8
.. index:: pair: compound; statement
10
Compound statements contain (groups of) other statements; they affect or control
11
the execution of those other statements in some way. In general, compound
12
statements span multiple lines, although in simple incarnations a whole compound
13
statement may be contained in one line.
15
The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement
16
traditional control flow constructs. :keyword:`try` specifies exception
17
handlers and/or cleanup code for a group of statements. Function and class
18
definitions are also syntactically compound statements.
24
Compound statements consist of one or more 'clauses.' A clause consists of a
25
header and a 'suite.' The clause headers of a particular compound statement are
26
all at the same indentation level. Each clause header begins with a uniquely
27
identifying keyword and ends with a colon. A suite is a group of statements
28
controlled by a clause. A suite can be one or more semicolon-separated simple
29
statements on the same line as the header, following the header's colon, or it
30
can be one or more indented statements on subsequent lines. Only the latter
31
form of suite can contain nested compound statements; the following is illegal,
32
mostly because it wouldn't be clear to which :keyword:`if` clause a following
33
:keyword:`else` clause would belong: ::
35
if test1: if test2: print x
37
Also note that the semicolon binds tighter than the colon in this context, so
38
that in the following example, either all or none of the :keyword:`print`
39
statements are executed::
41
if x < y < z: print x; print y; print z
46
compound_stmt: `if_stmt`
54
suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
55
statement: `stmt_list` NEWLINE | `compound_stmt`
56
stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
63
Note that statements always end in a ``NEWLINE`` possibly followed by a
64
``DEDENT``. Also note that optional continuation clauses always begin with a
65
keyword that cannot start a statement, thus there are no ambiguities (the
66
'dangling :keyword:`else`' problem is solved in Python by requiring nested
67
:keyword:`if` statements to be indented).
69
The formatting of the grammar rules in the following sections places each clause
70
on a separate line for clarity.
77
The :keyword:`if` statement
78
===========================
85
The :keyword:`if` statement is used for conditional execution:
88
if_stmt: "if" `expression` ":" `suite`
89
: ( "elif" `expression` ":" `suite` )*
90
: ["else" ":" `suite`]
92
It selects exactly one of the suites by evaluating the expressions one by one
93
until one is found to be true (see section :ref:`booleans` for the definition of
94
true and false); then that suite is executed (and no other part of the
95
:keyword:`if` statement is executed or evaluated). If all expressions are
96
false, the suite of the :keyword:`else` clause, if present, is executed.
101
The :keyword:`while` statement
102
==============================
106
pair: loop; statement
109
The :keyword:`while` statement is used for repeated execution as long as an
113
while_stmt: "while" `expression` ":" `suite`
114
: ["else" ":" `suite`]
116
This repeatedly tests the expression and, if it is true, executes the first
117
suite; if the expression is false (which may be the first time it is tested) the
118
suite of the :keyword:`else` clause, if present, is executed and the loop
125
A :keyword:`break` statement executed in the first suite terminates the loop
126
without executing the :keyword:`else` clause's suite. A :keyword:`continue`
127
statement executed in the first suite skips the rest of the suite and goes back
128
to testing the expression.
133
The :keyword:`for` statement
134
============================
138
pair: loop; statement
144
The :keyword:`for` statement is used to iterate over the elements of a sequence
145
(such as a string, tuple or list) or other iterable object:
148
for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`
149
: ["else" ":" `suite`]
151
The expression list is evaluated once; it should yield an iterable object. An
152
iterator is created for the result of the ``expression_list``. The suite is
153
then executed once for each item provided by the iterator, in the order of
154
ascending indices. Each item in turn is assigned to the target list using the
155
standard rules for assignments, and then the suite is executed. When the items
156
are exhausted (which is immediately when the sequence is empty), the suite in
157
the :keyword:`else` clause, if present, is executed, and the loop terminates.
163
A :keyword:`break` statement executed in the first suite terminates the loop
164
without executing the :keyword:`else` clause's suite. A :keyword:`continue`
165
statement executed in the first suite skips the rest of the suite and continues
166
with the next item, or with the :keyword:`else` clause if there was no next
169
The suite may assign to the variable(s) in the target list; this does not affect
170
the next item assigned to it.
174
pair: Pascal; language
176
The target list is not deleted when the loop is finished, but if the sequence is
177
empty, it will not have been assigned to at all by the loop. Hint: the built-in
178
function :func:`range` returns a sequence of integers suitable to emulate the
179
effect of Pascal's ``for i := a to b do``; e.g., ``range(3)`` returns the list
185
single: loop; over mutable sequence
186
single: mutable sequence; loop over
188
There is a subtlety when the sequence is being modified by the loop (this can
189
only occur for mutable sequences, i.e. lists). An internal counter is used to
190
keep track of which item is used next, and this is incremented on each
191
iteration. When this counter has reached the length of the sequence the loop
192
terminates. This means that if the suite deletes the current (or a previous)
193
item from the sequence, the next item will be skipped (since it gets the index
194
of the current item which has already been treated). Likewise, if the suite
195
inserts an item in the sequence before the current item, the current item will
196
be treated again the next time through the loop. This can lead to nasty bugs
197
that can be avoided by making a temporary copy using a slice of the whole
203
if x < 0: a.remove(x)
210
The :keyword:`try` statement
211
============================
218
The :keyword:`try` statement specifies exception handlers and/or cleanup code
219
for a group of statements:
222
try_stmt: try1_stmt | try2_stmt
223
try1_stmt: "try" ":" `suite`
224
: ("except" [`expression` [("as" | ",") `target`]] ":" `suite`)+
225
: ["else" ":" `suite`]
226
: ["finally" ":" `suite`]
227
try2_stmt: "try" ":" `suite`
228
: "finally" ":" `suite`
230
.. versionchanged:: 2.5
231
In previous versions of Python, :keyword:`try`...\ :keyword:`except`...\
232
:keyword:`finally` did not work. :keyword:`try`...\ :keyword:`except` had to be
233
nested in :keyword:`try`...\ :keyword:`finally`.
235
The :keyword:`except` clause(s) specify one or more exception handlers. When no
236
exception occurs in the :keyword:`try` clause, no exception handler is executed.
237
When an exception occurs in the :keyword:`try` suite, a search for an exception
238
handler is started. This search inspects the except clauses in turn until one
239
is found that matches the exception. An expression-less except clause, if
240
present, must be last; it matches any exception. For an except clause with an
241
expression, that expression is evaluated, and the clause matches the exception
242
if the resulting object is "compatible" with the exception. An object is
243
compatible with an exception if it is the class or a base class of the exception
244
object, a tuple containing an item compatible with the exception, or, in the
245
(deprecated) case of string exceptions, is the raised string itself (note that
246
the object identities must match, i.e. it must be the same string object, not
247
just a string with the same value).
249
If no except clause matches the exception, the search for an exception handler
250
continues in the surrounding code and on the invocation stack. [#]_
252
If the evaluation of an expression in the header of an except clause raises an
253
exception, the original search for a handler is canceled and a search starts for
254
the new exception in the surrounding code and on the call stack (it is treated
255
as if the entire :keyword:`try` statement raised the exception).
257
When a matching except clause is found, the exception is assigned to the target
258
specified in that except clause, if present, and the except clause's suite is
259
executed. All except clauses must have an executable block. When the end of
260
this block is reached, execution continues normally after the entire try
261
statement. (This means that if two nested handlers exist for the same
262
exception, and the exception occurs in the try clause of the inner handler, the
263
outer handler will not handle the exception.)
268
single: exc_type (in module sys)
269
single: exc_value (in module sys)
270
single: exc_traceback (in module sys)
272
Before an except clause's suite is executed, details about the exception are
273
assigned to three variables in the :mod:`sys` module: ``sys.exc_type`` receives
274
the object identifying the exception; ``sys.exc_value`` receives the exception's
275
parameter; ``sys.exc_traceback`` receives a traceback object (see section
276
:ref:`types`) identifying the point in the program where the exception
277
occurred. These details are also available through the :func:`sys.exc_info`
278
function, which returns a tuple ``(exc_type, exc_value, exc_traceback)``. Use
279
of the corresponding variables is deprecated in favor of this function, since
280
their use is unsafe in a threaded program. As of Python 1.5, the variables are
281
restored to their previous values (before the call) when returning from a
282
function that handled an exception.
290
The optional :keyword:`else` clause is executed if and when control flows off
291
the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
292
clause are not handled by the preceding :keyword:`except` clauses.
294
.. index:: keyword: finally
296
If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
297
:keyword:`try` clause is executed, including any :keyword:`except` and
298
:keyword:`else` clauses. If an exception occurs in any of the clauses and is
299
not handled, the exception is temporarily saved. The :keyword:`finally` clause
300
is executed. If there is a saved exception, it is re-raised at the end of the
301
:keyword:`finally` clause. If the :keyword:`finally` clause raises another
302
exception or executes a :keyword:`return` or :keyword:`break` statement, the
303
saved exception is lost. The exception information is not available to the
304
program during execution of the :keyword:`finally` clause.
311
When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
312
executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
313
statement, the :keyword:`finally` clause is also executed 'on the way out.' A
314
:keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
315
reason is a problem with the current implementation --- this restriction may be
316
lifted in the future).
318
Additional information on exceptions can be found in section :ref:`exceptions`,
319
and information on using the :keyword:`raise` statement to generate exceptions
320
may be found in section :ref:`raise`.
326
The :keyword:`with` statement
327
=============================
329
.. index:: statement: with
331
.. versionadded:: 2.5
333
The :keyword:`with` statement is used to wrap the execution of a block with
334
methods defined by a context manager (see section :ref:`context-managers`). This
335
allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` usage
336
patterns to be encapsulated for convenient reuse.
339
with_stmt: "with" `expression` ["as" `target`] ":" `suite`
341
The execution of the :keyword:`with` statement proceeds as follows:
343
#. The context expression is evaluated to obtain a context manager.
345
#. The context manager's :meth:`__enter__` method is invoked.
347
#. If a target was included in the :keyword:`with` statement, the return value
348
from :meth:`__enter__` is assigned to it.
352
The :keyword:`with` statement guarantees that if the :meth:`__enter__` method
353
returns without an error, then :meth:`__exit__` will always be called. Thus, if
354
an error occurs during the assignment to the target list, it will be treated the
355
same as an error occurring within the suite would be. See step 5 below.
357
#. The suite is executed.
359
#. The context manager's :meth:`__exit__` method is invoked. If an exception
360
caused the suite to be exited, its type, value, and traceback are passed as
361
arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
364
If the suite was exited due to an exception, and the return value from the
365
:meth:`__exit__` method was false, the exception is reraised. If the return
366
value was true, the exception is suppressed, and execution continues with the
367
statement following the :keyword:`with` statement.
369
If the suite was exited for any reason other than an exception, the return value
370
from :meth:`__exit__` is ignored, and execution proceeds at the normal location
371
for the kind of exit that was taken.
375
In Python 2.5, the :keyword:`with` statement is only allowed when the
376
``with_statement`` feature has been enabled. It is always enabled in
381
:pep:`0343` - The "with" statement
382
The specification, background, and examples for the Python :keyword:`with`
394
pair: function; definition
397
object: user-defined function
400
A function definition defines a user-defined function object (see section
404
decorated: decorators (classdef | funcdef)
405
decorators: `decorator`+
406
decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE
407
funcdef: "def" `funcname` "(" [`parameter_list`] ")" ":" `suite`
408
dotted_name: `identifier` ("." `identifier`)*
409
parameter_list: (`defparameter` ",")*
410
: ( "*" `identifier` [, "**" `identifier`]
411
: | "**" `identifier`
412
: | `defparameter` [","] )
413
defparameter: `parameter` ["=" `expression`]
414
sublist: `parameter` ("," `parameter`)* [","]
415
parameter: `identifier` | "(" `sublist` ")"
416
funcname: `identifier`
418
A function definition is an executable statement. Its execution binds the
419
function name in the current local namespace to a function object (a wrapper
420
around the executable code for the function). This function object contains a
421
reference to the current global namespace as the global namespace to be used
422
when the function is called.
424
The function definition does not execute the function body; this gets executed
425
only when the function is called. [#]_
430
A function definition may be wrapped by one or more :term:`decorator` expressions.
431
Decorator expressions are evaluated when the function is defined, in the scope
432
that contains the function definition. The result must be a callable, which is
433
invoked with the function object as the only argument. The returned value is
434
bound to the function name instead of the function object. Multiple decorators
435
are applied in nested fashion. For example, the following code::
444
func = f1(arg)(f2(func))
446
.. index:: triple: default; parameter; value
448
When one or more top-level parameters have the form *parameter* ``=``
449
*expression*, the function is said to have "default parameter values." For a
450
parameter with a default value, the corresponding argument may be omitted from a
451
call, in which case the parameter's default value is substituted. If a
452
parameter has a default value, all following parameters must also have a default
453
value --- this is a syntactic restriction that is not expressed by the grammar.
455
**Default parameter values are evaluated when the function definition is
456
executed.** This means that the expression is evaluated once, when the function
457
is defined, and that that same "pre-computed" value is used for each call. This
458
is especially important to understand when a default parameter is a mutable
459
object, such as a list or a dictionary: if the function modifies the object
460
(e.g. by appending an item to a list), the default value is in effect modified.
461
This is generally not what was intended. A way around this is to use ``None``
462
as the default, and explicitly test for it in the body of the function, e.g.::
464
def whats_on_the_telly(penguin=None):
467
penguin.append("property of the zoo")
474
Function call semantics are described in more detail in section :ref:`calls`. A
475
function call always assigns values to all parameters mentioned in the parameter
476
list, either from position arguments, from keyword arguments, or from default
477
values. If the form "``*identifier``" is present, it is initialized to a tuple
478
receiving any excess positional parameters, defaulting to the empty tuple. If
479
the form "``**identifier``" is present, it is initialized to a new dictionary
480
receiving any excess keyword arguments, defaulting to a new empty dictionary.
482
.. index:: pair: lambda; form
484
It is also possible to create anonymous functions (functions not bound to a
485
name), for immediate use in expressions. This uses lambda forms, described in
486
section :ref:`lambda`. Note that the lambda form is merely a shorthand for a
487
simplified function definition; a function defined in a ":keyword:`def`"
488
statement can be passed around or assigned to another name just like a function
489
defined by a lambda form. The ":keyword:`def`" form is actually more powerful
490
since it allows the execution of multiple statements.
492
**Programmer's note:** Functions are first-class objects. A "``def``" form
493
executed inside a function definition defines a local function that can be
494
returned or passed around. Free variables used in the nested function can
495
access the local variables of the function containing the def. See section
496
:ref:`naming` for details.
507
pair: class; definition
510
pair: execution; frame
514
A class definition defines a class object (see section :ref:`types`):
517
classdef: "class" `classname` [`inheritance`] ":" `suite`
518
inheritance: "(" [`expression_list`] ")"
519
classname: `identifier`
521
A class definition is an executable statement. It first evaluates the
522
inheritance list, if present. Each item in the inheritance list should evaluate
523
to a class object or class type which allows subclassing. The class's suite is
524
then executed in a new execution frame (see section :ref:`naming`), using a
525
newly created local namespace and the original global namespace. (Usually, the
526
suite contains only function definitions.) When the class's suite finishes
527
execution, its execution frame is discarded but its local namespace is
528
saved. [#]_ A class object is then created using the inheritance list for the
529
base classes and the saved local namespace for the attribute dictionary. The
530
class name is bound to this class object in the original local namespace.
532
**Programmer's note:** Variables defined in the class definition are class
533
variables; they are shared by all instances. To create instance variables, they
534
can be set in a method with ``self.name = value``. Both class and instance
535
variables are accessible through the notation "``self.name``", and an instance
536
variable hides a class variable with the same name when accessed in this way.
537
Class variables can be used as defaults for instance variables, but using
538
mutable values there can lead to unexpected results. For :term:`new-style
539
class`\es, descriptors can be used to create instance variables with different
540
implementation details.
542
Class definitions, like function definitions, may be wrapped by one or more
543
:term:`decorator` expressions. The evaluation rules for the decorator
544
expressions are the same as for functions. The result must be a class object,
545
which is then bound to the class name.
547
.. rubric:: Footnotes
549
.. [#] The exception is propagated to the invocation stack only if there is no
550
:keyword:`finally` clause that negates the exception.
552
.. [#] Currently, control "flows off the end" except in the case of an exception or the
553
execution of a :keyword:`return`, :keyword:`continue`, or :keyword:`break`
556
.. [#] A string literal appearing as the first statement in the function body is
557
transformed into the function's ``__doc__`` attribute and therefore the
558
function's :term:`docstring`.
560
.. [#] A string literal appearing as the first statement in the class body is
561
transformed into the namespace's ``__doc__`` item and therefore the class's
added
removed
Doc/reference/compound_stmts.rst
