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+% Format this file with latex.
+
+\documentstyle{article}
+
+% Page lay-out parameters
+\textwidth =      150mm
+\textheight =     240mm
+\topmargin =      -11mm
+\oddsidemargin =    5mm
+\evensidemargin =   5mm
+
+% Macros for e.g. and E.g. if you want them italicized:
+% \newcommand{\eg}{{\it e.g.}}
+% \newcommand{\Eg}{{\it E.g.}}
+% If you don't want them italicized:
+\newcommand{\eg}{e.g.}
+\newcommand{\Eg}{E.g.}
+
+% Frequently used system names
+\newcommand{\Python}{{\em Python}}
+\newcommand{\UNIX}{U{\sc nix}}
+
+% Code environment
+\newenvironment{code}{\begin{itemize}\samepage}{\end{itemize}}
+
+\title{\bf
+	Python Tutorial \\
+	(DRAFT)
+}
+
+\author{
+	Guido van Rossum \\
+	Dept. CST, CWI, Kruislaan 413 \\
+	1098 SJ Amsterdam, The Netherlands \\
+	E-mail: {\tt guido@cwi.nl}
+}
+
+\begin{document}
+
+\pagenumbering{roman}
+
+\maketitle
+
+\begin{abstract}
+
+\noindent
+\Python\ is a simple, yet powerful programming language that bridges the
+gap between C and shell programming, and is thus ideally suited for rapid
+prototyping.
+It is put together from constructs borrowed from a variety of other
+languages; most prominent are influences from ABC, C, Modula-3 and Icon.
+
+The \Python\ interpreter is easily extended with new functions and data
+types implemented in C.
+\Python\ is also suitable as an extension language for highly
+customizable C applications such as editors or window managers.
+
+\Python\ is available for various operating systems, amongst which
+several flavors of \UNIX, Amoeba, and the Apple Macintosh O.S.
+
+This tutorial introduces the reader informally to the basic concepts and
+features of the \Python\ language and system.
+It helps to have a \Python\ interpreter handy for hands-on experience,
+but as the examples are self-contained, the tutorial can be read
+off-line as well.
+For a description of standard objects and modules, see the Library and
+Module Reference document.
+The Language Reference document gives a more formal reference to the
+language.
+
+\end{abstract}
+
+\pagebreak
+
+\tableofcontents
+
+\pagebreak
+
+\pagenumbering{arabic}
+
+\section{Whetting Your Appetite}
+
+If you ever wrote a large shell script, you probably know this feeling:
+you'd love to add yet another feature, but it's already so slow, and so
+big, and so complicated; or the feature involves a system call or other
+funcion that is only accessable from C...
+Usually the problem at hand isn't serious enough to warrant rewriting
+the script in C; perhaps because the problem requires variable-length
+strings or other data types (like sorted lists of file names) that
+are easy in the shell but lots of work to implement in C; or perhaps
+just because you're not sufficiently familiar with C.
+
+In all such cases, \Python\ is just the language for you.
+\Python\ is simple to use, but it is a real programming language, offering
+much more structure and support for large programs than the shell has.
+On the other hand, it also offers much more error checking than C, and,
+being a
+{\it very-high-level language},
+it has high-level data types built in, such as flexible arrays and
+dictionaries that would cost you days to implement efficiently in C.
+Because of its more general data types \Python\ is applicable to a
+much larger problem domain than
+{\it Awk}
+or even
+{\it Perl},
+yet most simple things are at least as easy in \Python\ as in those
+languages.
+
+\Python\ allows you to split up your program in modules that can be reused
+in other \Python\ programs.
+It comes with a large collection of standard modules that you can use as
+the basis for your programs --- or as examples to start learning to
+program in \Python.
+There are also built-in modules that provide things like file I/O,
+system calls, and even a generic interface to window systems (STDWIN).
+
+\Python\ is an interpreted language, which saves you considerable time
+during program development because no compilation and linking is
+necessary.
+The interpreter can be used interactively, which makes it easy to
+experiment with features of the language, to write throw-away programs,
+or to test functions during bottom-up program development.
+It is also a handy desk calculator.
+
+\Python\ allows writing very compact and readable programs.
+Programs written in \Python\ are typically much shorter than equivalent C
+programs:
+No declarations are necessary (all type checking is
+dynamic); statement grouping is done by indentation instead of begin/end
+brackets; and the high-level data types allow you to express complex
+operations in a single statement.
+
+\Python\ is
+{\it extensible}:
+if you know how to program in C it is easy to add a new built-in module
+to the interpreter, either to perform critical operations at maximum
+speed, or to link \Python\ programs to libraries that may be only available
+in binary form (such as a vendor-specific graphics library).
+Once you are really hooked, you can link the \Python\ interpreter into an
+application written in C and use it as an extension or command language.
+
+\subsection{Where From Here}
+
+Now that you are all excited about \Python, you'll want to examine it in
+some more detail.
+Since the best introduction to a language is using it, you are invited
+here to do so.
+
+In the next section, the mechanics of using the interpreter are
+explained.
+This is rather mundane information, but essential for trying out the
+examples shown later.
+The rest of the tutorial introduces various features of the \Python\
+language and system though examples, beginning with simple expressions,
+statements and data types, through functions and modules, and finally
+touching upon advanced concepts like exceptions and classes.
+
+\section{Using the Python Interpreter}
+
+The \Python\ interpreter is usually installed as
+{\tt /usr/local/python}
+on those machines where it is available; putting
+{\tt /usr/local}
+in your \UNIX\ shell's search path makes it possible to start it by
+typing the command
+\begin{code}\begin{verbatim}
+python
+\end{verbatim}\end{code}
+to the shell.
+Since the choice of the directory where the interpreter lives is an
+installation option, other places instead of
+{\tt /usr/local}
+are possible; check with your local \Python\ guru or system
+administrator.%
+\footnote{
+	At CWI, at the time of writing, the interpreter can be found in
+	the following places:
+	On the Amoeba Ultrix machines, use the standard path,
+	{\tt /usr/local/python}.
+	On the Sun file servers, use
+	{\tt /ufs/guido/bin/}{\it arch}{\tt /python},
+	where {\it arch} can be {\tt sgi} or {\tt sun4}.
+	On piring, use {\tt /userfs3/amoeba/bin/python}.
+	(If you can't find a binary advertised here, get in touch with me.)
+}
+
+The interpreter operates somewhat like the \UNIX\ shell: when called with
+standard input connected to a tty device, it reads and executes commands
+interactively; when called with a file name argument or with a file as
+standard input, it reads and executes a
+{\it script}
+from that file.%
+\footnote{
+	There is a difference between ``{\tt python file}'' and
+	``{\tt python $<$file}''.  In the latter case {\tt input()} and
+	{\tt raw\_input()} are satisfied from {\it file}, which has
+	already been read until the end by the parser, so they will read
+	EOF immediately.  In the former case (which is usually what was
+	intended) they are satisfied from whatever file or device is
+	connected to standard input of the \Python\ interpreter.
+}
+If available, the script name and additional arguments thereafter are
+passed to the script in the variable
+{\tt sys.argv},
+which is a list of strings.
+
+When standard input is a tty, the interpreter is said to be in
+{\it interactive\ mode}.
+In this mode it prompts for the next command with the
+{\it primary\ prompt},
+usually three greater-than signs ({\tt >>>}); for continuation lines
+it prompts with the
+{\it secondary\ prompt},
+by default three dots ({\tt ...}).
+Typing an EOF (\^{}D) at the primary prompt causes the interpreter to exit
+with a zero exit status.
+
+When an error occurs in interactive mode, the interpreter prints a
+message and returns to the primary prompt; with input from a file, it
+exits with a nonzero exit status.
+(Exceptions handled by an
+{\tt except}
+clause in a
+{\tt try}
+statement are not errors in this context.)
+Some errors are unconditionally fatal and cause an exit with a nonzero
+exit; this applies to internal inconsistencies and some cases of running
+out of memory.
+All error messages are written to the standard error stream; normal
+output from the executed commands is written to standard output.
+
+Typing an interrupt (normally Control-C or DEL) to the primary or
+secondary prompt cancels the input and returns to the primary prompt.
+Typing an interrupt while a command is being executed raises the
+{\tt KeyboardInterrupt}
+exception, which may be handled by a
+{\tt try}
+statement.
+
+When a module named
+{\tt foo}
+is imported, the interpreter searches for a file named
+{\tt foo.py}
+in a list of directories specified by the environment variable
+{\tt PYTHONPATH}.
+It has the same syntax as the \UNIX\ shell variable
+{\tt PATH},
+i.e., a list of colon-separated directory names.
+When
+{\tt PYTHONPATH}
+is not set, an installation-dependent default path is used, usually
+{\tt .:/usr/local/lib/python}.%
+\footnote{
+	Modules are really searched in the list of directories given by
+	the variable {\tt sys.path} which is initialized from
+	{\tt PYTHONPATH} or from the installation-dependent default.
+	See the section on Standard Modules below.
+}
+The built-in module
+{\tt stdwin},
+if supported at all, is only available if the interpreter is started
+with the
+{\bf --s}
+flag.
+If this flag is given, stdwin is initialized as soon as the interpreter
+is started, and in the case of X11 stdwin certain command line arguments
+(like
+{\bf --display} )
+are consumed by stdwin.
+
+On BSD'ish \UNIX\ systems, \Python\ scripts can be made directly executable,
+like shell scripts, by putting the line
+\begin{code}\begin{verbatim}
+#! /usr/local/python
+\end{verbatim}\end{code}
+(assuming that's the name of the interpreter) at the beginning of the
+script and giving the file an executable mode.
+(The
+{\tt \#!}
+must be the first two characters of the file.)
+For scripts that use the built-in module
+{\tt stdwin},
+use
+\begin{code}\begin{verbatim}
+#! /usr/local/python -s
+\end{verbatim}\end{code}
+
+\subsection{Interactive Input Editing and History Substitution}
+
+Some versions of the \Python\ interpreter support editing of the current
+input line and history substitution, similar to facilities found in the
+Korn shell and the GNU Bash shell.
+This is implemented using the
+{\it GNU\ Readline}
+library, which supports Emacs-style and vi-style editing.
+This library has its own documentation which I won't duplicate here;
+however, the basics are easily explained.
+
+If supported,%
+\footnote{
+	Perhaps the quickest check to see whether command line editing
+	is supported is typing Control-P to the first \Python\ prompt
+	you get.  If it beeps, you have command line editing.
+	If not, you can forget about the rest of this section.
+}
+input line editing is active whenever the interpreter prints a primary
+or secondary prompt (yes, you can turn it off by deleting
+{\tt sys.ps1},
+and no, it is not provided for
+{\tt input()}
+and
+{\tt raw\_input()}).
+The current line can be edited using the conventional Emacs control
+characters.
+The most important of these are:
+C-A (Control-A) moves the cursor to the beginning of the line, C-E to
+the end, C-B moves it one position to the left, C-F to the right.
+Backspace erases the character to the left of the cursor, C-D the
+character to its right.
+C-K kills (erases) the rest of the line to the right of the cursor, C-Y
+yanks back the last killed string.
+C-\_ undoes the last change you made; it can be repeated for cumulative
+effect.
+
+History substitution works as follows.
+All non-empty input lines issued so far are saved in a history buffer,
+and when a new prompt is given you are positioned on a new line at the
+bottom of this buffer.
+C-P moves one line up (back) in the history buffer, C-N moves one down.
+The current line in the history buffer can be edited; in this case an
+asterisk appears in front of the prompt to mark it as modified.
+Pressing the Return key passes the current line to the interpreter.
+C-R starts an incremental reverse search; C-S starts a forward search.
+
+The key bindings and some other parameters of the Readline library can
+be customized by placing commands in an initialization file called
+{\tt \$HOME/.initrc}.
+Key bindings have the form
+\begin{code}\begin{verbatim}
+key-name: function-name
+\end{verbatim}\end{code}
+and options can be set with
+\begin{code}\begin{verbatim}
+set option-name value
+\end{verbatim}\end{code}
+Example:
+\begin{code}\begin{verbatim}
+# I prefer vi-style editing:
+set editing-mode vi
+# Edit using a single line:
+set horizontal-scroll-mode On
+# Rebind some keys:
+Meta-h: backward-kill-word
+Control-u: universal-argument
+\end{verbatim}\end{code}
+Note that the default binding for TAB in \Python\ is to insert a TAB
+instead of Readline's default filename completion function.
+If you insist, you can override this by putting
+\begin{code}\begin{verbatim}
+TAB: complete
+\end{verbatim}\end{code}
+in your
+{\tt \$HOME/.inputrc}.
+Of course, this makes it hard to type indented continuation lines.
+
+This facility is an enormous step forward compared to previous versions of
+the interpreter; however, some wishes are left:
+It would be nice if the proper indentation were suggested on
+continuation lines (the parser knows if an indent token is required
+next).
+The completion mechanism might use the interpreter's symbol table.
+A function to check (or even suggest) matching parentheses, quotes
+etc. would also be useful.
+
+\section{An Informal Introduction to Python}
+
+In the following examples, input and output are distinguished by the
+presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat the
+example, you must type everything after the prompt, when the prompt
+appears; everything on lines that do not begin with a prompt is output
+from the interpreter.
+Note that a secondary prompt on a line by itself in an example means you
+must type a blank line; this is used to end a multi-line command.
+
+\subsection{Using Python as a Calculator}
+
+Let's try some simple \Python\ commands.
+Start the interpreter and wait for the primary prompt,
+{\tt >>>}.
+The interpreter acts as a simple calculator: you can type an expression
+at it and it will write the value.
+Expression syntax is straightforward: the operators
+{\tt +},
+{\tt -},
+{\tt *}
+and
+{\tt /}
+work just as in most other languages (e.g., Pascal or C); parentheses
+can be used for grouping.
+For example:
+\begin{code}\begin{verbatim}
+>>> # This is a comment
+>>> 2+2
+4
+>>> 
+>>> (50-5+5*6+25)/4
+25
+>>> # Division truncates towards zero:
+>>> 7/3
+2
+>>> 
+\end{verbatim}\end{code}
+As in C, the equal sign ({\tt =}) is used to assign a value to a variable.
+The value of an assignment is not written:
+\begin{code}\begin{verbatim}
+>>> width = 20
+>>> height = 5*9
+>>> width * height
+900
+>>> 
+\end{verbatim}\end{code}
+There is some support for floating point:
+\begin{code}\begin{verbatim}
+>>> 10.0 / 3.3
+3.0303030303
+>>> 
+\end{verbatim}\end{code}
+But you can't mix floating point and integral numbers in expression (yet).
+
+Besides numbers, \Python\ can also manipulate strings, enclosed in single
+quotes:
+\begin{code}\begin{verbatim}
+>>> 'foo bar'
+'foo bar'
+>>> 'doesn\'t'
+'doesn\'t'
+>>> 
+\end{verbatim}\end{code}
+Strings are written inside quotes and with quotes and other funny
+characters escaped by backslashes, to show the precise value.
+(There is also a way to write strings without quotes and escapes.)
+Strings can be concatenated (glued together) with the
+{\tt +}
+operator, and repeated with
+{\tt *}:
+\begin{code}\begin{verbatim}
+>>> word = 'Help' + 'A'
+>>> word
+'HelpA'
+>>> '<' + word*5 + '>'
+'<HelpAHelpAHelpAHelpAHelpA>'
+>>> 
+\end{verbatim}\end{code}
+Strings can be subscripted; as in C, the first character of a string has
+subscript 0.
+There is no separate character type; a character is simply a string of
+size one.
+As in Icon, substrings can be specified with the
+{\it slice}
+notation: two subscripts (indices) separated by a colon.
+\begin{code}\begin{verbatim}
+>>> word[4]
+'A'
+>>> word[0:2]
+'He'
+>>> word[2:4]
+'lp'
+>>> # Slice indices have useful defaults:
+>>> word[:2]    # Take first two characters
+'He'
+>>> word[2:]    # Skip first two characters
+'lpA'
+>>> # A useful invariant: s[:i] + s[i:] = s
+>>> word[:3] + word[3:]
+'HelpA'
+>>> 
+\end{verbatim}\end{code}
+Degenerate cases are handled gracefully: an index that is too large is
+replaced by the string size, an upper bound smaller than the lower bound
+returns an empty string.
+\begin{code}\begin{verbatim}
+>>> word[1:100]
+'elpA'
+>>> word[10:]
+''
+>>> word[2:1]
+''
+>>> 
+\end{verbatim}\end{code}
+Slice indices (but not simple subscripts) may be negative numbers, to
+start counting from the right.
+For example:
+\begin{code}\begin{verbatim}
+>>> word[-2:]    # Take last two characters
+'pA'
+>>> word[:-2]    # Skip last two characters
+'Hel'
+>>> # But -0 does not count from the right!
+>>> word[-0:]    # (since -0 equals 0)
+'HelpA'
+>>> 
+\end{verbatim}\end{code}
+The best way to remember how slices work is to think of the indices as
+pointing
+{\it between}
+characters, with the left edge of the first character numbered 0.
+Then the right edge of the last character of a string of
+{\tt n}
+characters has index
+{\tt n},
+for example:
+\begin{code}\begin{verbatim}
+ +---+---+---+---+---+ 
+ | H | e | l | p | A |
+ +---+---+---+---+---+ 
+ 0   1   2   3   4   5 
+-5  -4  -3  -2  -1
+\end{verbatim}\end{code}
+The first row of numbers gives the position of the indices 0...5 in the
+string; the second row gives the corresponding negative indices.
+For nonnegative indices, the length of a slice is the difference of the
+indices, if both are within bounds,
+{\it e.g.},
+the length of
+{\tt word[1:3]}
+is 3--1 = 2.
+
+Finally, the built-in function {\tt len()} computes the length of a
+string:
+\begin{code}\begin{verbatim}
+>>> s = 'supercalifragilisticexpialidocious'
+>>> len(s)
+34
+>>> 
+\end{verbatim}\end{code}
+
+\Python\ knows a number of
+{\it compound}
+data types, used to group together other values.
+The most versatile is the
+{\it list},
+which can be written as a list of comma-separated values between square
+brackets:
+\begin{code}\begin{verbatim}
+>>> a = ['foo', 'bar', 100, 1234]
+>>> a
+['foo', 'bar', 100, 1234]
+>>> 
+\end{verbatim}\end{code}
+As for strings, list subscripts start at 0:
+\begin{code}\begin{verbatim}
+>>> a[0]
+'foo'
+>>> a[3]
+1234
+>>> 
+\end{verbatim}\end{code}
+Lists can be sliced and concatenated like strings:
+\begin{code}\begin{verbatim}
+>>> a[1:3]
+['bar', 100]
+>>> a[:2] + ['bletch', 2*2]
+['foo', 'bar', 'bletch', 4]
+>>> 
+\end{verbatim}\end{code}
+Unlike strings, which are
+{\it immutable},
+it is possible to change individual elements of a list:
+\begin{code}\begin{verbatim}
+>>> a
+['foo', 'bar', 100, 1234]
+>>> a[2] = a[2] + 23
+>>> a
+['foo', 'bar', 123, 1234]
+>>>
+\end{verbatim}\end{code}
+Assignment to slices is also possible, and this may even change the size
+of the list:
+\begin{code}\begin{verbatim}
+>>> # Replace some items:
+>>> a[0:2] = [1, 12]
+>>> a
+[1, 12, 123, 1234]
+>>> # Remove some:
+>>> a[0:2] = []
+>>> a
+[123, 1234]
+>>> # Insert some:
+>>> a[1:1] = ['bletch', 'xyzzy']
+>>> a
+[123, 'bletch', 'xyzzy', 1234]
+>>> 
+\end{verbatim}\end{code}
+The built-in function {\tt len()} also applies to lists:
+\begin{code}\begin{verbatim}
+>>> len(a)
+4
+>>> 
+\end{verbatim}\end{code}
+
+\subsection{Simple and Compound Statements}
+
+Of course, we can use \Python\ for more complicated tasks than adding two
+and two together.
+For instance, we can write an initial subsequence of the
+{\it Fibonacci}
+series as follows:
+\begin{code}\begin{verbatim}
+>>> # Fibonacci series:
+>>> # the sum of two elements defines the next
+>>> a, b = 0, 1
+>>> while b < 100:
+...       print b
+...       a, b = b, a+b
+... 
+1
+1
+2
+3
+5
+8
+13
+21
+34
+55
+89
+>>> 
+\end{verbatim}\end{code}
+This example introduces several new features.
+\begin{itemize}
+\item
+The first line contains a
+{\it multiple\ assignment}:
+the variables
+{\tt a}
+and
+{\tt b}
+simultaneously get the new values 0 and 1.
+On the last line this is used again, demonstrating that the expressions
+on the right-hand side are all evaluated first before any of the
+assignments take place.
+\item
+The
+{\tt while}
+loop executes as long as the condition remains true.
+In \Python, as in C, any non-zero integer value is true; zero is false.
+The condition may also be a string or list value, in fact any sequence;
+anything with a non-zero length is true, empty sequences are false.
+The test used in the example is a simple comparison.
+The standard comparison operators are written as
+{\tt <},
+{\tt >},
+{\tt =},
+{\tt <=},
+{\tt >=}
+and
+{\tt <>}.%
+\footnote{
+	The ambiguity of using {\tt =}
+	for both assignment and equality is resolved by disallowing
+	unparenthesized conditions at the right hand side of assignments.
+}
+\item
+The
+{\it body}
+of the loop is
+{\it indented}
+by one tab stop: indentation is \Python's way of grouping statements.
+\Python\ does not (yet!) provide an intelligent input line editing
+facility, so you have to type a tab for each indented line.
+In practice you will prepare more complicated input for \Python\ with a
+text editor; most text editors have an auto-indent facility.
+When a compound statement is entered interactively, it must be
+followed by a blank line to indicate completion (otherwise the parser
+doesn't know that you have typed the last line).
+\item
+The
+{\tt print}
+statement writes the value of the expression(s) it is passed.
+It differs from just writing the expression you want to write (as we did
+earlier in the calculator examples) in the way it handles multiple
+expressions and strings.
+Strings are written without quotes and a space is inserted between
+items, so you can do things like this:
+\begin{code}\begin{verbatim}
+>>> i = 256*256
+>>> print 'The value of i is', i
+The value of i is 65536
+>>> 
+\end{verbatim}\end{code}
+A trailing comma avoids the newline after the output:
+\begin{code}\begin{verbatim}
+>>> a, b = 0, 1
+>>> while b < 1000:
+...     print b,
+...     a, b = b, a+b
+... 
+1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
+>>> 
+\end{verbatim}\end{code}
+Note that the interpreter inserts a newline before it prints the next
+prompt if the last line was not completed.
+\end{itemize}
+
+\subsection{Other Control Flow Statements}
+
+Besides {\tt while}, already introduced, \Python\ supports the usual
+control flow statements known from other languages, with some twists.
+
+\subsubsection{If Statements}
+
+Perhaps the most well-known statement type is the {\tt if} statement.
+For example:
+\begin{code}\begin{verbatim}
+>>> if x < 0:
+...      x = 0
+...      print 'Negative changed to zero'
+... elif x = 0:
+...      print 'Zero'
+... elif x = 1:
+...      print 'Single'
+... else:
+...      print 'More'
+... 
+\end{verbatim}\end{code}
+There can be zero or more {\tt elif} parts, and the {\tt else} part is
+optional.
+
+\subsubsection{For Statements}
+
+The {\tt for} statement in \Python\ differs a bit from what you may be
+used to in C or Pascal.
+Rather than always iterating over an arithmetic progression of numbers,
+as in Pascal, or leaving the user completely free in the iteration test
+and step, as in C, \Python's {\tt for} iterates over the items of any
+sequence (\it e.g.\rm%
+, a list or a string).
+An example {\tt for} statement:
+\begin{code}\begin{verbatim}
+>>> # Measure some strings:
+>>> a = ['cat', 'window', 'defenestrate']
+>>> for x in a:
+...     print x, len(x)
+... 
+cat 3
+window 6
+defenestrate 12
+>>> 
+\end{verbatim}\end{code}
+If you do need to iterate over a sequence of numbers, the built-in
+function {\tt range()} comes in handy.
+It generates lists containing arithmetic progressions,
+{\it e.g.}:
+\begin{code}\begin{verbatim}
+>>> range(10)
+[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+>>> 
+\end{verbatim}\end{code}
+The end point is never part of the generated list; {\tt range(10)}
+generates exactly the legal indices for items of a list or string of
+length 10.
+It is possible to let the range start at another number, or to specify a
+different increment (even negative):
+\begin{code}\begin{verbatim}
+>>> range(5, 10)
+[5, 6, 7, 8, 9]
+>>> range(0, 10, 3)
+[0, 3, 6, 9]
+>>> range(-10, -100, -30)
+[-10, -40, -70]
+>>> 
+\end{verbatim}\end{code}
+To iterate over the indices of a list or string, combine {\tt range()}
+and {\tt len()} as follows:
+\begin{code}\begin{verbatim}
+>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
+>>> for i in range(len(a)):
+...     print i, a[i]
+... 
+0 Mary
+1 had
+2 a
+3 little
+4 lamb
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{Break Statements and Else Clauses on Loops}
+
+The {\tt break} statement breaks out of the smallest enclosing {\tt for}
+or {\tt while} loop.
+Loop statements may have an {\tt else} clause; it is executed when the
+loop terminates through exhaustion of the list (for {\tt for}) or when
+the condition becomes false (for {\tt while}) but not when the loop is
+terminated by a {\tt break} statement.
+This is exemplified by the following loop, which searches for a list
+item of value 0:
+\begin{code}\begin{verbatim}
+>>> a = [1, 10, 0, 5, 12]
+>>> for i in a:
+...     if i = 0:
+...        print '*** Found a zero'
+...        break
+... else:
+...     print '*** No zero found'
+... 
+*** Found a zero
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{Pass Statements}
+
+The {\tt pass} statement does nothing, similar to {\tt skip} in Algol-68
+or an empty statement in C.
+It can be used when a statement is required syntactically but the
+program requires no action.
+For example:
+\begin{code}\begin{verbatim}
+>>> while 1:
+...       pass # Busy-wait for keyboard interrupt
+... 
+\end{verbatim}\end{code}
+
+\subsection{Defining Functions}
+
+We can create a function that writes the Fibonacci series to an
+arbitrary boundary:
+\begin{code}\begin{verbatim}
+>>> def fib(n):    # write Fibonacci series up to n
+...     a, b = 0, 1
+...     while b <= n:
+...           print b,
+...           a, b = b, a+b
+... 
+>>> # Now call the function we just defined:
+>>> fib(2000)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
+>>> 
+\end{verbatim}\end{code}
+The keyword
+{\tt def}
+introduces a function
+{\it definition}.
+It must be followed by the function name and the parenthesized list of
+formal parameters.
+The statements that form the body of the function starts at the next
+line, indented by a tab stop.
+The
+{\it execution}
+of a function introduces a new symbol table used for the local variables
+of the function.
+More precisely, all variable assignments in a function store the value
+in the local symbol table; variable references first look in the local
+symbol table, then in the global symbol table, and then in the table of
+built-in names.
+Thus, the global symbol table is
+{\it read-only}
+within a function; the built-in symbol table is always read-only.
+The actual parameters (arguments) to a function call are introduced in
+the local symbol table of the called function when it is called;
+thus, arguments are passed using
+{\it call\ by\ value}.%
+\footnote{
+	 Actually, {\it call  by  object reference} would be a better
+	 name, since if a mutable object is passed, the caller will see
+	 any changes the callee makes to it.
+}
+When a function calls another function, a new local symbol table is
+created for that call.
+
+A function definition introduces the function name in the global symbol
+table.
+The value has a type that is recognized by the interpreter as a
+user-defined function.
+This value can be assigned to another name which can then also be used
+as a function.
+This serves as a general renaming mechanism:
+\begin{code}\begin{verbatim}
+>>> fib
+<user function 'fib'>
+>>> f = fib
+>>> f(100)
+1 1 2 3 5 8 13 21 34 55 89
+>>> 
+\end{verbatim}\end{code}
+You might object that
+{\tt fib}
+is not a function but a procedure.
+In \Python, as in C, procedures are just functions that don't return a
+value.
+In fact, technically speaking, procedures do return a value, albeit a
+rather boring one.
+This value is called {\tt None} (it's a built-in name).
+Writing the value {\tt None} is normally suppressed by the interpreter
+if it would be the only value written.
+You can see it if you really want to:
+\begin{code}\begin{verbatim}
+>>> print fib(0)
+None
+>>> 
+\end{verbatim}\end{code}
+It is simple to write a function that returns a list of the numbers of
+the Fibonacci series, instead of printing it:
+\begin{code}\begin{verbatim}
+>>> def fib2(n): # return Fibonacci series up to n
+...     ret = []
+...     a, b = 0, 1
+...     while b <= n:
+...           ret.append(b)    # see below
+...           a, b = b, a+b
+...     return ret
+... 
+>>> f100 = fib2(100)    # call it
+>>> f100                # write the result
+[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
+>>> 
+\end{verbatim}\end{code}
+This example, as usual, demonstrates some new \Python\ features:
+\begin{itemize}
+\item
+The
+{\tt return}
+statement returns with a value from a function.
+{\tt return}
+without an expression argument is used to return from the middle of a
+procedure (falling off the end also returns from a proceduce).
+\item
+The statement
+{\tt ret.append(b)}
+calls a
+{\it method}
+of the list object
+{\tt ret}.
+A method is a function that `belongs' to an object and is named
+{\tt obj.methodname},
+where
+{\tt obj}
+is some object (this may be an expression), and
+{\tt methodname}
+is the name of a method that is defined by the object's type.
+Different types define different methods.
+Methods of different types may have the same name without causing
+ambiguity.
+See the section on classes, later, to find out how you can define your
+own object types and methods.
+The method
+{\tt append}
+shown in the example, is defined for list objects; it adds a new element
+at the end of the list.
+In this case it is equivalent to
+{\tt ret = ret + [b]},
+but more efficient.%
+\footnote{
+	There is a subtle semantic difference if the object
+	is referenced from more than one place.
+}
+\end{itemize}
+The list object type has two more methods:
+\begin{list}{}{\labelwidth=4cm}
+\item[{\tt insert(i, x)}]
+Inserts an item at a given position.
+The first argument is the index of the element before which to insert,
+so {\tt a.insert(0, x)} inserts at the front of the list, and
+{\tt a.insert(len(a), x)} is equivalent to {\tt a.append(x)}.
+\item[{\tt sort()}]
+Sorts the elements of the list.
+\end{list}
+For example:
+\begin{code}\begin{verbatim}
+>>> a = [10, 100, 1, 1000]
+>>> a.insert(2, -1)
+>>> a
+[10, 100, -1, 1, 1000]
+>>> a.sort()
+>>> a
+[-1, 1, 10, 100, 1000]
+>>> # Strings are sorted according to ASCII:
+>>> b = ['Mary', 'had', 'a', 'little', 'lamb']
+>>> b.sort()
+>>> b
+['Mary', 'a', 'had', 'lamb', 'little']
+>>> 
+\end{verbatim}\end{code}
+
+\subsection{Modules}
+
+If you quit from the \Python\ interpreter and enter it again, the
+definitions you have made (functions and variables) are lost.
+Therefore, if you want to write a somewhat longer program, you are
+better off using a text editor to prepare the input for the interpreter
+and run it with that file as input instead.
+This is known as creating a
+{\it script}.
+As your program gets longer, you may want to split it into several files
+for easier maintenance.
+You may also want to use a handy function that you've written in several
+programs without copying its definition into each program.
+To support this, \Python\ has a way to put definitions in a file and use
+them in a script or in an interactive instance of the interpreter.
+Such a file is called a
+{\it module};
+definitions from a module can be
+{\it imported}
+into other modules or into the
+{\it main}
+module (the collection of variables that you have access to in
+a script and in calculator mode).
+
+A module is a file containing \Python\ definitions and statements.
+The file name is the module name with the suffix
+{\tt .py}
+appended.
+For instance, use your favorite text editor to create a file called
+{\tt fibo.py}
+in the current directory with the following contents:
+\begin{code}\begin{verbatim}
+# Fibonacci numbers module
+
+def fib(n):    # write Fibonacci series up to n
+    a, b = 0, 1
+    while b <= n:
+          print b,
+          a, b = b, a+b
+
+def fib2(n): # return Fibonacci series up to n
+    ret = []
+    a, b = 0, 1
+    while b <= n:
+          ret.append(b)
+          a, b = b, a+b
+    return ret
+\end{verbatim}\end{code}
+Now enter the \Python\ interpreter and import this module with the
+following command:
+\begin{code}\begin{verbatim}
+>>> import fibo
+>>> 
+\end{verbatim}\end{code}
+This does not enter the names of the functions defined in
+{\tt fibo}
+directly in the symbol table; it only enters the module name
+{\tt fibo}
+there.
+Using the module name you can access the functions:
+\begin{code}\begin{verbatim}
+>>> fibo.fib(1000)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
+>>> fibo.fib2(100)
+[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
+>>> 
+\end{verbatim}\end{code}
+If you intend to use a function often you can assign it to a local name:
+\begin{code}\begin{verbatim}
+>>> fib = fibo.fib
+>>> fib(500)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{More About Modules}
+
+A module can contain executable statements as well as function
+definitions.
+These statements are intended to initialize the module.
+They are executed only the
+{\it first}
+time the module is imported somewhere.%
+\footnote{
+	In fact function definitions are also `statements' that are
+	`executed'; the execution enters the function name in the
+	module's global symbol table.
+}
+
+Each module has its own private symbol table, which is used as the
+global symbol table by all functions defined in the module.
+Thus, the author of a module can use global variables in the module
+without worrying about accidental clashes with a user's global
+variables.
+On the other hand, if you know what you are doing you can touch a
+module's global variables with the same notation used to refer to its
+functions,
+{\tt modname.itemname}.
+
+Modules can import other modules.
+It is customary but not required to place all
+{\tt import}
+statements at the beginning of a module (or script, for that matter).
+The imported module names are placed in the importing module's global
+symbol table.
+
+There is a variant of the
+{\tt import}
+statement that imports names from a module directly into the importing
+module's symbol table.
+For example:
+\begin{code}\begin{verbatim}
+>>> from fibo import fib, fib2
+>>> fib(500)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377
+>>> 
+\end{verbatim}\end{code}
+This does not introduce the module name from which the imports are taken
+in the local symbol table (so in the example, {\tt fibo} is not
+defined).
+
+There is even a variant to import all names that a module defines:
+\begin{code}\begin{verbatim}
+>>> from fibo import *
+>>> fib(500)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377
+>>> 
+\end{verbatim}\end{code}
+This imports all names except those beginning with an underscore
+({\tt \_}).
+
+\subsubsection{Standard Modules}
+
+\Python\ comes with a library of standard modules, described in a separate
+document (Python Library and Module Reference).
+Some modules are built into the interpreter; these provide access to
+operations that are not part of the core of the language but are
+nevertheless built in, either for efficiency or to provide access to
+operating system primitives such as system calls.
+The set of such modules is a configuration option; e.g., the
+{\tt amoeba}
+module is only provided on systems that somehow support Amoeba
+primitives.
+One particular module deserves some attention:
+{\tt sys},
+which is built into every \Python\ interpreter.
+The variables
+{\tt sys.ps1}
+and
+{\tt sys.ps2}
+define the strings used as primary and secondary prompts:
+\begin{code}\begin{verbatim}
+>>> import sys
+>>> sys.ps1
+'>>> '
+>>> sys.ps2
+'... '
+>>> sys.ps1 = 'C> '
+C> print 'Yuck!'
+Yuck!
+C> 
+\end{verbatim}\end{code}
+These two variables are only defined if the interpreter is in
+interactive mode.
+
+The variable
+{\tt sys.path}
+is a list of strings that determine the interpreter's search path for
+modules.
+It is initialized to a default path taken from the environment variable
+{\tt PYTHONPATH},
+or from a built-in default if
+{\tt PYTHONPATH}
+is not set.
+You can modify it using standard list operations, e.g.:
+\begin{code}\begin{verbatim}
+>>> import sys
+>>> sys.path.append('/ufs/guido/lib/python')
+>>> 
+\end{verbatim}\end{code}
+
+\subsection{Errors and Exceptions}
+
+Until now error messages haven't yet been mentioned, but if you have
+tried out the examples you have probably seen some.
+There are (at least) two distinguishable kinds of errors:
+{\it syntax\ errors}
+and
+{\it exceptions}.
+
+\subsubsection{Syntax Errors}
+
+Syntax errors, also known as parsing errors, are perhaps the most common
+kind of complaint you get while you are still learning \Python:
+\begin{code}\begin{verbatim}
+>>> while 1 print 'Hello world'
+Parsing error at line 1:
+while 1 print 'Hello world'
+             \^
+>>> 
+\end{verbatim}\end{code}
+The parser repeats the offending line and displays a little `arrow'
+pointing at the earliest point in the line where the error was detected.
+The error is caused by (or at least detected at) the token
+{\it preceding}
+the arrow: in the example, the error is detected at the keyword
+{\tt print}, since a colon ({\tt :}) is missing before it.
+The line number is printed so you know where to look in case the input
+came from a script.
+
+\subsubsection{Exceptions}
+
+Even if a statement or expression is syntactically correct, it may cause
+an error when an attempt is made to execute it:
+\begin{code}\begin{verbatim}
+>>> 10 * (1/0)
+Unhandled exception: run-time error: domain error or
+zero division
+Context: 1 / 0 
+>>> 4 + foo*3
+Unhandled exception: undefined name: foo
+Context: 4 + foo * 3 
+>>> '2' + 2
+Unhandled exception: type error: invalid argument type
+Context: '2' + 2 
+>>> 
+\end{verbatim}\end{code}
+Errors detected during execution are called
+{\it exceptions}
+and are not unconditionally fatal: you will soon learn how to handle
+them in \Python\ programs.
+Most exceptions are not handled by programs, however, and result
+in error messages as shown here.
+
+The first line of the error message indicates what happened.
+Exceptions come in different types, and the type is printed as part of
+the message: the types in the example are
+{\tt run-time error},
+{\tt undefined name}
+and
+{\tt type error}.
+The rest of the line is a detail whose interpretation depends on the
+exception type.
+
+The second line of the error message shows the context where the
+exception happened.
+As you can see, this is usually a sub-expression enclosing the actual
+failing operation.%
+\footnote{
+	The context is reconstructed from the parse tree, so it may look
+	a little odd.  A stack trace should really be printed at this
+	point; this will be implemented in a future version of the
+	interpreter.  The context is suppressed for keyboard interrupts.
+}
+
+Here is a summary of the most common exceptions:
+\begin{itemize}
+\item
+{\it Run-time\ errors}
+are generally caused by wrong data used by the program; this can be the
+programmer's fault or caused by bad input.
+The detail states the cause of the error in more detail.
+\item
+{\it Undefined\ name}
+errors are more serious: these are usually caused by misspelled
+identifiers.%
+\footnote{
+	The parser does not check whether names used in a program are at
+	all defined elsewhere in the program, so such checks are
+	postponed until run-time.  The same holds for type checking.
+}
+The detail is the offending identifier.
+\item
+{\it Type\ errors}
+are also pretty serious: this is another case of using wrong data (or
+better, using data the wrong way), but here the error can be glanced
+from the object type(s) alone.
+The detail shows in what context the error was detected.
+\end{itemize}
+
+\subsubsection{Handling Exceptions}
+
+It is possible to write programs that handle selected exceptions.
+Look at the following example, which prints a table of inverses of
+some floating point numbers:
+\begin{code}\begin{verbatim}
+>>> numbers = [0.3333, 2.5, 0.0, 10.0]
+>>> for x in numbers:
+...     print x,
+...     try:
+...         print 1.0 / x
+...     except RuntimeError:
+...         print '*** has no inverse ***'
+... 
+0.3333 3.00030003
+2.5 0.4
+0 *** has no inverse ***
+10 0.1
+>>> 
+\end{verbatim}\end{code}
+The {\tt try} statement works as follows.
+\begin{itemize}
+\item
+First, the
+{\it try\ clause}
+(the statement(s) between the {\tt try} and {\tt except} keywords) is
+executed.
+\item
+If no exception occurs, the
+{\it except\ clause}
+is skipped and execution of the {\tt try} statement is finished.
+\item
+If an exception occurs during execution of the try clause, and its
+type matches the exception named after the {\tt except} keyword, the
+rest of the try clause is skipped, the except clause is executed, and
+then execution continues after the {\tt try} statement.
+\item
+If an exception occurs which does not match the exception named in the
+except clause, it is passed on to outer try statements; if no handler is
+found, it is an
+{\it unhandled\ exception}
+and execution stops with a message as shown above.
+\end{itemize}
+A {\tt try} statement may have more than one except clause, to specify
+handlers for different exceptions.
+At most one handler will be executed.
+Handlers only handle exceptions that occur in the corresponding try
+clause, not in other handlers of the same {\tt try} statement.
+An except clause may name multiple exceptions as a parenthesized list,
+{\it e.g.}:
+\begin{code}\begin{verbatim}
+... except (RuntimeError, TypeError, NameError):
+...     pass
+\end{verbatim}\end{code}
+The last except clause may omit the exception name(s), to serve as a
+wildcard.
+Use this with extreme caution!
+
+When an exception occurs, it may have an associated value, also known as
+the exceptions's
+{\it argument}.
+The presence and type of the argument depend on the exception type.
+For exception types which have an argument, the except clause may
+specify a variable after the exception name (or list) to receive the
+argument's value, as follows:
+\begin{code}\begin{verbatim}
+>>> try:
+...     foo()
+... except NameError, x:
+...     print x, 'undefined'
+... 
+foo undefined
+>>> 
+\end{verbatim}\end{code}
+If an exception has an argument, it is printed as the third part
+(`detail') of the message for unhandled exceptions.
+
+Standard exception names are built-in identifiers (not reserved
+keywords).
+These are in fact string objects whose
+{\it object\ identity}
+(not their value!) identifies the exceptions.%
+\footnote{
+	There should really be a separate exception type; it is pure
+	laziness that exceptions are identified by strings, and this may
+	be fixed in the future.
+}
+The string is printed as the second part of the message for unhandled
+exceptions.
+Their names and values are:
+\begin{code}\begin{verbatim}
+EOFError              'end-of-file read'
+KeyboardInterrupt     'keyboard interrupt'
+MemoryError           'out of memory'           *
+NameError             'undefined name'          *
+RuntimeError          'run-time error'          *
+SystemError           'system error'            *
+TypeError             'type error'              *
+\end{verbatim}\end{code}
+The meanings should be clear enough.
+Those exceptions with a {\tt *} in the third column have an argument.
+
+Exception handlers don't just handle exceptions if they occur
+immediately in the try clause, but also if they occur inside functions
+that are called (even indirectly) in the try clause.
+For example:
+\begin{code}\begin{verbatim}
+>>> def this_fails():
+...     x = 1/0
+... 
+>>> try:
+...     this_fails()
+... except RuntimeError, detail:
+...     print 'Handling run-time error:', detail
+... 
+Handling run-time error: domain error or zero division
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{Raising Exceptions}
+
+The {\tt raise} statement allows the programmer to force a specified
+exception to occur.
+For example:
+\begin{code}\begin{verbatim}
+>>> raise KeyboardInterrupt
+Unhandled exception: keyboard interrupt
+>>> raise NameError, 'Hi There!'
+Unhandled exception: undefined name: Hi There!
+Context: raise NameError , 'Hi There!' 
+
+>>> 
+\end{verbatim}\end{code}
+The first argument to {\tt raise} names the exception to be raised.
+The optional second argument specifies the exception's argument.
+
+\subsubsection{User-defined Exceptions}
+
+Programs may name their own exceptions by assigning a string to a
+variable.
+For example:
+\begin{code}\begin{verbatim}
+>>> my_exc = 'nobody likes me!'
+>>> try:
+...     raise my_exc, 2*2
+... except my_exc, val:
+...     print 'My exception occured, value:', val
+... 
+My exception occured, value: 4
+>>> raise my_exc, 1
+Unhandled exception: nobody likes me!: 1
+Context: raise my_exc , 1
+
+>>> 
+\end{verbatim}\end{code}
+Many standard modules use this to report errors that may occur in
+functions they define.
+
+\subsubsection{Defining Clean-up Actions}
+
+The {\tt try} statement has another optional clause which is intended to
+define clean-up actions that must be executed under all circumstances.
+For example:
+\begin{code}\begin{verbatim}
+>>> try:
+...     raise KeyboardInterrupt
+... finally:
+...     print 'Goodbye, world!'
+... 
+Goodbye, world!
+Unhandled exception: keyboard interrupt
+>>> 
+\end{verbatim}\end{code}
+The
+{\it finally\ clause}
+must follow the except clauses(s), if any.
+It is executed whether or not an exception occurred.
+If the exception is handled, the finally clause is executed after the
+handler (and even if another exception occurred in the handler).
+It is also executed when the {\tt try} statement is left via a
+{\tt break} or {\tt return} statement.
+
+\subsection{Classes}
+
+Classes in \Python\ make it possible to play the game of encapsulation in a
+somewhat different way than it is played with modules.
+Classes are an advanced topic and are probably best skipped on the first
+encounter with \Python.
+
+\subsubsection{Prologue}
+
+\Python's class mechanism is not particularly elegant, but quite powerful.
+It is a mixture of the class mechanisms found in C++ and Modula-3.
+As is true for modules, classes in \Python\ do not put an absolute barrier
+between definition and user, but rather rely on the politeness of the
+user not to ``break into the definition.''
+The most important features of classes are retained with full power,
+however: the class inheritance mechanism allows multiple base classes,
+a derived class can override any method of its base class(es), a method
+can call the method of a base class with the same name.
+Objects can contain an arbitrary amount of private data.
+
+In C++ terminology, all class members (including data members) are
+{\it public},
+and all member functions (methods) are
+{\it virtual}.
+There are no special constructors or destructors.
+As in Modula-3, there are no shorthands for referencing the object's
+members from its methods: the method function is declared with an
+explicit first argument representing the object, which is provided
+implicitly by the call.
+As in Smalltalk, classes themselves are objects, albeit in the wider
+sense of the word: in \Python, all data types are objects.
+This provides semantics for renaming or aliasing.
+But, just like in C++ or Modula-3, the built-in types cannot be used as
+base classes for extension by the user.
+Also, like Modula-3 but unlike C++, the built-in operators with special
+syntax (arithmetic operators, subscripting etc.) cannot be redefined for
+class members.%
+\footnote{
+	They can be redefined for new object types implemented in C in
+	extensions to the interpreter, however.  It would require only a
+	naming convention and a relatively small change to the
+	interpreter to allow operator overloading for classes, so
+	perhaps someday...
+}
+
+\subsubsection{A Simple Example}
+
+Consider the following example, which defines a class {\tt Set}
+representing a (finite) mathematical set with operations to add and
+remove elements, a membership test, and a request for the size of the
+set.
+\begin{code}\begin{verbatim}
+class Set():
+    def new(self):
+        self.elements = []
+        return self
+    def add(self, e):
+        if e not in self.elements:
+            self.elements.append(e)
+    def remove(self, e):
+        if e in self.elements:
+            for i in range(len(self.elements)):
+                if self.elements[i] = e:
+                    del self.elements[i]
+                    break
+    def is_element(self, e):
+        return e in self.elements
+    def size(self):
+        return len(self.elements)
+\end{verbatim}\end{code}
+Note that the class definition looks like a big compound statement,
+with all the function definitons indented repective to the
+{\tt class}
+keyword.
+
+Let's assume that this
+{\it class\ definition}
+is the only contents of the module file
+{\tt SetClass.py}.
+We can then use it in a \Python\ program as follows:
+\begin{code}\begin{verbatim}
+>>> from SetClass import Set
+>>> a = Set().new() # create a Set object
+>>> a.add(2)
+>>> a.add(3)
+>>> a.add(1)
+>>> a.add(1)
+>>> if a.is_element(3): print '3 is in the set'
+... 
+3 is in the set
+>>> if not a.is_element(4): print '4 is not in the set'
+... 
+4 is not in the set
+>>> print 'a has', a.size(), 'elements'
+a has 3 elements
+>>> a.remove(1)
+>>> print 'now a has', a.size(), 'elements'
+>>> 
+now a has 2 elements
+>>> 
+\end{verbatim}\end{code}
+From the example we learn in the first place that the functions defined
+in the class (e.g.,
+{\tt add})
+can be called using the
+{\it member}
+notation for the object
+{\tt a}.
+The member function is called with one less argument than it is defined:
+the object is implicitly passed as the first argument.
+Thus, the call
+{\tt a.add(2)}
+is equivalent to
+{\tt Set.add(a, 2)}.
+
+
+\section{XXX P.M.}
+
+The {\tt del} statement.
+
+The {\tt dir()} function.
+
+Tuples.
+
+Dictionaries.
+
+Objects and types in general.
+
+Backquotes.
+
+And/Or/Not.
+
+\end{document}
diff --git a/Doc/tut/tut.tex b/Doc/tut/tut.tex
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+% Format this file with latex.
+
+\documentstyle{article}
+
+% Page lay-out parameters
+\textwidth =      150mm
+\textheight =     240mm
+\topmargin =      -11mm
+\oddsidemargin =    5mm
+\evensidemargin =   5mm
+
+% Macros for e.g. and E.g. if you want them italicized:
+% \newcommand{\eg}{{\it e.g.}}
+% \newcommand{\Eg}{{\it E.g.}}
+% If you don't want them italicized:
+\newcommand{\eg}{e.g.}
+\newcommand{\Eg}{E.g.}
+
+% Frequently used system names
+\newcommand{\Python}{{\em Python}}
+\newcommand{\UNIX}{U{\sc nix}}
+
+% Code environment
+\newenvironment{code}{\begin{itemize}\samepage}{\end{itemize}}
+
+\title{\bf
+	Python Tutorial \\
+	(DRAFT)
+}
+
+\author{
+	Guido van Rossum \\
+	Dept. CST, CWI, Kruislaan 413 \\
+	1098 SJ Amsterdam, The Netherlands \\
+	E-mail: {\tt guido@cwi.nl}
+}
+
+\begin{document}
+
+\pagenumbering{roman}
+
+\maketitle
+
+\begin{abstract}
+
+\noindent
+\Python\ is a simple, yet powerful programming language that bridges the
+gap between C and shell programming, and is thus ideally suited for rapid
+prototyping.
+It is put together from constructs borrowed from a variety of other
+languages; most prominent are influences from ABC, C, Modula-3 and Icon.
+
+The \Python\ interpreter is easily extended with new functions and data
+types implemented in C.
+\Python\ is also suitable as an extension language for highly
+customizable C applications such as editors or window managers.
+
+\Python\ is available for various operating systems, amongst which
+several flavors of \UNIX, Amoeba, and the Apple Macintosh O.S.
+
+This tutorial introduces the reader informally to the basic concepts and
+features of the \Python\ language and system.
+It helps to have a \Python\ interpreter handy for hands-on experience,
+but as the examples are self-contained, the tutorial can be read
+off-line as well.
+For a description of standard objects and modules, see the Library and
+Module Reference document.
+The Language Reference document gives a more formal reference to the
+language.
+
+\end{abstract}
+
+\pagebreak
+
+\tableofcontents
+
+\pagebreak
+
+\pagenumbering{arabic}
+
+\section{Whetting Your Appetite}
+
+If you ever wrote a large shell script, you probably know this feeling:
+you'd love to add yet another feature, but it's already so slow, and so
+big, and so complicated; or the feature involves a system call or other
+funcion that is only accessable from C...
+Usually the problem at hand isn't serious enough to warrant rewriting
+the script in C; perhaps because the problem requires variable-length
+strings or other data types (like sorted lists of file names) that
+are easy in the shell but lots of work to implement in C; or perhaps
+just because you're not sufficiently familiar with C.
+
+In all such cases, \Python\ is just the language for you.
+\Python\ is simple to use, but it is a real programming language, offering
+much more structure and support for large programs than the shell has.
+On the other hand, it also offers much more error checking than C, and,
+being a
+{\it very-high-level language},
+it has high-level data types built in, such as flexible arrays and
+dictionaries that would cost you days to implement efficiently in C.
+Because of its more general data types \Python\ is applicable to a
+much larger problem domain than
+{\it Awk}
+or even
+{\it Perl},
+yet most simple things are at least as easy in \Python\ as in those
+languages.
+
+\Python\ allows you to split up your program in modules that can be reused
+in other \Python\ programs.
+It comes with a large collection of standard modules that you can use as
+the basis for your programs --- or as examples to start learning to
+program in \Python.
+There are also built-in modules that provide things like file I/O,
+system calls, and even a generic interface to window systems (STDWIN).
+
+\Python\ is an interpreted language, which saves you considerable time
+during program development because no compilation and linking is
+necessary.
+The interpreter can be used interactively, which makes it easy to
+experiment with features of the language, to write throw-away programs,
+or to test functions during bottom-up program development.
+It is also a handy desk calculator.
+
+\Python\ allows writing very compact and readable programs.
+Programs written in \Python\ are typically much shorter than equivalent C
+programs:
+No declarations are necessary (all type checking is
+dynamic); statement grouping is done by indentation instead of begin/end
+brackets; and the high-level data types allow you to express complex
+operations in a single statement.
+
+\Python\ is
+{\it extensible}:
+if you know how to program in C it is easy to add a new built-in module
+to the interpreter, either to perform critical operations at maximum
+speed, or to link \Python\ programs to libraries that may be only available
+in binary form (such as a vendor-specific graphics library).
+Once you are really hooked, you can link the \Python\ interpreter into an
+application written in C and use it as an extension or command language.
+
+\subsection{Where From Here}
+
+Now that you are all excited about \Python, you'll want to examine it in
+some more detail.
+Since the best introduction to a language is using it, you are invited
+here to do so.
+
+In the next section, the mechanics of using the interpreter are
+explained.
+This is rather mundane information, but essential for trying out the
+examples shown later.
+The rest of the tutorial introduces various features of the \Python\
+language and system though examples, beginning with simple expressions,
+statements and data types, through functions and modules, and finally
+touching upon advanced concepts like exceptions and classes.
+
+\section{Using the Python Interpreter}
+
+The \Python\ interpreter is usually installed as
+{\tt /usr/local/python}
+on those machines where it is available; putting
+{\tt /usr/local}
+in your \UNIX\ shell's search path makes it possible to start it by
+typing the command
+\begin{code}\begin{verbatim}
+python
+\end{verbatim}\end{code}
+to the shell.
+Since the choice of the directory where the interpreter lives is an
+installation option, other places instead of
+{\tt /usr/local}
+are possible; check with your local \Python\ guru or system
+administrator.%
+\footnote{
+	At CWI, at the time of writing, the interpreter can be found in
+	the following places:
+	On the Amoeba Ultrix machines, use the standard path,
+	{\tt /usr/local/python}.
+	On the Sun file servers, use
+	{\tt /ufs/guido/bin/}{\it arch}{\tt /python},
+	where {\it arch} can be {\tt sgi} or {\tt sun4}.
+	On piring, use {\tt /userfs3/amoeba/bin/python}.
+	(If you can't find a binary advertised here, get in touch with me.)
+}
+
+The interpreter operates somewhat like the \UNIX\ shell: when called with
+standard input connected to a tty device, it reads and executes commands
+interactively; when called with a file name argument or with a file as
+standard input, it reads and executes a
+{\it script}
+from that file.%
+\footnote{
+	There is a difference between ``{\tt python file}'' and
+	``{\tt python $<$file}''.  In the latter case {\tt input()} and
+	{\tt raw\_input()} are satisfied from {\it file}, which has
+	already been read until the end by the parser, so they will read
+	EOF immediately.  In the former case (which is usually what was
+	intended) they are satisfied from whatever file or device is
+	connected to standard input of the \Python\ interpreter.
+}
+If available, the script name and additional arguments thereafter are
+passed to the script in the variable
+{\tt sys.argv},
+which is a list of strings.
+
+When standard input is a tty, the interpreter is said to be in
+{\it interactive\ mode}.
+In this mode it prompts for the next command with the
+{\it primary\ prompt},
+usually three greater-than signs ({\tt >>>}); for continuation lines
+it prompts with the
+{\it secondary\ prompt},
+by default three dots ({\tt ...}).
+Typing an EOF (\^{}D) at the primary prompt causes the interpreter to exit
+with a zero exit status.
+
+When an error occurs in interactive mode, the interpreter prints a
+message and returns to the primary prompt; with input from a file, it
+exits with a nonzero exit status.
+(Exceptions handled by an
+{\tt except}
+clause in a
+{\tt try}
+statement are not errors in this context.)
+Some errors are unconditionally fatal and cause an exit with a nonzero
+exit; this applies to internal inconsistencies and some cases of running
+out of memory.
+All error messages are written to the standard error stream; normal
+output from the executed commands is written to standard output.
+
+Typing an interrupt (normally Control-C or DEL) to the primary or
+secondary prompt cancels the input and returns to the primary prompt.
+Typing an interrupt while a command is being executed raises the
+{\tt KeyboardInterrupt}
+exception, which may be handled by a
+{\tt try}
+statement.
+
+When a module named
+{\tt foo}
+is imported, the interpreter searches for a file named
+{\tt foo.py}
+in a list of directories specified by the environment variable
+{\tt PYTHONPATH}.
+It has the same syntax as the \UNIX\ shell variable
+{\tt PATH},
+i.e., a list of colon-separated directory names.
+When
+{\tt PYTHONPATH}
+is not set, an installation-dependent default path is used, usually
+{\tt .:/usr/local/lib/python}.%
+\footnote{
+	Modules are really searched in the list of directories given by
+	the variable {\tt sys.path} which is initialized from
+	{\tt PYTHONPATH} or from the installation-dependent default.
+	See the section on Standard Modules below.
+}
+The built-in module
+{\tt stdwin},
+if supported at all, is only available if the interpreter is started
+with the
+{\bf --s}
+flag.
+If this flag is given, stdwin is initialized as soon as the interpreter
+is started, and in the case of X11 stdwin certain command line arguments
+(like
+{\bf --display} )
+are consumed by stdwin.
+
+On BSD'ish \UNIX\ systems, \Python\ scripts can be made directly executable,
+like shell scripts, by putting the line
+\begin{code}\begin{verbatim}
+#! /usr/local/python
+\end{verbatim}\end{code}
+(assuming that's the name of the interpreter) at the beginning of the
+script and giving the file an executable mode.
+(The
+{\tt \#!}
+must be the first two characters of the file.)
+For scripts that use the built-in module
+{\tt stdwin},
+use
+\begin{code}\begin{verbatim}
+#! /usr/local/python -s
+\end{verbatim}\end{code}
+
+\subsection{Interactive Input Editing and History Substitution}
+
+Some versions of the \Python\ interpreter support editing of the current
+input line and history substitution, similar to facilities found in the
+Korn shell and the GNU Bash shell.
+This is implemented using the
+{\it GNU\ Readline}
+library, which supports Emacs-style and vi-style editing.
+This library has its own documentation which I won't duplicate here;
+however, the basics are easily explained.
+
+If supported,%
+\footnote{
+	Perhaps the quickest check to see whether command line editing
+	is supported is typing Control-P to the first \Python\ prompt
+	you get.  If it beeps, you have command line editing.
+	If not, you can forget about the rest of this section.
+}
+input line editing is active whenever the interpreter prints a primary
+or secondary prompt (yes, you can turn it off by deleting
+{\tt sys.ps1},
+and no, it is not provided for
+{\tt input()}
+and
+{\tt raw\_input()}).
+The current line can be edited using the conventional Emacs control
+characters.
+The most important of these are:
+C-A (Control-A) moves the cursor to the beginning of the line, C-E to
+the end, C-B moves it one position to the left, C-F to the right.
+Backspace erases the character to the left of the cursor, C-D the
+character to its right.
+C-K kills (erases) the rest of the line to the right of the cursor, C-Y
+yanks back the last killed string.
+C-\_ undoes the last change you made; it can be repeated for cumulative
+effect.
+
+History substitution works as follows.
+All non-empty input lines issued so far are saved in a history buffer,
+and when a new prompt is given you are positioned on a new line at the
+bottom of this buffer.
+C-P moves one line up (back) in the history buffer, C-N moves one down.
+The current line in the history buffer can be edited; in this case an
+asterisk appears in front of the prompt to mark it as modified.
+Pressing the Return key passes the current line to the interpreter.
+C-R starts an incremental reverse search; C-S starts a forward search.
+
+The key bindings and some other parameters of the Readline library can
+be customized by placing commands in an initialization file called
+{\tt \$HOME/.initrc}.
+Key bindings have the form
+\begin{code}\begin{verbatim}
+key-name: function-name
+\end{verbatim}\end{code}
+and options can be set with
+\begin{code}\begin{verbatim}
+set option-name value
+\end{verbatim}\end{code}
+Example:
+\begin{code}\begin{verbatim}
+# I prefer vi-style editing:
+set editing-mode vi
+# Edit using a single line:
+set horizontal-scroll-mode On
+# Rebind some keys:
+Meta-h: backward-kill-word
+Control-u: universal-argument
+\end{verbatim}\end{code}
+Note that the default binding for TAB in \Python\ is to insert a TAB
+instead of Readline's default filename completion function.
+If you insist, you can override this by putting
+\begin{code}\begin{verbatim}
+TAB: complete
+\end{verbatim}\end{code}
+in your
+{\tt \$HOME/.inputrc}.
+Of course, this makes it hard to type indented continuation lines.
+
+This facility is an enormous step forward compared to previous versions of
+the interpreter; however, some wishes are left:
+It would be nice if the proper indentation were suggested on
+continuation lines (the parser knows if an indent token is required
+next).
+The completion mechanism might use the interpreter's symbol table.
+A function to check (or even suggest) matching parentheses, quotes
+etc. would also be useful.
+
+\section{An Informal Introduction to Python}
+
+In the following examples, input and output are distinguished by the
+presence or absence of prompts ({\tt >>>} and {\tt ...}): to repeat the
+example, you must type everything after the prompt, when the prompt
+appears; everything on lines that do not begin with a prompt is output
+from the interpreter.
+Note that a secondary prompt on a line by itself in an example means you
+must type a blank line; this is used to end a multi-line command.
+
+\subsection{Using Python as a Calculator}
+
+Let's try some simple \Python\ commands.
+Start the interpreter and wait for the primary prompt,
+{\tt >>>}.
+The interpreter acts as a simple calculator: you can type an expression
+at it and it will write the value.
+Expression syntax is straightforward: the operators
+{\tt +},
+{\tt -},
+{\tt *}
+and
+{\tt /}
+work just as in most other languages (e.g., Pascal or C); parentheses
+can be used for grouping.
+For example:
+\begin{code}\begin{verbatim}
+>>> # This is a comment
+>>> 2+2
+4
+>>> 
+>>> (50-5+5*6+25)/4
+25
+>>> # Division truncates towards zero:
+>>> 7/3
+2
+>>> 
+\end{verbatim}\end{code}
+As in C, the equal sign ({\tt =}) is used to assign a value to a variable.
+The value of an assignment is not written:
+\begin{code}\begin{verbatim}
+>>> width = 20
+>>> height = 5*9
+>>> width * height
+900
+>>> 
+\end{verbatim}\end{code}
+There is some support for floating point:
+\begin{code}\begin{verbatim}
+>>> 10.0 / 3.3
+3.0303030303
+>>> 
+\end{verbatim}\end{code}
+But you can't mix floating point and integral numbers in expression (yet).
+
+Besides numbers, \Python\ can also manipulate strings, enclosed in single
+quotes:
+\begin{code}\begin{verbatim}
+>>> 'foo bar'
+'foo bar'
+>>> 'doesn\'t'
+'doesn\'t'
+>>> 
+\end{verbatim}\end{code}
+Strings are written inside quotes and with quotes and other funny
+characters escaped by backslashes, to show the precise value.
+(There is also a way to write strings without quotes and escapes.)
+Strings can be concatenated (glued together) with the
+{\tt +}
+operator, and repeated with
+{\tt *}:
+\begin{code}\begin{verbatim}
+>>> word = 'Help' + 'A'
+>>> word
+'HelpA'
+>>> '<' + word*5 + '>'
+'<HelpAHelpAHelpAHelpAHelpA>'
+>>> 
+\end{verbatim}\end{code}
+Strings can be subscripted; as in C, the first character of a string has
+subscript 0.
+There is no separate character type; a character is simply a string of
+size one.
+As in Icon, substrings can be specified with the
+{\it slice}
+notation: two subscripts (indices) separated by a colon.
+\begin{code}\begin{verbatim}
+>>> word[4]
+'A'
+>>> word[0:2]
+'He'
+>>> word[2:4]
+'lp'
+>>> # Slice indices have useful defaults:
+>>> word[:2]    # Take first two characters
+'He'
+>>> word[2:]    # Skip first two characters
+'lpA'
+>>> # A useful invariant: s[:i] + s[i:] = s
+>>> word[:3] + word[3:]
+'HelpA'
+>>> 
+\end{verbatim}\end{code}
+Degenerate cases are handled gracefully: an index that is too large is
+replaced by the string size, an upper bound smaller than the lower bound
+returns an empty string.
+\begin{code}\begin{verbatim}
+>>> word[1:100]
+'elpA'
+>>> word[10:]
+''
+>>> word[2:1]
+''
+>>> 
+\end{verbatim}\end{code}
+Slice indices (but not simple subscripts) may be negative numbers, to
+start counting from the right.
+For example:
+\begin{code}\begin{verbatim}
+>>> word[-2:]    # Take last two characters
+'pA'
+>>> word[:-2]    # Skip last two characters
+'Hel'
+>>> # But -0 does not count from the right!
+>>> word[-0:]    # (since -0 equals 0)
+'HelpA'
+>>> 
+\end{verbatim}\end{code}
+The best way to remember how slices work is to think of the indices as
+pointing
+{\it between}
+characters, with the left edge of the first character numbered 0.
+Then the right edge of the last character of a string of
+{\tt n}
+characters has index
+{\tt n},
+for example:
+\begin{code}\begin{verbatim}
+ +---+---+---+---+---+ 
+ | H | e | l | p | A |
+ +---+---+---+---+---+ 
+ 0   1   2   3   4   5 
+-5  -4  -3  -2  -1
+\end{verbatim}\end{code}
+The first row of numbers gives the position of the indices 0...5 in the
+string; the second row gives the corresponding negative indices.
+For nonnegative indices, the length of a slice is the difference of the
+indices, if both are within bounds,
+{\it e.g.},
+the length of
+{\tt word[1:3]}
+is 3--1 = 2.
+
+Finally, the built-in function {\tt len()} computes the length of a
+string:
+\begin{code}\begin{verbatim}
+>>> s = 'supercalifragilisticexpialidocious'
+>>> len(s)
+34
+>>> 
+\end{verbatim}\end{code}
+
+\Python\ knows a number of
+{\it compound}
+data types, used to group together other values.
+The most versatile is the
+{\it list},
+which can be written as a list of comma-separated values between square
+brackets:
+\begin{code}\begin{verbatim}
+>>> a = ['foo', 'bar', 100, 1234]
+>>> a
+['foo', 'bar', 100, 1234]
+>>> 
+\end{verbatim}\end{code}
+As for strings, list subscripts start at 0:
+\begin{code}\begin{verbatim}
+>>> a[0]
+'foo'
+>>> a[3]
+1234
+>>> 
+\end{verbatim}\end{code}
+Lists can be sliced and concatenated like strings:
+\begin{code}\begin{verbatim}
+>>> a[1:3]
+['bar', 100]
+>>> a[:2] + ['bletch', 2*2]
+['foo', 'bar', 'bletch', 4]
+>>> 
+\end{verbatim}\end{code}
+Unlike strings, which are
+{\it immutable},
+it is possible to change individual elements of a list:
+\begin{code}\begin{verbatim}
+>>> a
+['foo', 'bar', 100, 1234]
+>>> a[2] = a[2] + 23
+>>> a
+['foo', 'bar', 123, 1234]
+>>>
+\end{verbatim}\end{code}
+Assignment to slices is also possible, and this may even change the size
+of the list:
+\begin{code}\begin{verbatim}
+>>> # Replace some items:
+>>> a[0:2] = [1, 12]
+>>> a
+[1, 12, 123, 1234]
+>>> # Remove some:
+>>> a[0:2] = []
+>>> a
+[123, 1234]
+>>> # Insert some:
+>>> a[1:1] = ['bletch', 'xyzzy']
+>>> a
+[123, 'bletch', 'xyzzy', 1234]
+>>> 
+\end{verbatim}\end{code}
+The built-in function {\tt len()} also applies to lists:
+\begin{code}\begin{verbatim}
+>>> len(a)
+4
+>>> 
+\end{verbatim}\end{code}
+
+\subsection{Simple and Compound Statements}
+
+Of course, we can use \Python\ for more complicated tasks than adding two
+and two together.
+For instance, we can write an initial subsequence of the
+{\it Fibonacci}
+series as follows:
+\begin{code}\begin{verbatim}
+>>> # Fibonacci series:
+>>> # the sum of two elements defines the next
+>>> a, b = 0, 1
+>>> while b < 100:
+...       print b
+...       a, b = b, a+b
+... 
+1
+1
+2
+3
+5
+8
+13
+21
+34
+55
+89
+>>> 
+\end{verbatim}\end{code}
+This example introduces several new features.
+\begin{itemize}
+\item
+The first line contains a
+{\it multiple\ assignment}:
+the variables
+{\tt a}
+and
+{\tt b}
+simultaneously get the new values 0 and 1.
+On the last line this is used again, demonstrating that the expressions
+on the right-hand side are all evaluated first before any of the
+assignments take place.
+\item
+The
+{\tt while}
+loop executes as long as the condition remains true.
+In \Python, as in C, any non-zero integer value is true; zero is false.
+The condition may also be a string or list value, in fact any sequence;
+anything with a non-zero length is true, empty sequences are false.
+The test used in the example is a simple comparison.
+The standard comparison operators are written as
+{\tt <},
+{\tt >},
+{\tt =},
+{\tt <=},
+{\tt >=}
+and
+{\tt <>}.%
+\footnote{
+	The ambiguity of using {\tt =}
+	for both assignment and equality is resolved by disallowing
+	unparenthesized conditions at the right hand side of assignments.
+}
+\item
+The
+{\it body}
+of the loop is
+{\it indented}
+by one tab stop: indentation is \Python's way of grouping statements.
+\Python\ does not (yet!) provide an intelligent input line editing
+facility, so you have to type a tab for each indented line.
+In practice you will prepare more complicated input for \Python\ with a
+text editor; most text editors have an auto-indent facility.
+When a compound statement is entered interactively, it must be
+followed by a blank line to indicate completion (otherwise the parser
+doesn't know that you have typed the last line).
+\item
+The
+{\tt print}
+statement writes the value of the expression(s) it is passed.
+It differs from just writing the expression you want to write (as we did
+earlier in the calculator examples) in the way it handles multiple
+expressions and strings.
+Strings are written without quotes and a space is inserted between
+items, so you can do things like this:
+\begin{code}\begin{verbatim}
+>>> i = 256*256
+>>> print 'The value of i is', i
+The value of i is 65536
+>>> 
+\end{verbatim}\end{code}
+A trailing comma avoids the newline after the output:
+\begin{code}\begin{verbatim}
+>>> a, b = 0, 1
+>>> while b < 1000:
+...     print b,
+...     a, b = b, a+b
+... 
+1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
+>>> 
+\end{verbatim}\end{code}
+Note that the interpreter inserts a newline before it prints the next
+prompt if the last line was not completed.
+\end{itemize}
+
+\subsection{Other Control Flow Statements}
+
+Besides {\tt while}, already introduced, \Python\ supports the usual
+control flow statements known from other languages, with some twists.
+
+\subsubsection{If Statements}
+
+Perhaps the most well-known statement type is the {\tt if} statement.
+For example:
+\begin{code}\begin{verbatim}
+>>> if x < 0:
+...      x = 0
+...      print 'Negative changed to zero'
+... elif x = 0:
+...      print 'Zero'
+... elif x = 1:
+...      print 'Single'
+... else:
+...      print 'More'
+... 
+\end{verbatim}\end{code}
+There can be zero or more {\tt elif} parts, and the {\tt else} part is
+optional.
+
+\subsubsection{For Statements}
+
+The {\tt for} statement in \Python\ differs a bit from what you may be
+used to in C or Pascal.
+Rather than always iterating over an arithmetic progression of numbers,
+as in Pascal, or leaving the user completely free in the iteration test
+and step, as in C, \Python's {\tt for} iterates over the items of any
+sequence (\it e.g.\rm%
+, a list or a string).
+An example {\tt for} statement:
+\begin{code}\begin{verbatim}
+>>> # Measure some strings:
+>>> a = ['cat', 'window', 'defenestrate']
+>>> for x in a:
+...     print x, len(x)
+... 
+cat 3
+window 6
+defenestrate 12
+>>> 
+\end{verbatim}\end{code}
+If you do need to iterate over a sequence of numbers, the built-in
+function {\tt range()} comes in handy.
+It generates lists containing arithmetic progressions,
+{\it e.g.}:
+\begin{code}\begin{verbatim}
+>>> range(10)
+[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+>>> 
+\end{verbatim}\end{code}
+The end point is never part of the generated list; {\tt range(10)}
+generates exactly the legal indices for items of a list or string of
+length 10.
+It is possible to let the range start at another number, or to specify a
+different increment (even negative):
+\begin{code}\begin{verbatim}
+>>> range(5, 10)
+[5, 6, 7, 8, 9]
+>>> range(0, 10, 3)
+[0, 3, 6, 9]
+>>> range(-10, -100, -30)
+[-10, -40, -70]
+>>> 
+\end{verbatim}\end{code}
+To iterate over the indices of a list or string, combine {\tt range()}
+and {\tt len()} as follows:
+\begin{code}\begin{verbatim}
+>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
+>>> for i in range(len(a)):
+...     print i, a[i]
+... 
+0 Mary
+1 had
+2 a
+3 little
+4 lamb
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{Break Statements and Else Clauses on Loops}
+
+The {\tt break} statement breaks out of the smallest enclosing {\tt for}
+or {\tt while} loop.
+Loop statements may have an {\tt else} clause; it is executed when the
+loop terminates through exhaustion of the list (for {\tt for}) or when
+the condition becomes false (for {\tt while}) but not when the loop is
+terminated by a {\tt break} statement.
+This is exemplified by the following loop, which searches for a list
+item of value 0:
+\begin{code}\begin{verbatim}
+>>> a = [1, 10, 0, 5, 12]
+>>> for i in a:
+...     if i = 0:
+...        print '*** Found a zero'
+...        break
+... else:
+...     print '*** No zero found'
+... 
+*** Found a zero
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{Pass Statements}
+
+The {\tt pass} statement does nothing, similar to {\tt skip} in Algol-68
+or an empty statement in C.
+It can be used when a statement is required syntactically but the
+program requires no action.
+For example:
+\begin{code}\begin{verbatim}
+>>> while 1:
+...       pass # Busy-wait for keyboard interrupt
+... 
+\end{verbatim}\end{code}
+
+\subsection{Defining Functions}
+
+We can create a function that writes the Fibonacci series to an
+arbitrary boundary:
+\begin{code}\begin{verbatim}
+>>> def fib(n):    # write Fibonacci series up to n
+...     a, b = 0, 1
+...     while b <= n:
+...           print b,
+...           a, b = b, a+b
+... 
+>>> # Now call the function we just defined:
+>>> fib(2000)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
+>>> 
+\end{verbatim}\end{code}
+The keyword
+{\tt def}
+introduces a function
+{\it definition}.
+It must be followed by the function name and the parenthesized list of
+formal parameters.
+The statements that form the body of the function starts at the next
+line, indented by a tab stop.
+The
+{\it execution}
+of a function introduces a new symbol table used for the local variables
+of the function.
+More precisely, all variable assignments in a function store the value
+in the local symbol table; variable references first look in the local
+symbol table, then in the global symbol table, and then in the table of
+built-in names.
+Thus, the global symbol table is
+{\it read-only}
+within a function; the built-in symbol table is always read-only.
+The actual parameters (arguments) to a function call are introduced in
+the local symbol table of the called function when it is called;
+thus, arguments are passed using
+{\it call\ by\ value}.%
+\footnote{
+	 Actually, {\it call  by  object reference} would be a better
+	 name, since if a mutable object is passed, the caller will see
+	 any changes the callee makes to it.
+}
+When a function calls another function, a new local symbol table is
+created for that call.
+
+A function definition introduces the function name in the global symbol
+table.
+The value has a type that is recognized by the interpreter as a
+user-defined function.
+This value can be assigned to another name which can then also be used
+as a function.
+This serves as a general renaming mechanism:
+\begin{code}\begin{verbatim}
+>>> fib
+<user function 'fib'>
+>>> f = fib
+>>> f(100)
+1 1 2 3 5 8 13 21 34 55 89
+>>> 
+\end{verbatim}\end{code}
+You might object that
+{\tt fib}
+is not a function but a procedure.
+In \Python, as in C, procedures are just functions that don't return a
+value.
+In fact, technically speaking, procedures do return a value, albeit a
+rather boring one.
+This value is called {\tt None} (it's a built-in name).
+Writing the value {\tt None} is normally suppressed by the interpreter
+if it would be the only value written.
+You can see it if you really want to:
+\begin{code}\begin{verbatim}
+>>> print fib(0)
+None
+>>> 
+\end{verbatim}\end{code}
+It is simple to write a function that returns a list of the numbers of
+the Fibonacci series, instead of printing it:
+\begin{code}\begin{verbatim}
+>>> def fib2(n): # return Fibonacci series up to n
+...     ret = []
+...     a, b = 0, 1
+...     while b <= n:
+...           ret.append(b)    # see below
+...           a, b = b, a+b
+...     return ret
+... 
+>>> f100 = fib2(100)    # call it
+>>> f100                # write the result
+[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
+>>> 
+\end{verbatim}\end{code}
+This example, as usual, demonstrates some new \Python\ features:
+\begin{itemize}
+\item
+The
+{\tt return}
+statement returns with a value from a function.
+{\tt return}
+without an expression argument is used to return from the middle of a
+procedure (falling off the end also returns from a proceduce).
+\item
+The statement
+{\tt ret.append(b)}
+calls a
+{\it method}
+of the list object
+{\tt ret}.
+A method is a function that `belongs' to an object and is named
+{\tt obj.methodname},
+where
+{\tt obj}
+is some object (this may be an expression), and
+{\tt methodname}
+is the name of a method that is defined by the object's type.
+Different types define different methods.
+Methods of different types may have the same name without causing
+ambiguity.
+See the section on classes, later, to find out how you can define your
+own object types and methods.
+The method
+{\tt append}
+shown in the example, is defined for list objects; it adds a new element
+at the end of the list.
+In this case it is equivalent to
+{\tt ret = ret + [b]},
+but more efficient.%
+\footnote{
+	There is a subtle semantic difference if the object
+	is referenced from more than one place.
+}
+\end{itemize}
+The list object type has two more methods:
+\begin{list}{}{\labelwidth=4cm}
+\item[{\tt insert(i, x)}]
+Inserts an item at a given position.
+The first argument is the index of the element before which to insert,
+so {\tt a.insert(0, x)} inserts at the front of the list, and
+{\tt a.insert(len(a), x)} is equivalent to {\tt a.append(x)}.
+\item[{\tt sort()}]
+Sorts the elements of the list.
+\end{list}
+For example:
+\begin{code}\begin{verbatim}
+>>> a = [10, 100, 1, 1000]
+>>> a.insert(2, -1)
+>>> a
+[10, 100, -1, 1, 1000]
+>>> a.sort()
+>>> a
+[-1, 1, 10, 100, 1000]
+>>> # Strings are sorted according to ASCII:
+>>> b = ['Mary', 'had', 'a', 'little', 'lamb']
+>>> b.sort()
+>>> b
+['Mary', 'a', 'had', 'lamb', 'little']
+>>> 
+\end{verbatim}\end{code}
+
+\subsection{Modules}
+
+If you quit from the \Python\ interpreter and enter it again, the
+definitions you have made (functions and variables) are lost.
+Therefore, if you want to write a somewhat longer program, you are
+better off using a text editor to prepare the input for the interpreter
+and run it with that file as input instead.
+This is known as creating a
+{\it script}.
+As your program gets longer, you may want to split it into several files
+for easier maintenance.
+You may also want to use a handy function that you've written in several
+programs without copying its definition into each program.
+To support this, \Python\ has a way to put definitions in a file and use
+them in a script or in an interactive instance of the interpreter.
+Such a file is called a
+{\it module};
+definitions from a module can be
+{\it imported}
+into other modules or into the
+{\it main}
+module (the collection of variables that you have access to in
+a script and in calculator mode).
+
+A module is a file containing \Python\ definitions and statements.
+The file name is the module name with the suffix
+{\tt .py}
+appended.
+For instance, use your favorite text editor to create a file called
+{\tt fibo.py}
+in the current directory with the following contents:
+\begin{code}\begin{verbatim}
+# Fibonacci numbers module
+
+def fib(n):    # write Fibonacci series up to n
+    a, b = 0, 1
+    while b <= n:
+          print b,
+          a, b = b, a+b
+
+def fib2(n): # return Fibonacci series up to n
+    ret = []
+    a, b = 0, 1
+    while b <= n:
+          ret.append(b)
+          a, b = b, a+b
+    return ret
+\end{verbatim}\end{code}
+Now enter the \Python\ interpreter and import this module with the
+following command:
+\begin{code}\begin{verbatim}
+>>> import fibo
+>>> 
+\end{verbatim}\end{code}
+This does not enter the names of the functions defined in
+{\tt fibo}
+directly in the symbol table; it only enters the module name
+{\tt fibo}
+there.
+Using the module name you can access the functions:
+\begin{code}\begin{verbatim}
+>>> fibo.fib(1000)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
+>>> fibo.fib2(100)
+[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
+>>> 
+\end{verbatim}\end{code}
+If you intend to use a function often you can assign it to a local name:
+\begin{code}\begin{verbatim}
+>>> fib = fibo.fib
+>>> fib(500)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{More About Modules}
+
+A module can contain executable statements as well as function
+definitions.
+These statements are intended to initialize the module.
+They are executed only the
+{\it first}
+time the module is imported somewhere.%
+\footnote{
+	In fact function definitions are also `statements' that are
+	`executed'; the execution enters the function name in the
+	module's global symbol table.
+}
+
+Each module has its own private symbol table, which is used as the
+global symbol table by all functions defined in the module.
+Thus, the author of a module can use global variables in the module
+without worrying about accidental clashes with a user's global
+variables.
+On the other hand, if you know what you are doing you can touch a
+module's global variables with the same notation used to refer to its
+functions,
+{\tt modname.itemname}.
+
+Modules can import other modules.
+It is customary but not required to place all
+{\tt import}
+statements at the beginning of a module (or script, for that matter).
+The imported module names are placed in the importing module's global
+symbol table.
+
+There is a variant of the
+{\tt import}
+statement that imports names from a module directly into the importing
+module's symbol table.
+For example:
+\begin{code}\begin{verbatim}
+>>> from fibo import fib, fib2
+>>> fib(500)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377
+>>> 
+\end{verbatim}\end{code}
+This does not introduce the module name from which the imports are taken
+in the local symbol table (so in the example, {\tt fibo} is not
+defined).
+
+There is even a variant to import all names that a module defines:
+\begin{code}\begin{verbatim}
+>>> from fibo import *
+>>> fib(500)
+1 1 2 3 5 8 13 21 34 55 89 144 233 377
+>>> 
+\end{verbatim}\end{code}
+This imports all names except those beginning with an underscore
+({\tt \_}).
+
+\subsubsection{Standard Modules}
+
+\Python\ comes with a library of standard modules, described in a separate
+document (Python Library and Module Reference).
+Some modules are built into the interpreter; these provide access to
+operations that are not part of the core of the language but are
+nevertheless built in, either for efficiency or to provide access to
+operating system primitives such as system calls.
+The set of such modules is a configuration option; e.g., the
+{\tt amoeba}
+module is only provided on systems that somehow support Amoeba
+primitives.
+One particular module deserves some attention:
+{\tt sys},
+which is built into every \Python\ interpreter.
+The variables
+{\tt sys.ps1}
+and
+{\tt sys.ps2}
+define the strings used as primary and secondary prompts:
+\begin{code}\begin{verbatim}
+>>> import sys
+>>> sys.ps1
+'>>> '
+>>> sys.ps2
+'... '
+>>> sys.ps1 = 'C> '
+C> print 'Yuck!'
+Yuck!
+C> 
+\end{verbatim}\end{code}
+These two variables are only defined if the interpreter is in
+interactive mode.
+
+The variable
+{\tt sys.path}
+is a list of strings that determine the interpreter's search path for
+modules.
+It is initialized to a default path taken from the environment variable
+{\tt PYTHONPATH},
+or from a built-in default if
+{\tt PYTHONPATH}
+is not set.
+You can modify it using standard list operations, e.g.:
+\begin{code}\begin{verbatim}
+>>> import sys
+>>> sys.path.append('/ufs/guido/lib/python')
+>>> 
+\end{verbatim}\end{code}
+
+\subsection{Errors and Exceptions}
+
+Until now error messages haven't yet been mentioned, but if you have
+tried out the examples you have probably seen some.
+There are (at least) two distinguishable kinds of errors:
+{\it syntax\ errors}
+and
+{\it exceptions}.
+
+\subsubsection{Syntax Errors}
+
+Syntax errors, also known as parsing errors, are perhaps the most common
+kind of complaint you get while you are still learning \Python:
+\begin{code}\begin{verbatim}
+>>> while 1 print 'Hello world'
+Parsing error at line 1:
+while 1 print 'Hello world'
+             \^
+>>> 
+\end{verbatim}\end{code}
+The parser repeats the offending line and displays a little `arrow'
+pointing at the earliest point in the line where the error was detected.
+The error is caused by (or at least detected at) the token
+{\it preceding}
+the arrow: in the example, the error is detected at the keyword
+{\tt print}, since a colon ({\tt :}) is missing before it.
+The line number is printed so you know where to look in case the input
+came from a script.
+
+\subsubsection{Exceptions}
+
+Even if a statement or expression is syntactically correct, it may cause
+an error when an attempt is made to execute it:
+\begin{code}\begin{verbatim}
+>>> 10 * (1/0)
+Unhandled exception: run-time error: domain error or
+zero division
+Context: 1 / 0 
+>>> 4 + foo*3
+Unhandled exception: undefined name: foo
+Context: 4 + foo * 3 
+>>> '2' + 2
+Unhandled exception: type error: invalid argument type
+Context: '2' + 2 
+>>> 
+\end{verbatim}\end{code}
+Errors detected during execution are called
+{\it exceptions}
+and are not unconditionally fatal: you will soon learn how to handle
+them in \Python\ programs.
+Most exceptions are not handled by programs, however, and result
+in error messages as shown here.
+
+The first line of the error message indicates what happened.
+Exceptions come in different types, and the type is printed as part of
+the message: the types in the example are
+{\tt run-time error},
+{\tt undefined name}
+and
+{\tt type error}.
+The rest of the line is a detail whose interpretation depends on the
+exception type.
+
+The second line of the error message shows the context where the
+exception happened.
+As you can see, this is usually a sub-expression enclosing the actual
+failing operation.%
+\footnote{
+	The context is reconstructed from the parse tree, so it may look
+	a little odd.  A stack trace should really be printed at this
+	point; this will be implemented in a future version of the
+	interpreter.  The context is suppressed for keyboard interrupts.
+}
+
+Here is a summary of the most common exceptions:
+\begin{itemize}
+\item
+{\it Run-time\ errors}
+are generally caused by wrong data used by the program; this can be the
+programmer's fault or caused by bad input.
+The detail states the cause of the error in more detail.
+\item
+{\it Undefined\ name}
+errors are more serious: these are usually caused by misspelled
+identifiers.%
+\footnote{
+	The parser does not check whether names used in a program are at
+	all defined elsewhere in the program, so such checks are
+	postponed until run-time.  The same holds for type checking.
+}
+The detail is the offending identifier.
+\item
+{\it Type\ errors}
+are also pretty serious: this is another case of using wrong data (or
+better, using data the wrong way), but here the error can be glanced
+from the object type(s) alone.
+The detail shows in what context the error was detected.
+\end{itemize}
+
+\subsubsection{Handling Exceptions}
+
+It is possible to write programs that handle selected exceptions.
+Look at the following example, which prints a table of inverses of
+some floating point numbers:
+\begin{code}\begin{verbatim}
+>>> numbers = [0.3333, 2.5, 0.0, 10.0]
+>>> for x in numbers:
+...     print x,
+...     try:
+...         print 1.0 / x
+...     except RuntimeError:
+...         print '*** has no inverse ***'
+... 
+0.3333 3.00030003
+2.5 0.4
+0 *** has no inverse ***
+10 0.1
+>>> 
+\end{verbatim}\end{code}
+The {\tt try} statement works as follows.
+\begin{itemize}
+\item
+First, the
+{\it try\ clause}
+(the statement(s) between the {\tt try} and {\tt except} keywords) is
+executed.
+\item
+If no exception occurs, the
+{\it except\ clause}
+is skipped and execution of the {\tt try} statement is finished.
+\item
+If an exception occurs during execution of the try clause, and its
+type matches the exception named after the {\tt except} keyword, the
+rest of the try clause is skipped, the except clause is executed, and
+then execution continues after the {\tt try} statement.
+\item
+If an exception occurs which does not match the exception named in the
+except clause, it is passed on to outer try statements; if no handler is
+found, it is an
+{\it unhandled\ exception}
+and execution stops with a message as shown above.
+\end{itemize}
+A {\tt try} statement may have more than one except clause, to specify
+handlers for different exceptions.
+At most one handler will be executed.
+Handlers only handle exceptions that occur in the corresponding try
+clause, not in other handlers of the same {\tt try} statement.
+An except clause may name multiple exceptions as a parenthesized list,
+{\it e.g.}:
+\begin{code}\begin{verbatim}
+... except (RuntimeError, TypeError, NameError):
+...     pass
+\end{verbatim}\end{code}
+The last except clause may omit the exception name(s), to serve as a
+wildcard.
+Use this with extreme caution!
+
+When an exception occurs, it may have an associated value, also known as
+the exceptions's
+{\it argument}.
+The presence and type of the argument depend on the exception type.
+For exception types which have an argument, the except clause may
+specify a variable after the exception name (or list) to receive the
+argument's value, as follows:
+\begin{code}\begin{verbatim}
+>>> try:
+...     foo()
+... except NameError, x:
+...     print x, 'undefined'
+... 
+foo undefined
+>>> 
+\end{verbatim}\end{code}
+If an exception has an argument, it is printed as the third part
+(`detail') of the message for unhandled exceptions.
+
+Standard exception names are built-in identifiers (not reserved
+keywords).
+These are in fact string objects whose
+{\it object\ identity}
+(not their value!) identifies the exceptions.%
+\footnote{
+	There should really be a separate exception type; it is pure
+	laziness that exceptions are identified by strings, and this may
+	be fixed in the future.
+}
+The string is printed as the second part of the message for unhandled
+exceptions.
+Their names and values are:
+\begin{code}\begin{verbatim}
+EOFError              'end-of-file read'
+KeyboardInterrupt     'keyboard interrupt'
+MemoryError           'out of memory'           *
+NameError             'undefined name'          *
+RuntimeError          'run-time error'          *
+SystemError           'system error'            *
+TypeError             'type error'              *
+\end{verbatim}\end{code}
+The meanings should be clear enough.
+Those exceptions with a {\tt *} in the third column have an argument.
+
+Exception handlers don't just handle exceptions if they occur
+immediately in the try clause, but also if they occur inside functions
+that are called (even indirectly) in the try clause.
+For example:
+\begin{code}\begin{verbatim}
+>>> def this_fails():
+...     x = 1/0
+... 
+>>> try:
+...     this_fails()
+... except RuntimeError, detail:
+...     print 'Handling run-time error:', detail
+... 
+Handling run-time error: domain error or zero division
+>>> 
+\end{verbatim}\end{code}
+
+\subsubsection{Raising Exceptions}
+
+The {\tt raise} statement allows the programmer to force a specified
+exception to occur.
+For example:
+\begin{code}\begin{verbatim}
+>>> raise KeyboardInterrupt
+Unhandled exception: keyboard interrupt
+>>> raise NameError, 'Hi There!'
+Unhandled exception: undefined name: Hi There!
+Context: raise NameError , 'Hi There!' 
+
+>>> 
+\end{verbatim}\end{code}
+The first argument to {\tt raise} names the exception to be raised.
+The optional second argument specifies the exception's argument.
+
+\subsubsection{User-defined Exceptions}
+
+Programs may name their own exceptions by assigning a string to a
+variable.
+For example:
+\begin{code}\begin{verbatim}
+>>> my_exc = 'nobody likes me!'
+>>> try:
+...     raise my_exc, 2*2
+... except my_exc, val:
+...     print 'My exception occured, value:', val
+... 
+My exception occured, value: 4
+>>> raise my_exc, 1
+Unhandled exception: nobody likes me!: 1
+Context: raise my_exc , 1
+
+>>> 
+\end{verbatim}\end{code}
+Many standard modules use this to report errors that may occur in
+functions they define.
+
+\subsubsection{Defining Clean-up Actions}
+
+The {\tt try} statement has another optional clause which is intended to
+define clean-up actions that must be executed under all circumstances.
+For example:
+\begin{code}\begin{verbatim}
+>>> try:
+...     raise KeyboardInterrupt
+... finally:
+...     print 'Goodbye, world!'
+... 
+Goodbye, world!
+Unhandled exception: keyboard interrupt
+>>> 
+\end{verbatim}\end{code}
+The
+{\it finally\ clause}
+must follow the except clauses(s), if any.
+It is executed whether or not an exception occurred.
+If the exception is handled, the finally clause is executed after the
+handler (and even if another exception occurred in the handler).
+It is also executed when the {\tt try} statement is left via a
+{\tt break} or {\tt return} statement.
+
+\subsection{Classes}
+
+Classes in \Python\ make it possible to play the game of encapsulation in a
+somewhat different way than it is played with modules.
+Classes are an advanced topic and are probably best skipped on the first
+encounter with \Python.
+
+\subsubsection{Prologue}
+
+\Python's class mechanism is not particularly elegant, but quite powerful.
+It is a mixture of the class mechanisms found in C++ and Modula-3.
+As is true for modules, classes in \Python\ do not put an absolute barrier
+between definition and user, but rather rely on the politeness of the
+user not to ``break into the definition.''
+The most important features of classes are retained with full power,
+however: the class inheritance mechanism allows multiple base classes,
+a derived class can override any method of its base class(es), a method
+can call the method of a base class with the same name.
+Objects can contain an arbitrary amount of private data.
+
+In C++ terminology, all class members (including data members) are
+{\it public},
+and all member functions (methods) are
+{\it virtual}.
+There are no special constructors or destructors.
+As in Modula-3, there are no shorthands for referencing the object's
+members from its methods: the method function is declared with an
+explicit first argument representing the object, which is provided
+implicitly by the call.
+As in Smalltalk, classes themselves are objects, albeit in the wider
+sense of the word: in \Python, all data types are objects.
+This provides semantics for renaming or aliasing.
+But, just like in C++ or Modula-3, the built-in types cannot be used as
+base classes for extension by the user.
+Also, like Modula-3 but unlike C++, the built-in operators with special
+syntax (arithmetic operators, subscripting etc.) cannot be redefined for
+class members.%
+\footnote{
+	They can be redefined for new object types implemented in C in
+	extensions to the interpreter, however.  It would require only a
+	naming convention and a relatively small change to the
+	interpreter to allow operator overloading for classes, so
+	perhaps someday...
+}
+
+\subsubsection{A Simple Example}
+
+Consider the following example, which defines a class {\tt Set}
+representing a (finite) mathematical set with operations to add and
+remove elements, a membership test, and a request for the size of the
+set.
+\begin{code}\begin{verbatim}
+class Set():
+    def new(self):
+        self.elements = []
+        return self
+    def add(self, e):
+        if e not in self.elements:
+            self.elements.append(e)
+    def remove(self, e):
+        if e in self.elements:
+            for i in range(len(self.elements)):
+                if self.elements[i] = e:
+                    del self.elements[i]
+                    break
+    def is_element(self, e):
+        return e in self.elements
+    def size(self):
+        return len(self.elements)
+\end{verbatim}\end{code}
+Note that the class definition looks like a big compound statement,
+with all the function definitons indented repective to the
+{\tt class}
+keyword.
+
+Let's assume that this
+{\it class\ definition}
+is the only contents of the module file
+{\tt SetClass.py}.
+We can then use it in a \Python\ program as follows:
+\begin{code}\begin{verbatim}
+>>> from SetClass import Set
+>>> a = Set().new() # create a Set object
+>>> a.add(2)
+>>> a.add(3)
+>>> a.add(1)
+>>> a.add(1)
+>>> if a.is_element(3): print '3 is in the set'
+... 
+3 is in the set
+>>> if not a.is_element(4): print '4 is not in the set'
+... 
+4 is not in the set
+>>> print 'a has', a.size(), 'elements'
+a has 3 elements
+>>> a.remove(1)
+>>> print 'now a has', a.size(), 'elements'
+>>> 
+now a has 2 elements
+>>> 
+\end{verbatim}\end{code}
+From the example we learn in the first place that the functions defined
+in the class (e.g.,
+{\tt add})
+can be called using the
+{\it member}
+notation for the object
+{\tt a}.
+The member function is called with one less argument than it is defined:
+the object is implicitly passed as the first argument.
+Thus, the call
+{\tt a.add(2)}
+is equivalent to
+{\tt Set.add(a, 2)}.
+
+
+\section{XXX P.M.}
+
+The {\tt del} statement.
+
+The {\tt dir()} function.
+
+Tuples.
+
+Dictionaries.
+
+Objects and types in general.
+
+Backquotes.
+
+And/Or/Not.
+
+\end{document}