are converted to a
-common type.
-\indexii{bit-wise}{and}
-
-The \verb\"^"\ operator yields the bitwise XOR (exclusive OR) of its
-arguments, which must be plain or long integers.  The arguments are
-converted to a common type.
-\indexii{bit-wise}{xor}
-\indexii{exclusive}{or}
-
-The \verb\"|"\ operator yields the bitwise (inclusive) OR of its
-arguments, which must be plain or long integers.  The arguments are
-converted to a common type.
-\indexii{bit-wise}{or}
-\indexii{inclusive}{or}
-
-\section{Comparisons}
-\index{comparison}
-
-Contrary to C, all comparison operations in Python have the same
-priority, which is lower than that of any arithmetic, shifting or
-bitwise operation.  Also contrary to C, expressions like
-\verb\a < b < c\ have the interpretation that is conventional in
-mathematics:
-\index{C}
-
-\begin{verbatim}
-comparison:     or_expr (comp_operator or_expr)*
-comp_operator:  "<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"
-\end{verbatim}
-
-Comparisons yield integer values: 1 for true, 0 for false.
-
-Comparisons can be chained arbitrarily, e.g. $x < y <= z$ is
-equivalent to $x < y$ \verb\and\ $y <= z$, except that $y$ is
-evaluated only once (but in both cases $z$ is not evaluated at all
-when $x < y$ is found to be false).
-\indexii{chaining}{comparisons}
-
-Formally, $e_0 op_1 e_1 op_2 e_2 ...e_{n-1} op_n e_n$ is equivalent to
-$e_0 op_1 e_1$ \verb\and\ $e_1 op_2 e_2$ \verb\and\ ... \verb\and\
-$e_{n-1} op_n e_n$, except that each expression is evaluated at most once.
-
-Note that $e_0 op_1 e_1 op_2 e_2$ does not imply any kind of comparison
-between $e_0$ and $e_2$, e.g. $x < y > z$ is perfectly legal.
-
-The forms \verb\<>\ and \verb\!=\ are equivalent; for consistency with
-C, \verb\!=\ is preferred; where \verb\!=\ is mentioned below
-\verb\<>\ is also implied.
-
-The operators {\tt "<", ">", "==", ">=", "<="}, and {\tt "!="} compare
-the values of two objects.  The objects needn't have the same type.
-If both are numbers, they are coverted to a common type.  Otherwise,
-objects of different types {\em always} compare unequal, and are
-ordered consistently but arbitrarily.
-
-(This unusual definition of comparison is done to simplify the
-definition of operations like sorting and the \verb\in\ and \verb\not
-in\ operators.)
-
-Comparison of objects of the same type depends on the type:
-
-\begin{itemize}
-
-\item
-Numbers are compared arithmetically.
-
-\item
-Strings are compared lexicographically using the numeric equivalents
-(the result of the built-in function \verb\ord\) of their characters.
-
-\item
-Tuples and lists are compared lexicographically using comparison of
-corresponding items.
-
-\item
-Mappings (dictionaries) are compared through lexicographic
-comparison of their sorted (key, value) lists.%
-\footnote{This is expensive since it requires sorting the keys first,
-but about the only sensible definition.  It was tried to compare
-dictionaries by identity only, but this caused surprises because
-people expected to be able to test a dictionary for emptiness by
-comparing it to {\tt \{\}}.}
-
-\item
-Most other types compare unequal unless they are the same object;
-the choice whether one object is considered smaller or larger than
-another one is made arbitrarily but consistently within one
-execution of a program.
-
-\end{itemize}
-
-The operators \verb\in\ and \verb\not in\ test for sequence
-membership: if $y$ is a sequence, $x ~\verb\in\~ y$ is true if and
-only if there exists an index $i$ such that $x = y[i]$.
-$x ~\verb\not in\~ y$ yields the inverse truth value.  The exception
-\verb\TypeError\ is raised when $y$ is not a sequence, or when $y$ is
-a string and $x$ is not a string of length one.%
-\footnote{The latter restriction is sometimes a nuisance.}
-\opindex{in}
-\opindex{not in}
-\indexii{membership}{test}
-\obindex{sequence}
-
-The operators \verb\is\ and \verb\is not\ test for object identity:
-$x ~\verb\is\~ y$ is true if and only if $x$ and $y$ are the same
-object.  $x ~\verb\is not\~ y$ yields the inverse truth value.
-\opindex{is}
-\opindex{is not}
-\indexii{identity}{test}
-
-\section{Boolean operations} \label{Booleans}
-\indexii{Boolean}{operation}
-
-Boolean operations have the lowest priority of all Python operations:
-
-\begin{verbatim}
-condition:      or_test
-or_test:        and_test | or_test "or" and_test
-and_test:       not_test | and_test "and" not_test
-not_test:       comparison | "not" not_test
-\end{verbatim}
-
-In the context of Boolean operations, and also when conditions are
-used by control flow statements, the following values are interpreted
-as false: \verb\None\, numeric zero of all types, empty sequences
-(strings, tuples and lists), and empty mappings (dictionaries).  All
-other values are interpreted as true.
-
-The operator \verb\not\ yields 1 if its argument is false, 0 otherwise.
-\opindex{not}
-
-The condition $x ~\verb\and\~ y$ first evaluates $x$; if $x$ is false,
-its value is returned; otherwise, $y$ is evaluated and the resulting
-value is returned.
-\opindex{and}
-
-The condition $x ~\verb\or\~ y$ first evaluates $x$; if $x$ is true,
-its value is returned; otherwise, $y$ is evaluated and the resulting
-value is returned.
-\opindex{or}
-
-(Note that \verb\and\ and \verb\or\ do not restrict the value and type
-they return to 0 and 1, but rather return the last evaluated argument.
-This is sometimes useful, e.g. if \verb\s\ is a string that should be
-replaced by a default value if it is empty, the expression
-\verb\s or 'foo'\ yields the desired value.  Because \verb\not\ has to
-invent a value anyway, it does not bother to return a value of the
-same type as its argument, so e.g. \verb\not 'foo'\ yields \verb\0\,
-not \verb\''\.)
-
-\section{Expression lists and condition lists}
-\indexii{expression}{list}
-\indexii{condition}{list}
-
-\begin{verbatim}
-expr_list:      or_expr ("," or_expr)* [","]
-cond_list:      condition ("," condition)* [","]
-\end{verbatim}
-
-The only difference between expression lists and condition lists is
-the lowest priority of operators that can be used in them without
-being enclosed in parentheses; condition lists allow all operators,
-while expression lists don't allow comparisons and Boolean operators
-(they do allow bitwise and shift operators though).
-
-Expression lists are used in expression statements and assignments;
-condition lists are used everywhere else where a list of
-comma-separated values is required.
-
-An expression (condition) list containing at least one comma yields a
-tuple.  The length of the tuple is the number of expressions
-(conditions) in the list.  The expressions (conditions) are evaluated
-from left to right.  (Conditions lists are used syntactically is a few
-places where no tuple is constructed but a list of values is needed
-nevertheless.)
-\obindex{tuple}
-
-The trailing comma is required only to create a single tuple (a.k.a. a
-{\em singleton}); it is optional in all other cases.  A single
-expression (condition) without a trailing comma doesn't create a
-tuple, but rather yields the value of that expression (condition).
-\indexii{trailing}{comma}
-
-(To create an empty tuple, use an empty pair of parentheses:
-\verb\()\.)
-
-\chapter{Simple statements}
-\indexii{simple}{statement}
-
-Simple statements are comprised within a single logical line.
-Several simple statements may occur on a single line separated
-by semicolons.  The syntax for simple statements is:
-
-\begin{verbatim}
-simple_stmt:    expression_stmt
-              | assignment_stmt
-              | pass_stmt
-              | del_stmt
-              | print_stmt
-              | return_stmt
-              | raise_stmt
-              | break_stmt
-              | continue_stmt
-              | import_stmt
-              | global_stmt
-\end{verbatim}
-
-\section{Expression statements}
-\indexii{expression}{statement}
-
-Expression statements are used (mostly interactively) to compute and
-write a value, or (usually) to call a procedure (a function that
-returns no meaningful result; in Python, procedures return the value
-\verb\None\):
-
-\begin{verbatim}
-expression_stmt: expression_list
-\end{verbatim}
-
-An expression statement evaluates the expression list (which may be a
-single expression).  If the value is not \verb\None\, it is converted
-to a string using the rules for string conversions (expressions in
-reverse quotes), and the resulting string is written to standard
-output (see section \ref{print}) on a line by itself.
-\indexii{expression}{list}
-\ttindex{None}
-\indexii{string}{conversion}
-\index{output}
-\indexii{standard}{output}
-\indexii{writing}{values}
-
-(The exception for \verb\None\ is made so that procedure calls, which
-are syntactically equivalent to expressions, do not cause any output.
-A tuple with only \verb\None\ items is written normally.)
-\indexii{procedure}{call}
-
-\section{Assignment statements}
-\indexii{assignment}{statement}
-
-Assignment statements are used to (re)bind names to values and to
-modify attributes or items of mutable objects:
-\indexii{binding}{name}
-\indexii{rebinding}{name}
-\obindex{mutable}
-\indexii{attribute}{assignment}
-
-\begin{verbatim}
-assignment_stmt: (target_list "=")+ expression_list
-target_list:     target ("," target)* [","]
-target:          identifier | "(" target_list ")" | "[" target_list "]"
-               | attributeref | subscription | slicing
-\end{verbatim}
-
-(See section \ref{primaries} for the syntax definitions for the last
-three symbols.)
-
-An assignment statement evaluates the expression list (remember that
-this can be a single expression or a comma-separated list, the latter
-yielding a tuple) and assigns the single resulting object to each of
-the target lists, from left to right.
-\indexii{expression}{list}
-
-Assignment is defined recursively depending on the form of the target
-(list).  When a target is part of a mutable object (an attribute
-reference, subscription or slicing), the mutable object must
-ultimately perform the assignment and decide about its validity, and
-may raise an exception if the assignment is unacceptable.  The rules
-observed by various types and the exceptions raised are given with the
-definition of the object types (see section \ref{types}).
-\index{target}
-\indexii{target}{list}
-
-Assignment of an object to a target list is recursively defined as
-follows.
-\indexiii{target}{list}{assignment}
-
-\begin{itemize}
-\item
-If the target list is a single target: the object is assigned to that
-target.
-
-\item
-If the target list is a comma-separated list of targets: the object
-must be a tuple with the same number of items as the list contains
-targets, and the items are assigned, from left to right, to the
-corresponding targets.
-
-\end{itemize}
-
-Assignment of an object to a single target is recursively defined as
-follows.
-
-\begin{itemize} % nested
-
-\item
-If the target is an identifier (name):
-
-\begin{itemize}
-
-\item
-If the name does not occur in a \verb\global\ statement in the current
-code block: the name is bound to the object in the current local name
-space.
-\stindex{global}
-
-\item
-Otherwise: the name is bound to the object in the current global name
-space.
-
-\end{itemize} % nested
-
-The name is rebound if it was already bound.
-
-\item
-If the target is a target list enclosed in parentheses: the object is
-assigned to that target list as described above.
-
-\item
-If the target is a target list enclosed in square brackets: the object
-must be a list with the same number of items as the target list
-contains targets, and its items are assigned, from left to right, to
-the corresponding targets.
-
-\item
-If the target is an attribute reference: The primary expression in the
-reference is evaluated.  It should yield an object with assignable
-attributes; if this is not the case, \verb\TypeError\ is raised.  That
-object is then asked to assign the assigned object to the given
-attribute; if it cannot perform the assignment, it raises an exception
-(usually but not necessarily \verb\AttributeError\).
-\indexii{attribute}{assignment}
-
-\item
-If the target is a subscription: The primary expression in the
-reference is evaluated.  It should yield either a mutable sequence
-(list) object or a mapping (dictionary) object.  Next, the subscript
-expression is evaluated.
-\indexii{subscription}{assignment}
-\obindex{mutable}
-
-If the primary is a mutable sequence object (a list), the subscript
-must yield a plain integer.  If it is negative, the sequence's length
-is added to it.  The resulting value must be a nonnegative integer
-less than the sequence's length, and the sequence is asked to assign
-the assigned object to its item with that index.  If the index is out
-of range, \verb\IndexError\ is raised (assignment to a subscripted
-sequence cannot add new items to a list).
-\obindex{sequence}
-\obindex{list}
-
-If the primary is a mapping (dictionary) object, the subscript must
-have a type compatible with the mapping's key type, and the mapping is
-then asked to to create a key/datum pair which maps the subscript to
-the assigned object.  This can either replace an existing key/value
-pair with the same key value, or insert a new key/value pair (if no
-key with the same value existed).
-\obindex{mapping}
-\obindex{dictionary}
-
-\item
-If the target is a slicing: The primary expression in the reference is
-evaluated.  It should yield a mutable sequence (list) object.  The
-assigned object should be a sequence object of the same type.  Next,
-the lower and upper bound expressions are evaluated, insofar they are
-present; defaults are zero and the sequence's length.  The bounds
-should evaluate to (small) integers.  If either bound is negative, the
-sequence's length is added to it.  The resulting bounds are clipped to
-lie between zero and the sequence's length, inclusive.  Finally, the
-sequence object is asked to replace the items indicated by the slice
-with the items of the assigned sequence.  This may change the
-sequence's length, if it allows it.
-\indexii{slicing}{assignment}
-
-\end{itemize}
-	
-(In the original implementation, the syntax for targets is taken
-to be the same as for expressions, and invalid syntax is rejected
-during the code generation phase, causing less detailed error
-messages.)
-
-\section{The {\tt pass} statement}
-\stindex{pass}
-
-\begin{verbatim}
-pass_stmt:      "pass"
-\end{verbatim}
-
-\verb\pass\ is a null operation --- when it is executed, nothing
-happens.  It is useful as a placeholder when a statement is
-required syntactically, but no code needs to be executed, for example:
-\indexii{null}{operation}
-
-\begin{verbatim}
-def f(arg): pass    # a function that does nothing (yet)
-
-class C: pass       # an class with no methods (yet)
-\end{verbatim}
-
-\section{The {\tt del} statement}
-\stindex{del}
-
-\begin{verbatim}
-del_stmt:       "del" target_list
-\end{verbatim}
-
-Deletion is recursively defined very similar to the way assignment is
-defined. Rather that spelling it out in full details, here are some
-hints.
-\indexii{deletion}{target}
-\indexiii{deletion}{target}{list}
-
-Deletion of a target list recursively deletes each target, from left
-to right.
-
-Deletion of a name removes the binding of that name (which must exist)
-from the local or global name space, depending on whether the name
-occurs in a \verb\global\ statement in the same code block.
-\stindex{global}
-\indexii{unbinding}{name}
-
-Deletion of attribute references, subscriptions and slicings
-is passed to the primary object involved; deletion of a slicing
-is in general equivalent to assignment of an empty slice of the
-right type (but even this is determined by the sliced object).
-\indexii{attribute}{deletion}
-
-\section{The {\tt print} statement} \label{print}
-\stindex{print}
-
-\begin{verbatim}
-print_stmt:     "print" [ condition ("," condition)* [","] ]
-\end{verbatim}
-
-\verb\print\ evaluates each condition in turn and writes the resulting
-object to standard output (see below).  If an object is not a string,
-it is first converted to a string using the rules for string
-conversions.  The (resulting or original) string is then written.  A
-space is written before each object is (converted and) written, unless
-the output system believes it is positioned at the beginning of a
-line.  This is the case: (1) when no characters have yet been written
-to standard output; or (2) when the last character written to standard
-output is \verb/\n/; or (3) when the last write operation on standard
-output was not a \verb\print\ statement.  (In some cases it may be
-functional to write an empty string to standard output for this
-reason.)
-\index{output}
-\indexii{writing}{values}
-
-A \verb/"\n"/ character is written at the end, unless the \verb\print\
-statement ends with a comma.  This is the only action if the statement
-contains just the keyword \verb\print\.
-\indexii{trailing}{comma}
-\indexii{newline}{suppression}
-
-Standard output is defined as the file object named \verb\stdout\
-in the built-in module \verb\sys\.  If no such object exists,
-or if it is not a writable file, a \verb\RuntimeError\ exception is raised.
-(The original implementation attempts to write to the system's original
-standard output instead, but this is not safe, and should be fixed.)
-\indexii{standard}{output}
-\bimodindex{sys}
-\ttindex{stdout}
-\exindex{RuntimeError}
-
-\section{The {\tt return} statement}
-\stindex{return}
-
-\begin{verbatim}
-return_stmt:    "return" [condition_list]
-\end{verbatim}
-
-\verb\return\ may only occur syntactically nested in a function
-definition, not within a nested class definition.
-\indexii{function}{definition}
-\indexii{class}{definition}
-
-If a condition list is present, it is evaluated, else \verb\None\
-is substituted.
-
-\verb\return\ leaves the current function call with the condition
-list (or \verb\None\) as return value.
-
-When \verb\return\ passes control out of a \verb\try\ statement
-with a \verb\finally\ clause, that finally clause is executed
-before really leaving the function.
-\kwindex{finally}
-
-\section{The {\tt raise} statement}
-\stindex{raise}
-
-\begin{verbatim}
-raise_stmt:     "raise" condition ["," condition]
-\end{verbatim}
-
-\verb\raise\ evaluates its first condition, which must yield
-a string object.  If there is a second condition, this is evaluated,
-else \verb\None\ is substituted.
-\index{exception}
-\indexii{raising}{exception}
-
-It then raises the exception identified by the first object,
-with the second one (or \verb\None\) as its parameter.
-
-\section{The {\tt break} statement}
-\stindex{break}
-
-\begin{verbatim}
-break_stmt:     "break"
-\end{verbatim}
-
-\verb\break\ may only occur syntactically nested in a \verb\for\
-or \verb\while\ loop, not nested in a function or class definition.
-\stindex{for}
-\stindex{while}
-\indexii{loop}{statement}
-
-It terminates the neares enclosing loop, skipping the optional
-\verb\else\ clause if the loop has one.
-\kwindex{else}
-
-If a \verb\for\ loop is terminated by \verb\break\, the loop control
-target keeps its current value.
-\indexii{loop control}{target}
-
-When \verb\break\ passes control out of a \verb\try\ statement
-with a \verb\finally\ clause, that finally clause is executed
-before really leaving the loop.
-\kwindex{finally}
-
-\section{The {\tt continue} statement}
-\stindex{continue}
-
-\begin{verbatim}
-continue_stmt:  "continue"
-\end{verbatim}
-
-\verb\continue\ may only occur syntactically nested in a \verb\for\ or
-\verb\while\ loop, not nested in a function or class definition, and
-not nested in the \verb\try\ clause of a \verb\try\ statement with a
-\verb\finally\ clause (it may occur nested in a \verb\except\ or
-\verb\finally\ clause of a \verb\try\ statement though).
-\stindex{for}
-\stindex{while}
-\indexii{loop}{statement}
-\kwindex{finally}
-
-It continues with the next cycle of the nearest enclosing loop.
-
-\section{The {\tt import} statement} \label{import}
-\stindex{import}
-
-\begin{verbatim}
-import_stmt:    "import" identifier ("," identifier)*
-              | "from" identifier "import" identifier ("," identifier)*
-              | "from" identifier "import" "*"
-\end{verbatim}
-
-Import statements are executed in two steps: (1) find a module, and
-initialize it if necessary; (2) define a name or names in the local
-name space (of the scope where the \verb\import\ statement occurs).
-The first form (without \verb\from\) repeats these steps for each
-identifier in the list, the \verb\from\ form performs them once, with
-the first identifier specifying the module name.
-\indexii{importing}{module}
-\indexii{name}{binding}
-\kwindex{from}
-
-The system maintains a table of modules that have been initialized,
-indexed by module name.  (The current implementation makes this table
-accessible as \verb\sys.modules\.)  When a module name is found in
-this table, step (1) is finished.  If not, a search for a module
-definition is started.  This first looks for a built-in module
-definition, and if no built-in module if the given name is found, it
-searches a user-specified list of directories for a file whose name is
-the module name with extension \verb\".py"\.  (The current
-implementation uses the list of strings \verb\sys.path\ as the search
-path; it is initialized from the shell environment variable
-\verb\$PYTHONPATH\, with an installation-dependent default.)
-\ttindex{modules}
-\ttindex{sys.modules}
-\indexii{module}{name}
-\indexii{built-in}{module}
-\indexii{user-defined}{module}
-\bimodindex{sys}
-\ttindex{path}
-\ttindex{sys.path}
-\indexii{filename}{extension}
-
-If a built-in module is found, its built-in initialization code is
-executed and step (1) is finished.  If no matching file is found,
-\verb\ImportError\ is raised.  If a file is found, it is parsed,
-yielding an executable code block.  If a syntax error occurs,
-\verb\SyntaxError\ is raised.  Otherwise, an empty module of the given
-name is created and inserted in the module table, and then the code
-block is executed in the context of this module.  Exceptions during
-this execution terminate step (1).
-\indexii{module}{initialization}
-\exindex{SyntaxError}
-\exindex{ImportError}
-\index{code block}
-
-When step (1) finishes without raising an exception, step (2) can
-begin.
-
-The first form of \verb\import\ statement binds the module name in the
-local name space to the module object, and then goes on to import the
-next identifier, if any.  The \verb\from\ from does not bind the
-module name: it goes through the list of identifiers, looks each one
-of them up in the module found in step (1), and binds the name in the
-local name space to the object thus found.  If a name is not found,
-\verb\ImportError\ is raised.  If the list of identifiers is replaced
-by a star (\verb\*\), all names defined in the module are bound,
-except those beginning with an underscore(\verb\_\).
-\indexii{name}{binding}
-\exindex{ImportError}
-
-Names bound by import statements may not occur in \verb\global\
-statements in the same scope.
-\stindex{global}
-
-The \verb\from\ form with \verb\*\ may only occur in a module scope.
-\kwindex{from}
-\ttindex{from ... import *}
-
-(The current implementation does not enforce the latter two
-restrictions, but programs should not abuse this freedom, as future
-implementations may enforce them or silently change the meaning of the
-program.)
-
-\section{The {\tt global} statement} \label{global}
-\stindex{global}
-
-\begin{verbatim}
-global_stmt:    "global" identifier ("," identifier)*
-\end{verbatim}
-
-The \verb\global\ statement is a declaration which holds for the
-entire current scope.  It means that the listed identifiers are to be
-interpreted as globals.  While {\em using} global names is automatic
-if they are not defined in the local scope, {\em assigning} to global
-names would be impossible without \verb\global\.
-\indexiii{global}{name}{binding}
-
-Names listed in a \verb\global\ statement must not be used in the same
-scope before that \verb\global\ statement is executed.
-
-Name listed in a \verb\global\ statement must not be defined as formal
-parameters or in a \verb\for\ loop control target, \verb\class\
-definition, function definition, or \verb\import\ statement.
-
-(The current implementation does not enforce the latter two
-restrictions, but programs should not abuse this freedom, as future
-implementations may enforce them or silently change the meaning of the
-program.)
-
-\chapter{Compound statements}
-\indexii{compound}{statement}
-
-Compound statements contain (groups of) other statements; they affect
-or control the execution of those other statements in some way.  In
-general, compound statements span multiple lines, although in simple
-incarnations a whole compound statement may be contained in one line.
-
-The \verb\if\, \verb\while\ and \verb\for\ statements implement
-traditional control flow constructs.  \verb\try\ specifies exception
-handlers and/or cleanup code for a group of statements.  Function and
-class definitions are also syntactically compound statements.
-
-Compound statements consist of one or more `clauses'.  A clause
-consists of a header and a `suite'.  The clause headers of a
-particular compound statement are all at the same indentation level.
-Each clause header begins with a uniquely identifying keyword and ends
-with a colon.  A suite is a group of statements controlled by a
-clause.  A suite can be one or more semicolon-separated simple
-statements on the same line as the header, following the header's
-colon, or it can be one or more indented statements on subsequent
-lines.  Only the latter form of suite can contain nested compound
-statements; the following is illegal, mostly because it wouldn't be
-clear to which \verb\if\ clause a following \verb\else\ clause would
-belong:
-\index{clause}
-\index{suite}
-
-\begin{verbatim}
-if test1: if test2: print x
-\end{verbatim}
-
-Also note that the semicolon binds tighter that the colon in this
-context, so that in the following example, either all or none of the
-\verb\print\ statements are executed:
-
-\begin{verbatim}
-if x < y < z: print x; print y; print z
-\end{verbatim}
-
-Summarizing:
-
-\begin{verbatim}
-compound_stmt:  if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef
-suite:          stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
-statement:      stmt_list NEWLINE | compound_stmt
-stmt_list:      simple_stmt (";" simple_stmt)* [";"]
-\end{verbatim}
-
-Note that statements always ends in a \verb\NEWLINE\ possibly followed
-by a \verb\DEDENT\.
-\index{NEWLINE token}
-\index{DEDENT token}
-
-Also note that optional continuation clauses always begin with a
-keyword that cannot start a statement, thus there are no ambiguities
-(the `dangling \verb\else\' problem is solved in Python by requiring
-nested \verb\if\ statements to be indented).
-\indexii{dangling}{else}
-
-The formatting of the grammar rules in the following sections places
-each clause on a separate line for clarity.
-
-\section{The {\tt if} statement}
-\stindex{if}
-
-The \verb\if\ statement is used for conditional execution:
-
-\begin{verbatim}
-if_stmt:        "if" condition ":" suite
-               ("elif" condition ":" suite)*
-               ["else" ":" suite]
-\end{verbatim}
-
-It selects exactly one of the suites by evaluating the conditions one
-by one until one is found to be true (see section \ref{Booleans} for
-the definition of true and false); then that suite is executed (and no
-other part of the \verb\if\ statement is executed or evaluated).  If
-all conditions are false, the suite of the \verb\else\ clause, if
-present, is executed.
-\kwindex{elif}
-\kwindex{else}
-
-\section{The {\tt while} statement}
-\stindex{while}
-\indexii{loop}{statement}
-
-The \verb\while\ statement is used for repeated execution as long as a
-condition is true:
-
-\begin{verbatim}
-while_stmt:     "while" condition ":" suite
-               ["else" ":" suite]
-\end{verbatim}
-
-This repeatedly tests the condition and, if it is true, executes the
-first suite; if the condition is false (which may be the first time it
-is tested) the suite of the \verb\else\ clause, if present, is
-executed and the loop terminates.
-\kwindex{else}
-
-A \verb\break\ statement executed in the first suite terminates the
-loop without executing the \verb\else\ clause's suite.  A
-\verb\continue\ statement executed in the first suited skips the rest
-of the suite and goes back to testing the condition.
-\stindex{break}
-\stindex{continue}
-
-\section{The {\tt for} statement}
-\stindex{for}
-\indexii{loop}{statement}
-
-The \verb\for\ statement is used to iterate over the elements of a
-sequence (string, tuple or list):
-\obindex{sequence}
-
-\begin{verbatim}
-for_stmt:       "for" target_list "in" condition_list ":" suite
-               ["else" ":" suite]
-\end{verbatim}
-
-The condition list is evaluated once; it should yield a sequence.  The
-suite is then executed once for each item in the sequence, in the
-order of ascending indices.  Each item in turn is assigned to the
-target list using the standard rules for assignments, and then the
-suite is executed.  When the items are exhausted (which is immediately
-when the sequence is empty), the suite in the \verb\else\ clause, if
-present, is executed, and the loop terminates.
-\kwindex{in}
-\kwindex{else}
-\indexii{target}{list}
-
-A \verb\break\ statement executed in the first suite terminates the
-loop without executing the \verb\else\ clause's suite.  A
-\verb\continue\ statement executed in the first suited skips the rest
-of the suite and continues with the next item, or with the \verb\else\
-clause if there was no next item.
-\stindex{break}
-\stindex{continue}
-
-The suite may assign to the variable(s) in the target list; this does
-not affect the next item assigned to it.
-
-The target list is not deleted when the loop is finished, but if the
-sequence is empty, it will not have been assigned to at all by the
-loop.
-
-Hint: the built-in function \verb\range()\ returns a sequence of
-integers suitable to emulate the effect of Pascal's \verb\for i := a
-to b do\; e.g. \verb\range(3)\ returns the list \verb\[0, 1, 2]\.
-\bifuncindex{range}
-\index{Pascal}
-
-{\bf Warning:} There is a subtlety when the sequence is being modified
-by the loop (this can only occur for mutable sequences, i.e. lists).
-An internal counter is used to keep track of which item is used next,
-and this is incremented on each iteration.  When this counter has
-reached the length of the sequence the loop terminates.  This means that
-if the suite deletes the current (or a previous) item from the
-sequence, the next item will be skipped (since it gets the index of
-the current item which has already been treated).  Likewise, if the
-suite inserts an item in the sequence before the current item, the
-current item will be treated again the next time through the loop.
-This can lead to nasty bugs that can be avoided by making a temporary
-copy using a slice of the whole sequence, e.g.
-\index{loop!over mutable sequence}
-\index{mutable sequence!loop over}
-
-\begin{verbatim}
-for x in a[:]:
-    if x < 0: a.remove(x)
-\end{verbatim}
-
-\section{The {\tt try} statement}
-\stindex{try}
-
-The \verb\try\ statement specifies exception handlers and/or cleanup
-code for a group of statements:
-
-\begin{verbatim}
-try_stmt:       try_exc_stmt | try_fin_stmt
-try_exc_stmt:   "try" ":" suite
-               ("except" [condition ["," target]] ":" suite)+
-try_fin_stmt:   "try" ":" suite
-               "finally" ":" suite
-\end{verbatim}
-
-There are two forms of \verb\try\ statement: \verb\try...except\ and
-\verb\try...finally\.  These forms cannot be mixed.
-
-The \verb\try...except\ form specifies one or more exception handlers
-(the \verb\except\ clauses).  When no exception occurs in the
-\verb\try\ clause, no exception handler is executed.  When an
-exception occurs in the \verb\try\ suite, a search for an exception
-handler is started.  This inspects the except clauses in turn until
-one is found that matches the exception.  A condition-less except
-clause, if present, must be last; it matches any exception.  For an
-except clause with a condition, that condition is evaluated, and the
-clause matches the exception if the resulting object is ``compatible''
-with the exception.  An object is compatible with an exception if it
-is either the object that identifies the exception or it is a tuple
-containing an item that is compatible with the exception.  Note that
-the object identities must match, i.e. it must be the same object, not
-just an onject with the same value.
-\kwindex{except}
-
-If no except clause matches the exception, the search for an exception
-handler continues in the surrounding code and on the invocation stack.
-
-If the evaluation of a condition in the header of an except clause
-raises an exception, the original search for a handler is cancelled
-and a search starts for the new exception in the surrounding code and
-on the call stack (it is treated as if the entire \verb\try\ statement
-raised the exception).
-
-When a matching except clause is found, the exception's parameter is
-assigned to the target specified in that except clause, if present,
-and the except clause's suite is executed.  When the end of this suite
-is reached, execution continues normally after the entire try
-statement.  (This means that if two nested handlers exist for the same
-exception, and the exception occurs in the try clause of the inner
-handler, the outer handler will not handle the exception.)
-
-The \verb\try...finally\ form specifies a `cleanup' handler.  The
-\verb\try\ clause is executed.  When no exception occurs, the
-\verb\finally\ clause is executed.  When an exception occurs in the
-\verb\try\ clause, the exception is temporarily saved, the
-\verb\finally\ clause is executed, and then the saved exception is
-re-raised.  If the \verb\finally\ clause raises another exception or
-executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
-the saved exception is lost.
-\kwindex{finally}
-
-When a \verb\return\ or \verb\break\ statement is executed in the
-\verb\try\ suite of a \verb\try...finally\ statement, the
-\verb\finally\ clause is also executed `on the way out'.  A
-\verb\continue\ statement is illegal in the \verb\try\ clause.  (The
-reason is a problem with the current implementation --- this
-restriction may be lifted in the future).
-\stindex{return}
-\stindex{break}
-\stindex{continue}
-
-\section{Function definitions} \label{function}
-\indexii{function}{definition}
-
-A function definition defines a user-defined function object (see
-section \ref{types}):
-\obindex{user-defined function}
-\obindex{function}
-
-\begin{verbatim}
-funcdef:        "def" funcname "(" [parameter_list] ")" ":" suite
-parameter_list: (parameter ",")* ("*" identifier | parameter [","])
-sublist:        parameter ("," parameter)* [","]
-parameter:      identifier | "(" sublist ")"
-funcname:       identifier
-\end{verbatim}
-
-A function definition is an executable statement.  Its execution binds
-the function name in the current local name space to a function object
-(a wrapper around the executable code for the function).  This
-function object contains a reference to the current global name space
-as the global name space to be used when the function is called.
-\indexii{function}{name}
-\indexii{name}{binding}
-
-The function definition does not execute the function body; this gets
-executed only when the function is called.
-
-Function call semantics are described in section \ref{calls}.  When a
-user-defined function is called, the arguments (a.k.a. actual
-parameters) are bound to the (formal) parameters, as follows:
-\indexii{function}{call}
-\indexiii{user-defined}{function}{call}
-\index{parameter}
-\index{argument}
-\indexii{parameter}{formal}
-\indexii{parameter}{actual}
-
-\begin{itemize}
-
-\item
-If there are no formal parameters, there must be no arguments.
-
-\item
-If the formal parameter list does not end in a star followed by an
-identifier, there must be exactly as many arguments as there are
-parameters in the formal parameter list (at the top level); the
-arguments are assigned to the formal parameters one by one.  Note that
-the presence or absence of a trailing comma at the top level in either
-the formal or the actual parameter list makes no difference.  The
-assignment to a formal parameter is performed as if the parameter
-occurs on the left hand side of an assignment statement whose right
-hand side's value is that of the argument.
-
-\item
-If the formal parameter list ends in a star followed by an identifier,
-preceded by zero or more comma-followed parameters, there must be at
-least as many arguments as there are parameters preceding the star.
-Call this number {\em N}.  The first {\em N} arguments are assigned to
-the corresponding formal parameters in the way descibed above.  A
-tuple containing the remaining arguments, if any, is then assigned to
-the identifier following the star.  This variable will always be a
-tuple: if there are no extra arguments, its value is \verb\()\, if
-there is just one extra argument, it is a singleton tuple.
-\indexii{variable length}{parameter list}
-
-\end{itemize}
-
-Note that the `variable length parameter list' feature only works at
-the top level of the parameter list; individual parameters use a model
-corresponding more closely to that of ordinary assignment.  While the
-latter model is generally preferable, because of the greater type
-safety it offers (wrong-sized tuples aren't silently mistreated),
-variable length parameter lists are a sufficiently accepted practice
-in most programming languages that a compromise has been worked out.
-(And anyway, assignment has no equivalent for empty argument lists.)
-
-\section{Class definitions} \label{class}
-\indexii{class}{definition}
-
-A class definition defines a class object (see section \ref{types}):
-\obindex{class}
-
-\begin{verbatim}
-classdef:       "class" classname [inheritance] ":" suite
-inheritance:    "(" [condition_list] ")"
-classname:      identifier
-\end{verbatim}
-
-A class definition is an executable statement.  It first evaluates the
-inheritance list, if present.  Each item in the inheritance list
-should evaluate to a class object.  The class's suite is then executed
-in a new execution frame (see section \ref{execframes}), using a newly
-created local name space and the original global name space.
-(Usually, the suite contains only function definitions.)  When the
-class's suite finishes execution, its execution frame is discarded but
-its local name space is saved.  A class object is then created using
-the inheritance list for the base classes and the saved local name
-space for the attribute dictionary.  The class name is bound to this
-class object in the original local name space.
-\index{inheritance}
-\indexii{class}{name}
-\indexii{name}{binding}
-\indexii{execution}{frame}
-
-\chapter{Top-level components}
-
-The Python interpreter can get its input from a number of sources:
-from a script passed to it as standard input or as program argument,
-typed in interactively, from a module source file, etc.  This chapter
-gives the syntax used in these cases.
-\index{interpreter}
-
-\section{Complete Python programs}
-\index{program}
-
-While a language specification need not prescribe how the language
-interpreter is invoked, it is useful to have a notion of a complete
-Python program.  A complete Python program is executed in a minimally
-initialized environment: all built-in and standard modules are
-available, but none have been initialized, except for \verb\sys\
-(various system services), \verb\builtin\ (built-in functions,
-exceptions and \verb\None\) and \verb\__main__\.  The latter is used
-to provide the local and global name space for execution of the
-complete program.
-\bimodindex{sys}
-\bimodindex{__main__}
-\bimodindex{builtin}
-
-The syntax for a complete Python program is that for file input,
-described in the next section.
-
-The interpreter may also be invoked in interactive mode; in this case,
-it does not read and execute a complete program but reads and executes
-one statement (possibly compound) at a time.  The initial environment
-is identical to that of a complete program; each statement is executed
-in the name space of \verb\__main__\.
-\index{interactive mode}
-
-Under {\UNIX}, a complete program can be passed to the interpreter in
-three forms: with the {\bf -c} {\it string} command line option, as a
-file passed as the first command line argument, or as standard input.
-If the file or standard input is a tty device, the interpreter enters
-interactive mode; otherwise, it executes the file as a complete
-program.
-\index{UNIX}
-\index{command line}
-\index{standard input}
-
-\section{File input}
-
-All input read from non-interactive files has the same form:
-
-\begin{verbatim}
-file_input:     (NEWLINE | statement)*
-\end{verbatim}
-
-This syntax is used in the following situations:
-
-\begin{itemize}
-
-\item when parsing a complete Python program (from a file or from a string);
-
-\item when parsing a module;
-
-\item when parsing a string passed to \verb\exec()\;
-\bifuncindex{exec}
-
-\item when parsing a file passed to \verb\execfile()\;
-\bifuncindex{execfile}
-
-\end{itemize}
-
-\section{Interactive input}
-
-Input in interactive mode is parsed using the following grammar:
-
-\begin{verbatim}
-interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
-\end{verbatim}
-
-Note that a (top-level) compound statement must be followed by a blank
-line in interactive mode; this is needed to help the parser detect the
-end of the input.
-
-\section{Expression input}
-\index{input}
-
-There are two forms of expression input.  Both ignore leading
-whitespace.
- 
-The string argument to \verb\eval()\ must have the following form:
-\bifuncindex{eval}
-
-\begin{verbatim}
-eval_input:     condition_list NEWLINE*
-\end{verbatim}
-
-The input line read by \verb\input()\ must have the following form:
-\bifuncindex{input}
-
-\begin{verbatim}
-input_input:    condition_list NEWLINE
-\end{verbatim}
-
-Note: to read `raw' input line without interpretation, you can use the
-built-in function \verb\raw_input()\  or the \verb\readline()\ method
-of file objects.
-\obindex{file}
-\index{input!raw}
-\index{raw input}
-\bifuncindex{raw_index}
-\ttindex{readline}
-
-\input{ref.ind}		% The index
+\include{ref1}
+\include{ref2}
+\include{ref3}
+\include{ref4}
+\include{ref5}
+\include{ref6}
+\include{ref7}
+\include{ref8}
+\inclide{ref.ind}
 
 \end{document}