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

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\chapter{Compound statements}
\indexii{compound}{statement}
Compound statements contain (groups of) other statements; they affect
or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The \verb\if\, \verb\while\ and \verb\for\ statements implement
traditional control flow constructs. \verb\try\ specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
Compound statements consist of one or more `clauses'. A clause
consists of a header and a `suite'. The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon. A suite is a group of statements controlled by a
clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which \verb\if\ clause a following \verb\else\ clause would
belong:
\index{clause}
\index{suite}
\begin{verbatim}
if test1: if test2: print x
\end{verbatim}
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
\verb\print\ statements are executed:
\begin{verbatim}
if x < y < z: print x; print y; print z
\end{verbatim}
Summarizing:
\begin{verbatim}
compound_stmt: if_stmt | while_stmt | for_stmt
| try_stmt | funcdef | classdef
suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
\end{verbatim}
Note that statements always end in a \verb\NEWLINE\ possibly followed
by a \verb\DEDENT\.
\index{NEWLINE token}
\index{DEDENT token}
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling \verb\else\' problem is solved in Python by requiring
nested \verb\if\ statements to be indented).
\indexii{dangling}{else}
The formatting of the grammar rules in the following sections places
each clause on a separate line for clarity.
\section{The {\tt if} statement}
\stindex{if}
The \verb\if\ statement is used for conditional execution:
\begin{verbatim}
if_stmt: "if" condition ":" suite
("elif" condition ":" suite)*
["else" ":" suite]
\end{verbatim}
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section \ref{Booleans} for
the definition of true and false); then that suite is executed (and no
other part of the \verb\if\ statement is executed or evaluated). If
all conditions are false, the suite of the \verb\else\ clause, if
present, is executed.
\kwindex{elif}
\kwindex{else}
\section{The {\tt while} statement}
\stindex{while}
\indexii{loop}{statement}
The \verb\while\ statement is used for repeated execution as long as a
condition is true:
\begin{verbatim}
while_stmt: "while" condition ":" suite
["else" ":" suite]
\end{verbatim}
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the \verb\else\ clause, if present, is
executed and the loop terminates.
\kwindex{else}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
\stindex{break}
\stindex{continue}
\section{The {\tt for} statement}
\stindex{for}
\indexii{loop}{statement}
The \verb\for\ statement is used to iterate over the elements of a
sequence (string, tuple or list):
\obindex{sequence}
\begin{verbatim}
for_stmt: "for" target_list "in" condition_list ":" suite
["else" ":" suite]
\end{verbatim}
The condition list is evaluated once; it should yield a sequence. The
suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the \verb\else\ clause, if
present, is executed, and the loop terminates.
\kwindex{in}
\kwindex{else}
\indexii{target}{list}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb\else\
clause if there was no next item.
\stindex{break}
\stindex{continue}
The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.
The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function \verb\range()\ returns a sequence of
integers suitable to emulate the effect of Pascal's \verb\for i := a
to b do\; e.g. \verb\range(3)\ returns the list \verb\[0, 1, 2]\.
\bifuncindex{range}
\index{Pascal}
{\bf Warning:} There is a subtlety when the sequence is being modified
by the loop (this can only occur for mutable sequences, i.e. lists).
An internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has
reached the length of the sequence the loop terminates. This means that
if the suite deletes the current (or a previous) item from the
sequence, the next item will be skipped (since it gets the index of
the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the
current item will be treated again the next time through the loop.
This can lead to nasty bugs that can be avoided by making a temporary
copy using a slice of the whole sequence, e.g.
\index{loop!over mutable sequence}
\index{mutable sequence!loop over}
\begin{verbatim}
for x in a[:]:
if x < 0: a.remove(x)
\end{verbatim}
\section{The {\tt try} statement}
\stindex{try}
The \verb\try\ statement specifies exception handlers and/or cleanup
code for a group of statements:
\begin{verbatim}
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
try_fin_stmt: "try" ":" suite
"finally" ":" suite
\end{verbatim}
There are two forms of \verb\try\ statement: \verb\try...except\ and
\verb\try...finally\. These forms cannot be mixed.
The \verb\try...except\ form specifies one or more exception handlers
(the \verb\except\ clauses). When no exception occurs in the
\verb\try\ clause, no exception handler is executed. When an
exception occurs in the \verb\try\ suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
except clause with a condition, that condition is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
is either the object that identifies the exception or it is a tuple
containing an item that is compatible with the exception. Note that
the object identities must match, i.e. it must be the same object, not
just an object with the same value.
\kwindex{except}
If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire \verb\try\ statement
raised the exception).
When a matching except clause is found, the exception's parameter is
assigned to the target specified in that except clause, if present,
and the except clause's suite is executed. When the end of this suite
is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
The \verb\try...finally\ form specifies a `cleanup' handler. The
\verb\try\ clause is executed. When no exception occurs, the
\verb\finally\ clause is executed. When an exception occurs in the
\verb\try\ clause, the exception is temporarily saved, the
\verb\finally\ clause is executed, and then the saved exception is
re-raised. If the \verb\finally\ clause raises another exception or
executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
the saved exception is lost.
\kwindex{finally}
When a \verb\return\ or \verb\break\ statement is executed in the
\verb\try\ suite of a \verb\try...finally\ statement, the
\verb\finally\ clause is also executed `on the way out'. A
\verb\continue\ statement is illegal in the \verb\try\ clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
\stindex{return}
\stindex{break}
\stindex{continue}
\section{Function definitions} \label{function}
\indexii{function}{definition}
A function definition defines a user-defined function object (see
section \ref{types}):
\obindex{user-defined function}
\obindex{function}
\begin{verbatim}
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (parameter ",")* ("*" identifier | parameter [","])
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
\end{verbatim}
A function definition is an executable statement. Its execution binds
the function name in the current local name space to a function object
(a wrapper around the executable code for the function). This
function object contains a reference to the current global name space
as the global name space to be used when the function is called.
\indexii{function}{name}
\indexii{name}{binding}
The function definition does not execute the function body; this gets
executed only when the function is called.
Function call semantics are described in section \ref{calls}. When a
user-defined function is called, the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
\indexii{function}{call}
\indexiii{user-defined}{function}{call}
\index{parameter}
\index{argument}
\indexii{parameter}{formal}
\indexii{parameter}{actual}
\begin{itemize}
\item
If there are no formal parameters, there must be no arguments.
\item
If the formal parameter list does not end in a star followed by an
identifier, there must be exactly as many arguments as there are
parameters in the formal parameter list (at the top level); the
arguments are assigned to the formal parameters one by one. Note that
the presence or absence of a trailing comma at the top level in either
the formal or the actual parameter list makes no difference. The
assignment to a formal parameter is performed as if the parameter
occurs on the left hand side of an assignment statement whose right
hand side's value is that of the argument.
\item
If the formal parameter list ends in a star followed by an identifier,
preceded by zero or more comma-followed parameters, there must be at
least as many arguments as there are parameters preceding the star.
Call this number {\em N}. The first {\em N} arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is \verb\()\, if
there is just one extra argument, it is a singleton tuple.
\indexii{variable length}{parameter list}
\end{itemize}
Note that the `variable length parameter list' feature only works at
the top level of the parameter list; individual parameters use a model
corresponding more closely to that of ordinary assignment. While the
latter model is generally preferable, because of the greater type
safety it offers (wrong-sized tuples aren't silently mistreated),
variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
\section{Class definitions} \label{class}
\indexii{class}{definition}
A class definition defines a class object (see section \ref{types}):
\obindex{class}
\begin{verbatim}
classdef: "class" classname [inheritance] ":" suite
inheritance: "(" [condition_list] ")"
classname: identifier
\end{verbatim}
A class definition is an executable statement. It first evaluates the
inheritance list, if present. Each item in the inheritance list
should evaluate to a class object. The class's suite is then executed
in a new execution frame (see section \ref{execframes}), using a newly
created local name space and the original global name space.
(Usually, the suite contains only function definitions.) When the
class's suite finishes execution, its execution frame is discarded but
its local name space is saved. A class object is then created using
the inheritance list for the base classes and the saved local name
space for the attribute dictionary. The class name is bound to this
class object in the original local name space.
\index{inheritance}
\indexii{class}{name}
\indexii{name}{binding}
\indexii{execution}{frame}

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\chapter{Top-level components}
The Python interpreter can get its input from a number of sources:
from a script passed to it as standard input or as program argument,
typed in interactively, from a module source file, etc. This chapter
gives the syntax used in these cases.
\index{interpreter}
\section{Complete Python programs}
\index{program}
While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for \verb\sys\
(various system services), \verb\builtin\ (built-in functions,
exceptions and \verb\None\) and \verb\__main__\. The latter is used
to provide the local and global name space for execution of the
complete program.
\bimodindex{sys}
\bimodindex{__main__}
\bimodindex{builtin}
The syntax for a complete Python program is that for file input,
described in the next section.
The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of \verb\__main__\.
\index{interactive mode}
Under {\UNIX}, a complete program can be passed to the interpreter in
three forms: with the {\bf -c} {\it string} command line option, as a
file passed as the first command line argument, or as standard input.
If the file or standard input is a tty device, the interpreter enters
interactive mode; otherwise, it executes the file as a complete
program.
\index{UNIX}
\index{command line}
\index{standard input}
\section{File input}
All input read from non-interactive files has the same form:
\begin{verbatim}
file_input: (NEWLINE | statement)*
\end{verbatim}
This syntax is used in the following situations:
\begin{itemize}
\item when parsing a complete Python program (from a file or from a string);
\item when parsing a module;
\item when parsing a string passed to \verb\exec()\;
\bifuncindex{exec}
\item when parsing a file passed to \verb\execfile()\;
\bifuncindex{execfile}
\end{itemize}
\section{Interactive input}
Input in interactive mode is parsed using the following grammar:
\begin{verbatim}
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
\end{verbatim}
Note that a (top-level) compound statement must be followed by a blank
line in interactive mode; this is needed to help the parser detect the
end of the input.
\section{Expression input}
\index{input}
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to \verb\eval()\ must have the following form:
\bifuncindex{eval}
\begin{verbatim}
eval_input: condition_list NEWLINE*
\end{verbatim}
The input line read by \verb\input()\ must have the following form:
\bifuncindex{input}
\begin{verbatim}
input_input: condition_list NEWLINE
\end{verbatim}
Note: to read `raw' input line without interpretation, you can use the
built-in function \verb\raw_input()\ or the \verb\readline()\ method
of file objects.
\obindex{file}
\index{input!raw}
\index{raw input}
\bifuncindex{raw_index}
\ttindex{readline}

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

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\chapter{Compound statements}
\indexii{compound}{statement}
Compound statements contain (groups of) other statements; they affect
or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The \verb\if\, \verb\while\ and \verb\for\ statements implement
traditional control flow constructs. \verb\try\ specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
Compound statements consist of one or more `clauses'. A clause
consists of a header and a `suite'. The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon. A suite is a group of statements controlled by a
clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which \verb\if\ clause a following \verb\else\ clause would
belong:
\index{clause}
\index{suite}
\begin{verbatim}
if test1: if test2: print x
\end{verbatim}
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
\verb\print\ statements are executed:
\begin{verbatim}
if x < y < z: print x; print y; print z
\end{verbatim}
Summarizing:
\begin{verbatim}
compound_stmt: if_stmt | while_stmt | for_stmt
| try_stmt | funcdef | classdef
suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
\end{verbatim}
Note that statements always end in a \verb\NEWLINE\ possibly followed
by a \verb\DEDENT\.
\index{NEWLINE token}
\index{DEDENT token}
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling \verb\else\' problem is solved in Python by requiring
nested \verb\if\ statements to be indented).
\indexii{dangling}{else}
The formatting of the grammar rules in the following sections places
each clause on a separate line for clarity.
\section{The {\tt if} statement}
\stindex{if}
The \verb\if\ statement is used for conditional execution:
\begin{verbatim}
if_stmt: "if" condition ":" suite
("elif" condition ":" suite)*
["else" ":" suite]
\end{verbatim}
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section \ref{Booleans} for
the definition of true and false); then that suite is executed (and no
other part of the \verb\if\ statement is executed or evaluated). If
all conditions are false, the suite of the \verb\else\ clause, if
present, is executed.
\kwindex{elif}
\kwindex{else}
\section{The {\tt while} statement}
\stindex{while}
\indexii{loop}{statement}
The \verb\while\ statement is used for repeated execution as long as a
condition is true:
\begin{verbatim}
while_stmt: "while" condition ":" suite
["else" ":" suite]
\end{verbatim}
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the \verb\else\ clause, if present, is
executed and the loop terminates.
\kwindex{else}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
\stindex{break}
\stindex{continue}
\section{The {\tt for} statement}
\stindex{for}
\indexii{loop}{statement}
The \verb\for\ statement is used to iterate over the elements of a
sequence (string, tuple or list):
\obindex{sequence}
\begin{verbatim}
for_stmt: "for" target_list "in" condition_list ":" suite
["else" ":" suite]
\end{verbatim}
The condition list is evaluated once; it should yield a sequence. The
suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the \verb\else\ clause, if
present, is executed, and the loop terminates.
\kwindex{in}
\kwindex{else}
\indexii{target}{list}
A \verb\break\ statement executed in the first suite terminates the
loop without executing the \verb\else\ clause's suite. A
\verb\continue\ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb\else\
clause if there was no next item.
\stindex{break}
\stindex{continue}
The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.
The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function \verb\range()\ returns a sequence of
integers suitable to emulate the effect of Pascal's \verb\for i := a
to b do\; e.g. \verb\range(3)\ returns the list \verb\[0, 1, 2]\.
\bifuncindex{range}
\index{Pascal}
{\bf Warning:} There is a subtlety when the sequence is being modified
by the loop (this can only occur for mutable sequences, i.e. lists).
An internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has
reached the length of the sequence the loop terminates. This means that
if the suite deletes the current (or a previous) item from the
sequence, the next item will be skipped (since it gets the index of
the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the
current item will be treated again the next time through the loop.
This can lead to nasty bugs that can be avoided by making a temporary
copy using a slice of the whole sequence, e.g.
\index{loop!over mutable sequence}
\index{mutable sequence!loop over}
\begin{verbatim}
for x in a[:]:
if x < 0: a.remove(x)
\end{verbatim}
\section{The {\tt try} statement}
\stindex{try}
The \verb\try\ statement specifies exception handlers and/or cleanup
code for a group of statements:
\begin{verbatim}
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
try_fin_stmt: "try" ":" suite
"finally" ":" suite
\end{verbatim}
There are two forms of \verb\try\ statement: \verb\try...except\ and
\verb\try...finally\. These forms cannot be mixed.
The \verb\try...except\ form specifies one or more exception handlers
(the \verb\except\ clauses). When no exception occurs in the
\verb\try\ clause, no exception handler is executed. When an
exception occurs in the \verb\try\ suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
except clause with a condition, that condition is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
is either the object that identifies the exception or it is a tuple
containing an item that is compatible with the exception. Note that
the object identities must match, i.e. it must be the same object, not
just an object with the same value.
\kwindex{except}
If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire \verb\try\ statement
raised the exception).
When a matching except clause is found, the exception's parameter is
assigned to the target specified in that except clause, if present,
and the except clause's suite is executed. When the end of this suite
is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
The \verb\try...finally\ form specifies a `cleanup' handler. The
\verb\try\ clause is executed. When no exception occurs, the
\verb\finally\ clause is executed. When an exception occurs in the
\verb\try\ clause, the exception is temporarily saved, the
\verb\finally\ clause is executed, and then the saved exception is
re-raised. If the \verb\finally\ clause raises another exception or
executes a \verb\return\, \verb\break\ or \verb\continue\ statement,
the saved exception is lost.
\kwindex{finally}
When a \verb\return\ or \verb\break\ statement is executed in the
\verb\try\ suite of a \verb\try...finally\ statement, the
\verb\finally\ clause is also executed `on the way out'. A
\verb\continue\ statement is illegal in the \verb\try\ clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
\stindex{return}
\stindex{break}
\stindex{continue}
\section{Function definitions} \label{function}
\indexii{function}{definition}
A function definition defines a user-defined function object (see
section \ref{types}):
\obindex{user-defined function}
\obindex{function}
\begin{verbatim}
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (parameter ",")* ("*" identifier | parameter [","])
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
\end{verbatim}
A function definition is an executable statement. Its execution binds
the function name in the current local name space to a function object
(a wrapper around the executable code for the function). This
function object contains a reference to the current global name space
as the global name space to be used when the function is called.
\indexii{function}{name}
\indexii{name}{binding}
The function definition does not execute the function body; this gets
executed only when the function is called.
Function call semantics are described in section \ref{calls}. When a
user-defined function is called, the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
\indexii{function}{call}
\indexiii{user-defined}{function}{call}
\index{parameter}
\index{argument}
\indexii{parameter}{formal}
\indexii{parameter}{actual}
\begin{itemize}
\item
If there are no formal parameters, there must be no arguments.
\item
If the formal parameter list does not end in a star followed by an
identifier, there must be exactly as many arguments as there are
parameters in the formal parameter list (at the top level); the
arguments are assigned to the formal parameters one by one. Note that
the presence or absence of a trailing comma at the top level in either
the formal or the actual parameter list makes no difference. The
assignment to a formal parameter is performed as if the parameter
occurs on the left hand side of an assignment statement whose right
hand side's value is that of the argument.
\item
If the formal parameter list ends in a star followed by an identifier,
preceded by zero or more comma-followed parameters, there must be at
least as many arguments as there are parameters preceding the star.
Call this number {\em N}. The first {\em N} arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is \verb\()\, if
there is just one extra argument, it is a singleton tuple.
\indexii{variable length}{parameter list}
\end{itemize}
Note that the `variable length parameter list' feature only works at
the top level of the parameter list; individual parameters use a model
corresponding more closely to that of ordinary assignment. While the
latter model is generally preferable, because of the greater type
safety it offers (wrong-sized tuples aren't silently mistreated),
variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
\section{Class definitions} \label{class}
\indexii{class}{definition}
A class definition defines a class object (see section \ref{types}):
\obindex{class}
\begin{verbatim}
classdef: "class" classname [inheritance] ":" suite
inheritance: "(" [condition_list] ")"
classname: identifier
\end{verbatim}
A class definition is an executable statement. It first evaluates the
inheritance list, if present. Each item in the inheritance list
should evaluate to a class object. The class's suite is then executed
in a new execution frame (see section \ref{execframes}), using a newly
created local name space and the original global name space.
(Usually, the suite contains only function definitions.) When the
class's suite finishes execution, its execution frame is discarded but
its local name space is saved. A class object is then created using
the inheritance list for the base classes and the saved local name
space for the attribute dictionary. The class name is bound to this
class object in the original local name space.
\index{inheritance}
\indexii{class}{name}
\indexii{name}{binding}
\indexii{execution}{frame}

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\chapter{Top-level components}
The Python interpreter can get its input from a number of sources:
from a script passed to it as standard input or as program argument,
typed in interactively, from a module source file, etc. This chapter
gives the syntax used in these cases.
\index{interpreter}
\section{Complete Python programs}
\index{program}
While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for \verb\sys\
(various system services), \verb\builtin\ (built-in functions,
exceptions and \verb\None\) and \verb\__main__\. The latter is used
to provide the local and global name space for execution of the
complete program.
\bimodindex{sys}
\bimodindex{__main__}
\bimodindex{builtin}
The syntax for a complete Python program is that for file input,
described in the next section.
The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of \verb\__main__\.
\index{interactive mode}
Under {\UNIX}, a complete program can be passed to the interpreter in
three forms: with the {\bf -c} {\it string} command line option, as a
file passed as the first command line argument, or as standard input.
If the file or standard input is a tty device, the interpreter enters
interactive mode; otherwise, it executes the file as a complete
program.
\index{UNIX}
\index{command line}
\index{standard input}
\section{File input}
All input read from non-interactive files has the same form:
\begin{verbatim}
file_input: (NEWLINE | statement)*
\end{verbatim}
This syntax is used in the following situations:
\begin{itemize}
\item when parsing a complete Python program (from a file or from a string);
\item when parsing a module;
\item when parsing a string passed to \verb\exec()\;
\bifuncindex{exec}
\item when parsing a file passed to \verb\execfile()\;
\bifuncindex{execfile}
\end{itemize}
\section{Interactive input}
Input in interactive mode is parsed using the following grammar:
\begin{verbatim}
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
\end{verbatim}
Note that a (top-level) compound statement must be followed by a blank
line in interactive mode; this is needed to help the parser detect the
end of the input.
\section{Expression input}
\index{input}
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to \verb\eval()\ must have the following form:
\bifuncindex{eval}
\begin{verbatim}
eval_input: condition_list NEWLINE*
\end{verbatim}
The input line read by \verb\input()\ must have the following form:
\bifuncindex{input}
\begin{verbatim}
input_input: condition_list NEWLINE
\end{verbatim}
Note: to read `raw' input line without interpretation, you can use the
built-in function \verb\raw_input()\ or the \verb\readline()\ method
of file objects.
\obindex{file}
\index{input!raw}
\index{raw input}
\bifuncindex{raw_index}
\ttindex{readline}