7 .. index:: pair: compound; statement
9 Compound statements contain (groups of) other statements; they affect or control
10 the execution of those other statements in some way. In general, compound
11 statements span multiple lines, although in simple incarnations a whole compound
12 statement may be contained in one line.
14 The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement
15 traditional control flow constructs. :keyword:`try` specifies exception
16 handlers and/or cleanup code for a group of statements. Function and class
17 definitions are also syntactically compound statements.
23 Compound statements consist of one or more 'clauses.' A clause consists of a
24 header and a 'suite.' The clause headers of a particular compound statement are
25 all at the same indentation level. Each clause header begins with a uniquely
26 identifying keyword and ends with a colon. A suite is a group of statements
27 controlled by a clause. A suite can be one or more semicolon-separated simple
28 statements on the same line as the header, following the header's colon, or it
29 can be one or more indented statements on subsequent lines. Only the latter
30 form of suite can contain nested compound statements; the following is illegal,
31 mostly because it wouldn't be clear to which :keyword:`if` clause a following
32 :keyword:`else` clause would belong: ::
34 if test1: if test2: print x
36 Also note that the semicolon binds tighter than the colon in this context, so
37 that in the following example, either all or none of the :keyword:`print`
38 statements are executed::
40 if x < y < z: print x; print y; print z
45 compound_stmt: `if_stmt`
53 suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
54 statement: `stmt_list` NEWLINE | `compound_stmt`
55 stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
62 Note that statements always end in a ``NEWLINE`` possibly followed by a
63 ``DEDENT``. Also note that optional continuation clauses always begin with a
64 keyword that cannot start a statement, thus there are no ambiguities (the
65 'dangling :keyword:`else`' problem is solved in Python by requiring nested
66 :keyword:`if` statements to be indented).
68 The formatting of the grammar rules in the following sections places each clause
69 on a separate line for clarity.
76 The :keyword:`if` statement
77 ===========================
84 The :keyword:`if` statement is used for conditional execution:
87 if_stmt: "if" `expression` ":" `suite`
88 : ( "elif" `expression` ":" `suite` )*
89 : ["else" ":" `suite`]
91 It selects exactly one of the suites by evaluating the expressions one by one
92 until one is found to be true (see section :ref:`booleans` for the definition of
93 true and false); then that suite is executed (and no other part of the
94 :keyword:`if` statement is executed or evaluated). If all expressions are
95 false, the suite of the :keyword:`else` clause, if present, is executed.
100 The :keyword:`while` statement
101 ==============================
105 pair: loop; statement
108 The :keyword:`while` statement is used for repeated execution as long as an
112 while_stmt: "while" `expression` ":" `suite`
113 : ["else" ":" `suite`]
115 This repeatedly tests the expression and, if it is true, executes the first
116 suite; if the expression is false (which may be the first time it is tested) the
117 suite of the :keyword:`else` clause, if present, is executed and the loop
124 A :keyword:`break` statement executed in the first suite terminates the loop
125 without executing the :keyword:`else` clause's suite. A :keyword:`continue`
126 statement executed in the first suite skips the rest of the suite and goes back
127 to testing the expression.
132 The :keyword:`for` statement
133 ============================
137 pair: loop; statement
143 The :keyword:`for` statement is used to iterate over the elements of a sequence
144 (such as a string, tuple or list) or other iterable object:
147 for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`
148 : ["else" ":" `suite`]
150 The expression list is evaluated once; it should yield an iterable object. An
151 iterator is created for the result of the ``expression_list``. The suite is
152 then executed once for each item provided by the iterator, in the order of
153 ascending indices. Each item in turn is assigned to the target list using the
154 standard rules for assignments, and then the suite is executed. When the items
155 are exhausted (which is immediately when the sequence is empty), the suite in
156 the :keyword:`else` clause, if present, is executed, and the loop terminates.
162 A :keyword:`break` statement executed in the first suite terminates the loop
163 without executing the :keyword:`else` clause's suite. A :keyword:`continue`
164 statement executed in the first suite skips the rest of the suite and continues
165 with the next item, or with the :keyword:`else` clause if there was no next
168 The suite may assign to the variable(s) in the target list; this does not affect
169 the next item assigned to it.
173 pair: Pascal; language
175 The target list is not deleted when the loop is finished, but if the sequence is
176 empty, it will not have been assigned to at all by the loop. Hint: the built-in
177 function :func:`range` returns a sequence of integers suitable to emulate the
178 effect of Pascal's ``for i := a to b do``; e.g., ``range(3)`` returns the list
184 single: loop; over mutable sequence
185 single: mutable sequence; loop over
187 There is a subtlety when the sequence is being modified by the loop (this can
188 only occur for mutable sequences, i.e. lists). An internal counter is used to
189 keep track of which item is used next, and this is incremented on each
190 iteration. When this counter has reached the length of the sequence the loop
191 terminates. This means that if the suite deletes the current (or a previous)
192 item from the sequence, the next item will be skipped (since it gets the index
193 of the current item which has already been treated). Likewise, if the suite
194 inserts an item in the sequence before the current item, the current item will
195 be treated again the next time through the loop. This can lead to nasty bugs
196 that can be avoided by making a temporary copy using a slice of the whole
200 if x < 0: a.remove(x)
207 The :keyword:`try` statement
208 ============================
215 The :keyword:`try` statement specifies exception handlers and/or cleanup code
216 for a group of statements:
219 try_stmt: try1_stmt | try2_stmt
220 try1_stmt: "try" ":" `suite`
221 : ("except" [`expression` [("as" | ",") `target`]] ":" `suite`)+
222 : ["else" ":" `suite`]
223 : ["finally" ":" `suite`]
224 try2_stmt: "try" ":" `suite`
225 : "finally" ":" `suite`
227 .. versionchanged:: 2.5
228 In previous versions of Python, :keyword:`try`...\ :keyword:`except`...\
229 :keyword:`finally` did not work. :keyword:`try`...\ :keyword:`except` had to be
230 nested in :keyword:`try`...\ :keyword:`finally`.
232 The :keyword:`except` clause(s) specify one or more exception handlers. When no
233 exception occurs in the :keyword:`try` clause, no exception handler is executed.
234 When an exception occurs in the :keyword:`try` suite, a search for an exception
235 handler is started. This search inspects the except clauses in turn until one
236 is found that matches the exception. An expression-less except clause, if
237 present, must be last; it matches any exception. For an except clause with an
238 expression, that expression is evaluated, and the clause matches the exception
239 if the resulting object is "compatible" with the exception. An object is
240 compatible with an exception if it is the class or a base class of the exception
241 object, a tuple containing an item compatible with the exception, or, in the
242 (deprecated) case of string exceptions, is the raised string itself (note that
243 the object identities must match, i.e. it must be the same string object, not
244 just a string with the same value).
246 If no except clause matches the exception, the search for an exception handler
247 continues in the surrounding code and on the invocation stack. [#]_
249 If the evaluation of an expression in the header of an except clause raises an
250 exception, the original search for a handler is canceled and a search starts for
251 the new exception in the surrounding code and on the call stack (it is treated
252 as if the entire :keyword:`try` statement raised the exception).
254 When a matching except clause is found, the exception is assigned to the target
255 specified in that except clause, if present, and the except clause's suite is
256 executed. All except clauses must have an executable block. When the end of
257 this block is reached, execution continues normally after the entire try
258 statement. (This means that if two nested handlers exist for the same
259 exception, and the exception occurs in the try clause of the inner handler, the
260 outer handler will not handle the exception.)
265 single: exc_type (in module sys)
266 single: exc_value (in module sys)
267 single: exc_traceback (in module sys)
269 Before an except clause's suite is executed, details about the exception are
270 assigned to three variables in the :mod:`sys` module: ``sys.exc_type`` receives
271 the object identifying the exception; ``sys.exc_value`` receives the exception's
272 parameter; ``sys.exc_traceback`` receives a traceback object (see section
273 :ref:`types`) identifying the point in the program where the exception
274 occurred. These details are also available through the :func:`sys.exc_info`
275 function, which returns a tuple ``(exc_type, exc_value, exc_traceback)``. Use
276 of the corresponding variables is deprecated in favor of this function, since
277 their use is unsafe in a threaded program. As of Python 1.5, the variables are
278 restored to their previous values (before the call) when returning from a
279 function that handled an exception.
287 The optional :keyword:`else` clause is executed if and when control flows off
288 the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
289 clause are not handled by the preceding :keyword:`except` clauses.
291 .. index:: keyword: finally
293 If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
294 :keyword:`try` clause is executed, including any :keyword:`except` and
295 :keyword:`else` clauses. If an exception occurs in any of the clauses and is
296 not handled, the exception is temporarily saved. The :keyword:`finally` clause
297 is executed. If there is a saved exception, it is re-raised at the end of the
298 :keyword:`finally` clause. If the :keyword:`finally` clause raises another
299 exception or executes a :keyword:`return` or :keyword:`break` statement, the
300 saved exception is lost. The exception information is not available to the
301 program during execution of the :keyword:`finally` clause.
308 When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
309 executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
310 statement, the :keyword:`finally` clause is also executed 'on the way out.' A
311 :keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
312 reason is a problem with the current implementation --- this restriction may be
313 lifted in the future).
315 Additional information on exceptions can be found in section :ref:`exceptions`,
316 and information on using the :keyword:`raise` statement to generate exceptions
317 may be found in section :ref:`raise`.
323 The :keyword:`with` statement
324 =============================
326 .. index:: statement: with
328 .. versionadded:: 2.5
330 The :keyword:`with` statement is used to wrap the execution of a block with
331 methods defined by a context manager (see section :ref:`context-managers`). This
332 allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` usage
333 patterns to be encapsulated for convenient reuse.
336 with_stmt: "with" with_item ("," with_item)* ":" `suite`
337 with_item: `expression` ["as" `target`]
339 The execution of the :keyword:`with` statement with one "item" proceeds as follows:
341 #. The context expression (the expression given in the :token:`with_item`) is
342 evaluated to obtain a context manager.
344 #. The context manager's :meth:`__exit__` is loaded for later use.
346 #. The context manager's :meth:`__enter__` method is invoked.
348 #. If a target was included in the :keyword:`with` statement, the return value
349 from :meth:`__enter__` is assigned to it.
353 The :keyword:`with` statement guarantees that if the :meth:`__enter__` method
354 returns without an error, then :meth:`__exit__` will always be called. Thus, if
355 an error occurs during the assignment to the target list, it will be treated the
356 same as an error occurring within the suite would be. See step 6 below.
358 #. The suite is executed.
360 #. The context manager's :meth:`__exit__` method is invoked. If an exception
361 caused the suite to be exited, its type, value, and traceback are passed as
362 arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
365 If the suite was exited due to an exception, and the return value from the
366 :meth:`__exit__` method was false, the exception is reraised. If the return
367 value was true, the exception is suppressed, and execution continues with the
368 statement following the :keyword:`with` statement.
370 If the suite was exited for any reason other than an exception, the return value
371 from :meth:`__exit__` is ignored, and execution proceeds at the normal location
372 for the kind of exit that was taken.
374 With more than one item, the context managers are processed as if multiple
375 :keyword:`with` statements were nested::
377 with A() as a, B() as b:
388 In Python 2.5, the :keyword:`with` statement is only allowed when the
389 ``with_statement`` feature has been enabled. It is always enabled in
392 .. versionchanged:: 2.7
393 Support for multiple context expressions.
397 :pep:`0343` - The "with" statement
398 The specification, background, and examples for the Python :keyword:`with`
410 pair: function; definition
413 object: user-defined function
416 A function definition defines a user-defined function object (see section
420 decorated: decorators (classdef | funcdef)
421 decorators: `decorator`+
422 decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE
423 funcdef: "def" `funcname` "(" [`parameter_list`] ")" ":" `suite`
424 dotted_name: `identifier` ("." `identifier`)*
425 parameter_list: (`defparameter` ",")*
426 : ( "*" `identifier` [, "**" `identifier`]
427 : | "**" `identifier`
428 : | `defparameter` [","] )
429 defparameter: `parameter` ["=" `expression`]
430 sublist: `parameter` ("," `parameter`)* [","]
431 parameter: `identifier` | "(" `sublist` ")"
432 funcname: `identifier`
434 A function definition is an executable statement. Its execution binds the
435 function name in the current local namespace to a function object (a wrapper
436 around the executable code for the function). This function object contains a
437 reference to the current global namespace as the global namespace to be used
438 when the function is called.
440 The function definition does not execute the function body; this gets executed
441 only when the function is called. [#]_
446 A function definition may be wrapped by one or more :term:`decorator` expressions.
447 Decorator expressions are evaluated when the function is defined, in the scope
448 that contains the function definition. The result must be a callable, which is
449 invoked with the function object as the only argument. The returned value is
450 bound to the function name instead of the function object. Multiple decorators
451 are applied in nested fashion. For example, the following code::
460 func = f1(arg)(f2(func))
462 .. index:: triple: default; parameter; value
464 When one or more top-level parameters have the form *parameter* ``=``
465 *expression*, the function is said to have "default parameter values." For a
466 parameter with a default value, the corresponding argument may be omitted from a
467 call, in which case the parameter's default value is substituted. If a
468 parameter has a default value, all following parameters must also have a default
469 value --- this is a syntactic restriction that is not expressed by the grammar.
471 **Default parameter values are evaluated when the function definition is
472 executed.** This means that the expression is evaluated once, when the function
473 is defined, and that that same "pre-computed" value is used for each call. This
474 is especially important to understand when a default parameter is a mutable
475 object, such as a list or a dictionary: if the function modifies the object
476 (e.g. by appending an item to a list), the default value is in effect modified.
477 This is generally not what was intended. A way around this is to use ``None``
478 as the default, and explicitly test for it in the body of the function, e.g.::
480 def whats_on_the_telly(penguin=None):
483 penguin.append("property of the zoo")
490 Function call semantics are described in more detail in section :ref:`calls`. A
491 function call always assigns values to all parameters mentioned in the parameter
492 list, either from position arguments, from keyword arguments, or from default
493 values. If the form "``*identifier``" is present, it is initialized to a tuple
494 receiving any excess positional parameters, defaulting to the empty tuple. If
495 the form "``**identifier``" is present, it is initialized to a new dictionary
496 receiving any excess keyword arguments, defaulting to a new empty dictionary.
498 .. index:: pair: lambda; form
500 It is also possible to create anonymous functions (functions not bound to a
501 name), for immediate use in expressions. This uses lambda forms, described in
502 section :ref:`lambda`. Note that the lambda form is merely a shorthand for a
503 simplified function definition; a function defined in a ":keyword:`def`"
504 statement can be passed around or assigned to another name just like a function
505 defined by a lambda form. The ":keyword:`def`" form is actually more powerful
506 since it allows the execution of multiple statements.
508 **Programmer's note:** Functions are first-class objects. A "``def``" form
509 executed inside a function definition defines a local function that can be
510 returned or passed around. Free variables used in the nested function can
511 access the local variables of the function containing the def. See section
512 :ref:`naming` for details.
523 pair: class; definition
526 pair: execution; frame
530 A class definition defines a class object (see section :ref:`types`):
533 classdef: "class" `classname` [`inheritance`] ":" `suite`
534 inheritance: "(" [`expression_list`] ")"
535 classname: `identifier`
537 A class definition is an executable statement. It first evaluates the
538 inheritance list, if present. Each item in the inheritance list should evaluate
539 to a class object or class type which allows subclassing. The class's suite is
540 then executed in a new execution frame (see section :ref:`naming`), using a
541 newly created local namespace and the original global namespace. (Usually, the
542 suite contains only function definitions.) When the class's suite finishes
543 execution, its execution frame is discarded but its local namespace is
544 saved. [#]_ A class object is then created using the inheritance list for the
545 base classes and the saved local namespace for the attribute dictionary. The
546 class name is bound to this class object in the original local namespace.
548 **Programmer's note:** Variables defined in the class definition are class
549 variables; they are shared by all instances. To create instance variables, they
550 can be set in a method with ``self.name = value``. Both class and instance
551 variables are accessible through the notation "``self.name``", and an instance
552 variable hides a class variable with the same name when accessed in this way.
553 Class variables can be used as defaults for instance variables, but using
554 mutable values there can lead to unexpected results. For :term:`new-style
555 class`\es, descriptors can be used to create instance variables with different
556 implementation details.
558 Class definitions, like function definitions, may be wrapped by one or more
559 :term:`decorator` expressions. The evaluation rules for the decorator
560 expressions are the same as for functions. The result must be a class object,
561 which is then bound to the class name.
563 .. rubric:: Footnotes
565 .. [#] The exception is propagated to the invocation stack only if there is no
566 :keyword:`finally` clause that negates the exception.
568 .. [#] Currently, control "flows off the end" except in the case of an exception or the
569 execution of a :keyword:`return`, :keyword:`continue`, or :keyword:`break`
572 .. [#] A string literal appearing as the first statement in the function body is
573 transformed into the function's ``__doc__`` attribute and therefore the
574 function's :term:`docstring`.
576 .. [#] A string literal appearing as the first statement in the class body is
577 transformed into the namespace's ``__doc__`` item and therefore the class's