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authorGuido van Rossum <guido@python.org>1996-10-22 20:00:02 (GMT)
committerGuido van Rossum <guido@python.org>1996-10-22 20:00:02 (GMT)
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-\chapter{Simple statements}
-\indexii{simple}{statement}
-
-Simple statements are comprised within a single logical line.
-Several simple statements may occur on a single line separated
-by semicolons. The syntax for simple statements is:
-
-\begin{verbatim}
-simple_stmt: expression_stmt
- | assignment_stmt
- | pass_stmt
- | del_stmt
- | print_stmt
- | return_stmt
- | raise_stmt
- | break_stmt
- | continue_stmt
- | import_stmt
- | global_stmt
- | exec_stmt
-\end{verbatim}
-
-\section{Expression statements}
-\indexii{expression}{statement}
-
-Expression statements are used (mostly interactively) to compute and
-write a value, or (usually) to call a procedure (a function that
-returns no meaningful result; in Python, procedures return the value
-\verb@None@):
-
-\begin{verbatim}
-expression_stmt: condition_list
-\end{verbatim}
-
-An expression statement evaluates the condition list (which may be a
-single condition).
-\indexii{expression}{list}
-
-In interactive mode, if the value is not \verb@None@, it is converted
-to a string using the rules for string conversions (expressions in
-reverse quotes), and the resulting string is written to standard
-output (see section \ref{print}) on a line by itself.
-(The exception for \verb@None@ is made so that procedure calls, which
-are syntactically equivalent to expressions, do not cause any output.)
-\ttindex{None}
-\indexii{string}{conversion}
-\index{output}
-\indexii{standard}{output}
-\indexii{writing}{values}
-\indexii{procedure}{call}
-
-\section{Assignment statements}
-\indexii{assignment}{statement}
-
-Assignment statements are used to (re)bind names to values and to
-modify attributes or items of mutable objects:
-\indexii{binding}{name}
-\indexii{rebinding}{name}
-\obindex{mutable}
-\indexii{attribute}{assignment}
-
-\begin{verbatim}
-assignment_stmt: (target_list "=")+ expression_list
-target_list: target ("," target)* [","]
-target: identifier | "(" target_list ")" | "[" target_list "]"
- | attributeref | subscription | slicing
-\end{verbatim}
-
-(See section \ref{primaries} for the syntax definitions for the last
-three symbols.)
-
-An assignment statement evaluates the expression list (remember that
-this can be a single expression or a comma-separated list, the latter
-yielding a tuple) and assigns the single resulting object to each of
-the target lists, from left to right.
-\indexii{expression}{list}
-
-Assignment is defined recursively depending on the form of the target
-(list). When a target is part of a mutable object (an attribute
-reference, subscription or slicing), the mutable object must
-ultimately perform the assignment and decide about its validity, and
-may raise an exception if the assignment is unacceptable. The rules
-observed by various types and the exceptions raised are given with the
-definition of the object types (see section \ref{types}).
-\index{target}
-\indexii{target}{list}
-
-Assignment of an object to a target list is recursively defined as
-follows.
-\indexiii{target}{list}{assignment}
-
-\begin{itemize}
-\item
-If the target list is a single target: the object is assigned to that
-target.
-
-\item
-If the target list is a comma-separated list of targets: the object
-must be a tuple with the same number of items as the list contains
-targets, and the items are assigned, from left to right, to the
-corresponding targets.
-
-\end{itemize}
-
-Assignment of an object to a single target is recursively defined as
-follows.
-
-\begin{itemize} % nested
-
-\item
-If the target is an identifier (name):
-
-\begin{itemize}
-
-\item
-If the name does not occur in a \verb@global@ statement in the current
-code block: the name is bound to the object in the current local name
-space.
-\stindex{global}
-
-\item
-Otherwise: the name is bound to the object in the current global name
-space.
-
-\end{itemize} % nested
-
-The name is rebound if it was already bound.
-
-\item
-If the target is a target list enclosed in parentheses: the object is
-assigned to that target list as described above.
-
-\item
-If the target is a target list enclosed in square brackets: the object
-must be a list with the same number of items as the target list
-contains targets, and its items are assigned, from left to right, to
-the corresponding targets.
-
-\item
-If the target is an attribute reference: The primary expression in the
-reference is evaluated. It should yield an object with assignable
-attributes; if this is not the case, \verb@TypeError@ is raised. That
-object is then asked to assign the assigned object to the given
-attribute; if it cannot perform the assignment, it raises an exception
-(usually but not necessarily \verb@AttributeError@).
-\indexii{attribute}{assignment}
-
-\item
-If the target is a subscription: The primary expression in the
-reference is evaluated. It should yield either a mutable sequence
-(list) object or a mapping (dictionary) object. Next, the subscript
-expression is evaluated.
-\indexii{subscription}{assignment}
-\obindex{mutable}
-
-If the primary is a mutable sequence object (a list), the subscript
-must yield a plain integer. If it is negative, the sequence's length
-is added to it. The resulting value must be a nonnegative integer
-less than the sequence's length, and the sequence is asked to assign
-the assigned object to its item with that index. If the index is out
-of range, \verb@IndexError@ is raised (assignment to a subscripted
-sequence cannot add new items to a list).
-\obindex{sequence}
-\obindex{list}
-
-If the primary is a mapping (dictionary) object, the subscript must
-have a type compatible with the mapping's key type, and the mapping is
-then asked to create a key/datum pair which maps the subscript to
-the assigned object. This can either replace an existing key/value
-pair with the same key value, or insert a new key/value pair (if no
-key with the same value existed).
-\obindex{mapping}
-\obindex{dictionary}
-
-\item
-If the target is a slicing: The primary expression in the reference is
-evaluated. It should yield a mutable sequence object (e.g. a list). The
-assigned object should be a sequence object of the same type. Next,
-the lower and upper bound expressions are evaluated, insofar they are
-present; defaults are zero and the sequence's length. The bounds
-should evaluate to (small) integers. If either bound is negative, the
-sequence's length is added to it. The resulting bounds are clipped to
-lie between zero and the sequence's length, inclusive. Finally, the
-sequence object is asked to replace the slice with the items of the
-assigned sequence. The length of the slice may be different from the
-length of the assigned sequence, thus changing the length of the
-target sequence, if the object allows it.
-\indexii{slicing}{assignment}
-
-\end{itemize}
-
-(In the current implementation, the syntax for targets is taken
-to be the same as for expressions, and invalid syntax is rejected
-during the code generation phase, causing less detailed error
-messages.)
-
-WARNING: Although the definition of assignment implies that overlaps
-between the left-hand side and the right-hand side are `safe' (e.g.
-\verb@a, b = b, a@ swaps two variables), overlaps within the
-collection of assigned-to variables are not safe! For instance, the
-following program prints \code@[0, 2]@:
-
-\begin{verbatim}
-x = [0, 1]
-i = 0
-i, x[i] = 1, 2
-print x
-\end{verbatim}
-
-
-\section{The {\tt pass} statement}
-\stindex{pass}
-
-\begin{verbatim}
-pass_stmt: "pass"
-\end{verbatim}
-
-\verb@pass@ is a null operation --- when it is executed, nothing
-happens. It is useful as a placeholder when a statement is
-required syntactically, but no code needs to be executed, for example:
-\indexii{null}{operation}
-
-\begin{verbatim}
-def f(arg): pass # a function that does nothing (yet)
-
-class C: pass # a class with no methods (yet)
-\end{verbatim}
-
-\section{The {\tt del} statement}
-\stindex{del}
-
-\begin{verbatim}
-del_stmt: "del" target_list
-\end{verbatim}
-
-Deletion is recursively defined very similar to the way assignment is
-defined. Rather that spelling it out in full details, here are some
-hints.
-\indexii{deletion}{target}
-\indexiii{deletion}{target}{list}
-
-Deletion of a target list recursively deletes each target, from left
-to right.
-
-Deletion of a name removes the binding of that name (which must exist)
-from the local or global name space, depending on whether the name
-occurs in a \verb@global@ statement in the same code block.
-\stindex{global}
-\indexii{unbinding}{name}
-
-Deletion of attribute references, subscriptions and slicings
-is passed to the primary object involved; deletion of a slicing
-is in general equivalent to assignment of an empty slice of the
-right type (but even this is determined by the sliced object).
-\indexii{attribute}{deletion}
-
-\section{The {\tt print} statement} \label{print}
-\stindex{print}
-
-\begin{verbatim}
-print_stmt: "print" [ condition ("," condition)* [","] ]
-\end{verbatim}
-
-\verb@print@ evaluates each condition in turn and writes the resulting
-object to standard output (see below). If an object is not a string,
-it is first converted to a string using the rules for string
-conversions. The (resulting or original) string is then written. A
-space is written before each object is (converted and) written, unless
-the output system believes it is positioned at the beginning of a
-line. This is the case: (1) when no characters have yet been written
-to standard output; or (2) when the last character written to standard
-output is \verb/\n/; or (3) when the last write operation on standard
-output was not a \verb@print@ statement. (In some cases it may be
-functional to write an empty string to standard output for this
-reason.)
-\index{output}
-\indexii{writing}{values}
-
-A \verb/"\n"/ character is written at the end, unless the \verb@print@
-statement ends with a comma. This is the only action if the statement
-contains just the keyword \verb@print@.
-\indexii{trailing}{comma}
-\indexii{newline}{suppression}
-
-Standard output is defined as the file object named \verb@stdout@
-in the built-in module \verb@sys@. If no such object exists,
-or if it is not a writable file, a \verb@RuntimeError@ exception is raised.
-(The original implementation attempts to write to the system's original
-standard output instead, but this is not safe, and should be fixed.)
-\indexii{standard}{output}
-\bimodindex{sys}
-\ttindex{stdout}
-\exindex{RuntimeError}
-
-\section{The {\tt return} statement}
-\stindex{return}
-
-\begin{verbatim}
-return_stmt: "return" [condition_list]
-\end{verbatim}
-
-\verb@return@ may only occur syntactically nested in a function
-definition, not within a nested class definition.
-\indexii{function}{definition}
-\indexii{class}{definition}
-
-If a condition list is present, it is evaluated, else \verb@None@
-is substituted.
-
-\verb@return@ leaves the current function call with the condition
-list (or \verb@None@) as return value.
-
-When \verb@return@ passes control out of a \verb@try@ statement
-with a \verb@finally@ clause, that finally clause is executed
-before really leaving the function.
-\kwindex{finally}
-
-\section{The {\tt raise} statement}
-\stindex{raise}
-
-\begin{verbatim}
-raise_stmt: "raise" condition ["," condition ["," condition]]
-\end{verbatim}
-
-\verb@raise@ evaluates its first condition, which must yield
-a string, class, or instance object. If there is a second condition,
-this is evaluated, else \verb@None@ is substituted. If the first
-condition is a class object, then the second condition must be an
-instance of that class or one of its derivatives. If the first
-condition is an instance object, the second condition must be
-\verb@None@.
-\index{exception}
-\indexii{raising}{exception}
-
-If the first object is a class or string, it then raises the exception
-identified by the first object, with the second one (or \verb@None@)
-as its parameter. If the first object is an instance, it raises the
-exception identified by the class of the object, with the instance as
-its parameter (and there should be no second object, or the second
-object should be \verb@None@).
-
-If a third object is present, and it it not \verb@None@, it should be
-a traceback object (see section \ref{traceback}), and it is
-substituted instead of the current location as the place where the
-exception occurred. This is useful to re-raise an exception
-transparently in an except clause.
-\obindex{traceback}
-
-\section{The {\tt break} statement}
-\stindex{break}
-
-\begin{verbatim}
-break_stmt: "break"
-\end{verbatim}
-
-\verb@break@ may only occur syntactically nested in a \verb@for@
-or \verb@while@ loop, but not nested in a function or class definition
-within that loop.
-\stindex{for}
-\stindex{while}
-\indexii{loop}{statement}
-
-It terminates the nearest enclosing loop, skipping the optional
-\verb@else@ clause if the loop has one.
-\kwindex{else}
-
-If a \verb@for@ loop is terminated by \verb@break@, the loop control
-target keeps its current value.
-\indexii{loop control}{target}
-
-When \verb@break@ passes control out of a \verb@try@ statement
-with a \verb@finally@ clause, that finally clause is executed
-before really leaving the loop.
-\kwindex{finally}
-
-\section{The {\tt continue} statement}
-\stindex{continue}
-
-\begin{verbatim}
-continue_stmt: "continue"
-\end{verbatim}
-
-\verb@continue@ may only occur syntactically nested in a \verb@for@ or
-\verb@while@ loop, but not nested in a function or class definition or
-\verb@try@ statement within that loop.\footnote{Except that it may
-currently occur within an {\tt except} clause.}
-\stindex{for}
-\stindex{while}
-\indexii{loop}{statement}
-\kwindex{finally}
-
-It continues with the next cycle of the nearest enclosing loop.
-
-\section{The {\tt import} statement} \label{import}
-\stindex{import}
-
-\begin{verbatim}
-import_stmt: "import" identifier ("," identifier)*
- | "from" identifier "import" identifier ("," identifier)*
- | "from" identifier "import" "*"
-\end{verbatim}
-
-Import statements are executed in two steps: (1) find a module, and
-initialize it if necessary; (2) define a name or names in the local
-name space (of the scope where the \verb@import@ statement occurs).
-The first form (without \verb@from@) repeats these steps for each
-identifier in the list, the \verb@from@ form performs them once, with
-the first identifier specifying the module name.
-\indexii{importing}{module}
-\indexii{name}{binding}
-\kwindex{from}
-
-The system maintains a table of modules that have been initialized,
-indexed by module name. (The current implementation makes this table
-accessible as \verb@sys.modules@.) When a module name is found in
-this table, step (1) is finished. If not, a search for a module
-definition is started. This first looks for a built-in module
-definition, and if no built-in module if the given name is found, it
-searches a user-specified list of directories for a file whose name is
-the module name with extension \verb@".py"@. (The current
-implementation uses the list of strings \verb@sys.path@ as the search
-path; it is initialized from the shell environment variable
-\verb@$PYTHONPATH@, with an installation-dependent default.)
-\ttindex{modules}
-\ttindex{sys.modules}
-\indexii{module}{name}
-\indexii{built-in}{module}
-\indexii{user-defined}{module}
-\bimodindex{sys}
-\ttindex{path}
-\ttindex{sys.path}
-\indexii{filename}{extension}
-
-If a built-in module is found, its built-in initialization code is
-executed and step (1) is finished. If no matching file is found,
-\verb@ImportError@ is raised. If a file is found, it is parsed,
-yielding an executable code block. If a syntax error occurs,
-\verb@SyntaxError@ is raised. Otherwise, an empty module of the given
-name is created and inserted in the module table, and then the code
-block is executed in the context of this module. Exceptions during
-this execution terminate step (1).
-\indexii{module}{initialization}
-\exindex{SyntaxError}
-\exindex{ImportError}
-\index{code block}
-
-When step (1) finishes without raising an exception, step (2) can
-begin.
-
-The first form of \verb@import@ statement binds the module name in the
-local name space to the module object, and then goes on to import the
-next identifier, if any. The \verb@from@ from does not bind the
-module name: it goes through the list of identifiers, looks each one
-of them up in the module found in step (1), and binds the name in the
-local name space to the object thus found. If a name is not found,
-\verb@ImportError@ is raised. If the list of identifiers is replaced
-by a star (\verb@*@), all names defined in the module are bound,
-except those beginning with an underscore(\verb@_@).
-\indexii{name}{binding}
-\exindex{ImportError}
-
-Names bound by import statements may not occur in \verb@global@
-statements in the same scope.
-\stindex{global}
-
-The \verb@from@ form with \verb@*@ may only occur in a module scope.
-\kwindex{from}
-\ttindex{from ... import *}
-
-(The current implementation does not enforce the latter two
-restrictions, but programs should not abuse this freedom, as future
-implementations may enforce them or silently change the meaning of the
-program.)
-
-\section{The {\tt global} statement} \label{global}
-\stindex{global}
-
-\begin{verbatim}
-global_stmt: "global" identifier ("," identifier)*
-\end{verbatim}
-
-The \verb@global@ statement is a declaration which holds for the
-entire current code block. It means that the listed identifiers are to be
-interpreted as globals. While {\em using} global names is automatic
-if they are not defined in the local scope, {\em assigning} to global
-names would be impossible without \verb@global@.
-\indexiii{global}{name}{binding}
-
-Names listed in a \verb@global@ statement must not be used in the same
-code block before that \verb@global@ statement is executed.
-
-Names listed in a \verb@global@ statement must not be defined as formal
-parameters or in a \verb@for@ loop control target, \verb@class@
-definition, function definition, or \verb@import@ statement.
-
-(The current implementation does not enforce the latter two
-restrictions, but programs should not abuse this freedom, as future
-implementations may enforce them or silently change the meaning of the
-program.)
-
-Note: the \verb@global@ is a directive to the parser. Therefore, it
-applies only to code parsed at the same time as the \verb@global@
-statement. In particular, a \verb@global@ statement contained in an
-\verb@exec@ statement does not affect the code block {\em containing}
-the \verb@exec@ statement, and code contained in an \verb@exec@
-statement is unaffected by \verb@global@ statements in the code
-containing the \verb@exec@ statement. The same applies to the
-\verb@eval()@, \verb@execfie()@ and \verb@compile()@ functions.
-\stindex{exec}
-\ttindex{eval}
-\ttindex{execfile}
-\ttindex{compile}