8
.. index:: pair: simple; statement
10
Simple statements are comprised within a single logical line. Several simple
11
statements may occur on a single line separated by semicolons. The syntax for
15
simple_stmt: `expression_stmt`
18
: | `augmented_assignment_stmt`
38
pair: expression; statement
39
pair: expression; list
41
Expression statements are used (mostly interactively) to compute and write a
42
value, or (usually) to call a procedure (a function that returns no meaningful
43
result; in Python, procedures return the value ``None``). Other uses of
44
expression statements are allowed and occasionally useful. The syntax for an
45
expression statement is:
48
expression_stmt: `expression_list`
50
An expression statement evaluates the expression list (which may be a single
56
pair: string; conversion
58
pair: standard; output
62
In interactive mode, if the value is not ``None``, it is converted to a string
63
using the built-in :func:`repr` function and the resulting string is written to
64
standard output (see section :ref:`print`) on a line by itself. (Expression
65
statements yielding ``None`` are not written, so that procedure calls do not
75
single: =; assignment statement
76
pair: assignment; statement
80
pair: attribute; assignment
82
Assignment statements are used to (re)bind names to values and to modify
83
attributes or items of mutable objects:
86
assignment_stmt: (`target_list` "=")+ (`expression_list` | `yield_expression`)
87
target_list: `target` ("," `target`)* [","]
89
: | "(" `target_list` ")"
90
: | "[" [`target_list`] "]"
95
(See section :ref:`primaries` for the syntax definitions for the last three
98
.. index:: pair: expression; list
100
An assignment statement evaluates the expression list (remember that this can be
101
a single expression or a comma-separated list, the latter yielding a tuple) and
102
assigns the single resulting object to each of the target lists, from left to
109
Assignment is defined recursively depending on the form of the target (list).
110
When a target is part of a mutable object (an attribute reference, subscription
111
or slicing), the mutable object must ultimately perform the assignment and
112
decide about its validity, and may raise an exception if the assignment is
113
unacceptable. The rules observed by various types and the exceptions raised are
114
given with the definition of the object types (see section :ref:`types`).
116
.. index:: triple: target; list; assignment
118
Assignment of an object to a target list is recursively defined as follows.
120
* If the target list is a single target: The object is assigned to that target.
122
* If the target list is a comma-separated list of targets: The object must be an
123
iterable with the same number of items as there are targets in the target list,
124
and the items are assigned, from left to right, to the corresponding targets.
126
Assignment of an object to a single target is recursively defined as follows.
128
* If the target is an identifier (name):
130
.. index:: statement: global
132
* If the name does not occur in a :keyword:`global` statement in the current
133
code block: the name is bound to the object in the current local namespace.
135
* Otherwise: the name is bound to the object in the current global namespace.
137
.. index:: single: destructor
139
The name is rebound if it was already bound. This may cause the reference count
140
for the object previously bound to the name to reach zero, causing the object to
141
be deallocated and its destructor (if it has one) to be called.
143
* If the target is a target list enclosed in parentheses or in square brackets:
144
The object must be an iterable with the same number of items as there are
145
targets in the target list, and its items are assigned, from left to right,
146
to the corresponding targets.
148
.. index:: pair: attribute; assignment
150
* If the target is an attribute reference: The primary expression in the
151
reference is evaluated. It should yield an object with assignable attributes;
152
if this is not the case, :exc:`TypeError` is raised. That object is then
153
asked to assign the assigned object to the given attribute; if it cannot
154
perform the assignment, it raises an exception (usually but not necessarily
155
:exc:`AttributeError`).
157
.. _attr-target-note:
159
Note: If the object is a class instance and the attribute reference occurs on
160
both sides of the assignment operator, the RHS expression, ``a.x`` can access
161
either an instance attribute or (if no instance attribute exists) a class
162
attribute. The LHS target ``a.x`` is always set as an instance attribute,
163
creating it if necessary. Thus, the two occurrences of ``a.x`` do not
164
necessarily refer to the same attribute: if the RHS expression refers to a
165
class attribute, the LHS creates a new instance attribute as the target of the
169
x = 3 # class variable
171
inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3
173
This description does not necessarily apply to descriptor attributes, such as
174
properties created with :func:`property`.
177
pair: subscription; assignment
180
* If the target is a subscription: The primary expression in the reference is
181
evaluated. It should yield either a mutable sequence object (such as a list) or
182
a mapping object (such as a dictionary). Next, the subscript expression is
189
If the primary is a mutable sequence object (such as a list), the subscript must
190
yield a plain integer. If it is negative, the sequence's length is added to it.
191
The resulting value must be a nonnegative integer less than the sequence's
192
length, and the sequence is asked to assign the assigned object to its item with
193
that index. If the index is out of range, :exc:`IndexError` is raised
194
(assignment to a subscripted sequence cannot add new items to a list).
200
If the primary is a mapping object (such as a dictionary), the subscript must
201
have a type compatible with the mapping's key type, and the mapping is then
202
asked to create a key/datum pair which maps the subscript to the assigned
203
object. This can either replace an existing key/value pair with the same key
204
value, or insert a new key/value pair (if no key with the same value existed).
206
.. index:: pair: slicing; assignment
208
* If the target is a slicing: The primary expression in the reference is
209
evaluated. It should yield a mutable sequence object (such as a list). The
210
assigned object should be a sequence object of the same type. Next, the lower
211
and upper bound expressions are evaluated, insofar they are present; defaults
212
are zero and the sequence's length. The bounds should evaluate to (small)
213
integers. If either bound is negative, the sequence's length is added to it.
214
The resulting bounds are clipped to lie between zero and the sequence's length,
215
inclusive. Finally, the sequence object is asked to replace the slice with the
216
items of the assigned sequence. The length of the slice may be different from
217
the length of the assigned sequence, thus changing the length of the target
218
sequence, if the object allows it.
222
In the current implementation, the syntax for targets is taken to be the same
223
as for expressions, and invalid syntax is rejected during the code generation
224
phase, causing less detailed error messages.
226
WARNING: Although the definition of assignment implies that overlaps between the
227
left-hand side and the right-hand side are 'safe' (for example ``a, b = b, a``
228
swaps two variables), overlaps *within* the collection of assigned-to variables
229
are not safe! For instance, the following program prints ``[0, 2]``::
239
Augmented assignment statements
240
-------------------------------
243
pair: augmented; assignment
244
single: statement; assignment, augmented
245
single: +=; augmented assignment
246
single: -=; augmented assignment
247
single: *=; augmented assignment
248
single: /=; augmented assignment
249
single: %=; augmented assignment
250
single: &=; augmented assignment
251
single: ^=; augmented assignment
252
single: |=; augmented assignment
253
single: **=; augmented assignment
254
single: //=; augmented assignment
255
single: >>=; augmented assignment
256
single: <<=; augmented assignment
258
Augmented assignment is the combination, in a single statement, of a binary
259
operation and an assignment statement:
262
augmented_assignment_stmt: `augtarget` `augop` (`expression_list` | `yield_expression`)
263
augtarget: `identifier` | `attributeref` | `subscription` | `slicing`
264
augop: "+=" | "-=" | "*=" | "/=" | "//=" | "%=" | "**="
265
: | ">>=" | "<<=" | "&=" | "^=" | "|="
267
(See section :ref:`primaries` for the syntax definitions for the last three
270
An augmented assignment evaluates the target (which, unlike normal assignment
271
statements, cannot be an unpacking) and the expression list, performs the binary
272
operation specific to the type of assignment on the two operands, and assigns
273
the result to the original target. The target is only evaluated once.
275
An augmented assignment expression like ``x += 1`` can be rewritten as ``x = x +
276
1`` to achieve a similar, but not exactly equal effect. In the augmented
277
version, ``x`` is only evaluated once. Also, when possible, the actual operation
278
is performed *in-place*, meaning that rather than creating a new object and
279
assigning that to the target, the old object is modified instead.
281
With the exception of assigning to tuples and multiple targets in a single
282
statement, the assignment done by augmented assignment statements is handled the
283
same way as normal assignments. Similarly, with the exception of the possible
284
*in-place* behavior, the binary operation performed by augmented assignment is
285
the same as the normal binary operations.
287
For targets which are attribute references, the same :ref:`caveat about class
288
and instance attributes <attr-target-note>` applies as for regular assignments.
293
The :keyword:`assert` statement
294
===============================
298
pair: debugging; assertions
300
Assert statements are a convenient way to insert debugging assertions into a
304
assert_stmt: "assert" `expression` ["," `expression`]
306
The simple form, ``assert expression``, is equivalent to ::
309
if not expression: raise AssertionError
311
The extended form, ``assert expression1, expression2``, is equivalent to ::
314
if not expression1: raise AssertionError(expression2)
318
exception: AssertionError
320
These equivalences assume that :const:`__debug__` and :exc:`AssertionError` refer to
321
the built-in variables with those names. In the current implementation, the
322
built-in variable :const:`__debug__` is ``True`` under normal circumstances,
323
``False`` when optimization is requested (command line option -O). The current
324
code generator emits no code for an assert statement when optimization is
325
requested at compile time. Note that it is unnecessary to include the source
326
code for the expression that failed in the error message; it will be displayed
327
as part of the stack trace.
329
Assignments to :const:`__debug__` are illegal. The value for the built-in variable
330
is determined when the interpreter starts.
335
The :keyword:`pass` statement
336
=============================
340
pair: null; operation
345
:keyword:`pass` is a null operation --- when it is executed, nothing happens.
346
It is useful as a placeholder when a statement is required syntactically, but no
347
code needs to be executed, for example::
349
def f(arg): pass # a function that does nothing (yet)
351
class C: pass # a class with no methods (yet)
356
The :keyword:`del` statement
357
============================
361
pair: deletion; target
362
triple: deletion; target; list
365
del_stmt: "del" `target_list`
367
Deletion is recursively defined very similar to the way assignment is defined.
368
Rather than spelling it out in full details, here are some hints.
370
Deletion of a target list recursively deletes each target, from left to right.
374
pair: unbinding; name
376
Deletion of a name removes the binding of that name from the local or global
377
namespace, depending on whether the name occurs in a :keyword:`global` statement
378
in the same code block. If the name is unbound, a :exc:`NameError` exception
381
.. index:: pair: free; variable
383
It is illegal to delete a name from the local namespace if it occurs as a free
384
variable in a nested block.
386
.. index:: pair: attribute; deletion
388
Deletion of attribute references, subscriptions and slicings is passed to the
389
primary object involved; deletion of a slicing is in general equivalent to
390
assignment of an empty slice of the right type (but even this is determined by
396
The :keyword:`print` statement
397
==============================
399
.. index:: statement: print
402
print_stmt: "print" ([`expression` ("," `expression`)* [","]]
403
: | ">>" `expression` [("," `expression`)+ [","]])
405
:keyword:`print` evaluates each expression in turn and writes the resulting
406
object to standard output (see below). If an object is not a string, it is
407
first converted to a string using the rules for string conversions. The
408
(resulting or original) string is then written. A space is written before each
409
object is (converted and) written, unless the output system believes it is
410
positioned at the beginning of a line. This is the case (1) when no characters
411
have yet been written to standard output, (2) when the last character written to
412
standard output is a whitespace character except ``' '``, or (3) when the last
413
write operation on standard output was not a :keyword:`print` statement.
414
(In some cases it may be functional to write an empty string to standard output
419
Objects which act like file objects but which are not the built-in file objects
420
often do not properly emulate this aspect of the file object's behavior, so it
421
is best not to rely on this.
425
pair: writing; values
426
pair: trailing; comma
427
pair: newline; suppression
429
A ``'\n'`` character is written at the end, unless the :keyword:`print`
430
statement ends with a comma. This is the only action if the statement contains
431
just the keyword :keyword:`print`.
434
pair: standard; output
436
single: stdout (in module sys)
437
exception: RuntimeError
439
Standard output is defined as the file object named ``stdout`` in the built-in
440
module :mod:`sys`. If no such object exists, or if it does not have a
441
:meth:`write` method, a :exc:`RuntimeError` exception is raised.
443
.. index:: single: extended print statement
445
:keyword:`print` also has an extended form, defined by the second portion of the
446
syntax described above. This form is sometimes referred to as ":keyword:`print`
447
chevron." In this form, the first expression after the ``>>`` must evaluate to a
448
"file-like" object, specifically an object that has a :meth:`write` method as
449
described above. With this extended form, the subsequent expressions are
450
printed to this file object. If the first expression evaluates to ``None``,
451
then ``sys.stdout`` is used as the file for output.
456
The :keyword:`return` statement
457
===============================
461
pair: function; definition
462
pair: class; definition
465
return_stmt: "return" [`expression_list`]
467
:keyword:`return` may only occur syntactically nested in a function definition,
468
not within a nested class definition.
470
If an expression list is present, it is evaluated, else ``None`` is substituted.
472
:keyword:`return` leaves the current function call with the expression list (or
473
``None``) as return value.
475
.. index:: keyword: finally
477
When :keyword:`return` passes control out of a :keyword:`try` statement with a
478
:keyword:`finally` clause, that :keyword:`finally` clause is executed before
479
really leaving the function.
481
In a generator function, the :keyword:`return` statement is not allowed to
482
include an :token:`expression_list`. In that context, a bare :keyword:`return`
483
indicates that the generator is done and will cause :exc:`StopIteration` to be
489
The :keyword:`yield` statement
490
==============================
494
single: generator; function
495
single: generator; iterator
496
single: function; generator
497
exception: StopIteration
500
yield_stmt: `yield_expression`
502
The :keyword:`yield` statement is only used when defining a generator function,
503
and is only used in the body of the generator function. Using a :keyword:`yield`
504
statement in a function definition is sufficient to cause that definition to
505
create a generator function instead of a normal function.
507
When a generator function is called, it returns an iterator known as a generator
508
iterator, or more commonly, a generator. The body of the generator function is
509
executed by calling the generator's :meth:`~generator.next` method repeatedly
510
until it raises an exception.
512
When a :keyword:`yield` statement is executed, the state of the generator is
513
frozen and the value of :token:`expression_list` is returned to
514
:meth:`~generator.next`'s caller. By "frozen" we mean that all local state is
515
retained, including the current bindings of local variables, the instruction
516
pointer, and the internal evaluation stack: enough information is saved so that
517
the next time :meth:`~generator.next` is invoked, the function can proceed
518
exactly as if the :keyword:`yield` statement were just another external call.
520
As of Python version 2.5, the :keyword:`yield` statement is now allowed in the
521
:keyword:`try` clause of a :keyword:`try` ... :keyword:`finally` construct. If
522
the generator is not resumed before it is finalized (by reaching a zero
523
reference count or by being garbage collected), the generator-iterator's
524
:meth:`close` method will be called, allowing any pending :keyword:`finally`
527
For full details of :keyword:`yield` semantics, refer to the :ref:`yieldexpr`
532
In Python 2.2, the :keyword:`yield` statement was only allowed when the
533
``generators`` feature has been enabled. This ``__future__``
534
import statement was used to enable the feature::
536
from __future__ import generators
541
:pep:`255` - Simple Generators
542
The proposal for adding generators and the :keyword:`yield` statement to Python.
544
:pep:`342` - Coroutines via Enhanced Generators
545
The proposal that, among other generator enhancements, proposed allowing
546
:keyword:`yield` to appear inside a :keyword:`try` ... :keyword:`finally` block.
551
The :keyword:`raise` statement
552
==============================
557
pair: raising; exception
560
raise_stmt: "raise" [`expression` ["," `expression` ["," `expression`]]]
562
If no expressions are present, :keyword:`raise` re-raises the last exception
563
that was active in the current scope. If no exception is active in the current
564
scope, a :exc:`TypeError` exception is raised indicating that this is an error
565
(if running under IDLE, a :exc:`Queue.Empty` exception is raised instead).
567
Otherwise, :keyword:`raise` evaluates the expressions to get three objects,
568
using ``None`` as the value of omitted expressions. The first two objects are
569
used to determine the *type* and *value* of the exception.
571
If the first object is an instance, the type of the exception is the class of
572
the instance, the instance itself is the value, and the second object must be
575
If the first object is a class, it becomes the type of the exception. The second
576
object is used to determine the exception value: If it is an instance of the
577
class, the instance becomes the exception value. If the second object is a
578
tuple, it is used as the argument list for the class constructor; if it is
579
``None``, an empty argument list is used, and any other object is treated as a
580
single argument to the constructor. The instance so created by calling the
581
constructor is used as the exception value.
583
.. index:: object: traceback
585
If a third object is present and not ``None``, it must be a traceback object
586
(see section :ref:`types`), and it is substituted instead of the current
587
location as the place where the exception occurred. If the third object is
588
present and not a traceback object or ``None``, a :exc:`TypeError` exception is
589
raised. The three-expression form of :keyword:`raise` is useful to re-raise an
590
exception transparently in an except clause, but :keyword:`raise` with no
591
expressions should be preferred if the exception to be re-raised was the most
592
recently active exception in the current scope.
594
Additional information on exceptions can be found in section :ref:`exceptions`,
595
and information about handling exceptions is in section :ref:`try`.
600
The :keyword:`break` statement
601
==============================
607
pair: loop; statement
612
:keyword:`break` may only occur syntactically nested in a :keyword:`for` or
613
:keyword:`while` loop, but not nested in a function or class definition within
616
.. index:: keyword: else
618
It terminates the nearest enclosing loop, skipping the optional :keyword:`else`
619
clause if the loop has one.
621
.. index:: pair: loop control; target
623
If a :keyword:`for` loop is terminated by :keyword:`break`, the loop control
624
target keeps its current value.
626
.. index:: keyword: finally
628
When :keyword:`break` passes control out of a :keyword:`try` statement with a
629
:keyword:`finally` clause, that :keyword:`finally` clause is executed before
630
really leaving the loop.
635
The :keyword:`continue` statement
636
=================================
642
pair: loop; statement
646
continue_stmt: "continue"
648
:keyword:`continue` may only occur syntactically nested in a :keyword:`for` or
649
:keyword:`while` loop, but not nested in a function or class definition or
650
:keyword:`finally` clause within that loop. It continues with the next
651
cycle of the nearest enclosing loop.
653
When :keyword:`continue` passes control out of a :keyword:`try` statement with a
654
:keyword:`finally` clause, that :keyword:`finally` clause is executed before
655
really starting the next loop cycle.
661
The :keyword:`import` statement
662
===============================
666
single: module; importing
669
single: as; import statement
672
import_stmt: "import" `module` ["as" `name`] ( "," `module` ["as" `name`] )*
673
: | "from" `relative_module` "import" `identifier` ["as" `name`]
674
: ( "," `identifier` ["as" `name`] )*
675
: | "from" `relative_module` "import" "(" `identifier` ["as" `name`]
676
: ( "," `identifier` ["as" `name`] )* [","] ")"
677
: | "from" `module` "import" "*"
678
module: (`identifier` ".")* `identifier`
679
relative_module: "."* `module` | "."+
682
Import statements are executed in two steps: (1) find a module, and initialize
683
it if necessary; (2) define a name or names in the local namespace (of the scope
684
where the :keyword:`import` statement occurs). The statement comes in two
685
forms differing on whether it uses the :keyword:`from` keyword. The first form
686
(without :keyword:`from`) repeats these steps for each identifier in the list.
687
The form with :keyword:`from` performs step (1) once, and then performs step
693
To understand how step (1) occurs, one must first understand how Python handles
694
hierarchical naming of modules. To help organize modules and provide a
695
hierarchy in naming, Python has a concept of packages. A package can contain
696
other packages and modules while modules cannot contain other modules or
697
packages. From a file system perspective, packages are directories and modules
703
Once the name of the module is known (unless otherwise specified, the term
704
"module" will refer to both packages and modules), searching
705
for the module or package can begin. The first place checked is
706
:data:`sys.modules`, the cache of all modules that have been imported
707
previously. If the module is found there then it is used in step (2) of import.
710
single: sys.meta_path
712
pair: finder; find_module
715
If the module is not found in the cache, then :data:`sys.meta_path` is searched
716
(the specification for :data:`sys.meta_path` can be found in :pep:`302`).
717
The object is a list of :term:`finder` objects which are queried in order as to
718
whether they know how to load the module by calling their :meth:`find_module`
719
method with the name of the module. If the module happens to be contained
720
within a package (as denoted by the existence of a dot in the name), then a
721
second argument to :meth:`find_module` is given as the value of the
722
:attr:`__path__` attribute from the parent package (everything up to the last
723
dot in the name of the module being imported). If a finder can find the module
724
it returns a :term:`loader` (discussed later) or returns ``None``.
727
single: sys.path_hooks
728
single: sys.path_importer_cache
731
If none of the finders on :data:`sys.meta_path` are able to find the module
732
then some implicitly defined finders are queried. Implementations of Python
733
vary in what implicit meta path finders are defined. The one they all do
734
define, though, is one that handles :data:`sys.path_hooks`,
735
:data:`sys.path_importer_cache`, and :data:`sys.path`.
737
The implicit finder searches for the requested module in the "paths" specified
738
in one of two places ("paths" do not have to be file system paths). If the
739
module being imported is supposed to be contained within a package then the
740
second argument passed to :meth:`find_module`, :attr:`__path__` on the parent
741
package, is used as the source of paths. If the module is not contained in a
742
package then :data:`sys.path` is used as the source of paths.
744
Once the source of paths is chosen it is iterated over to find a finder that
745
can handle that path. The dict at :data:`sys.path_importer_cache` caches
746
finders for paths and is checked for a finder. If the path does not have a
747
finder cached then :data:`sys.path_hooks` is searched by calling each object in
748
the list with a single argument of the path, returning a finder or raises
749
:exc:`ImportError`. If a finder is returned then it is cached in
750
:data:`sys.path_importer_cache` and then used for that path entry. If no finder
751
can be found but the path exists then a value of ``None`` is
752
stored in :data:`sys.path_importer_cache` to signify that an implicit,
753
file-based finder that handles modules stored as individual files should be
754
used for that path. If the path does not exist then a finder which always
755
returns ``None`` is placed in the cache for the path.
759
pair: loader; load_module
760
exception: ImportError
762
If no finder can find the module then :exc:`ImportError` is raised. Otherwise
763
some finder returned a loader whose :meth:`load_module` method is called with
764
the name of the module to load (see :pep:`302` for the original definition of
765
loaders). A loader has several responsibilities to perform on a module it
766
loads. First, if the module already exists in :data:`sys.modules` (a
767
possibility if the loader is called outside of the import machinery) then it
768
is to use that module for initialization and not a new module. But if the
769
module does not exist in :data:`sys.modules` then it is to be added to that
770
dict before initialization begins. If an error occurs during loading of the
771
module and it was added to :data:`sys.modules` it is to be removed from the
772
dict. If an error occurs but the module was already in :data:`sys.modules` it
782
The loader must set several attributes on the module. :data:`__name__` is to be
783
set to the name of the module. :data:`__file__` is to be the "path" to the file
784
unless the module is built-in (and thus listed in
785
:data:`sys.builtin_module_names`) in which case the attribute is not set.
786
If what is being imported is a package then :data:`__path__` is to be set to a
787
list of paths to be searched when looking for modules and packages contained
788
within the package being imported. :data:`__package__` is optional but should
789
be set to the name of package that contains the module or package (the empty
790
string is used for module not contained in a package). :data:`__loader__` is
791
also optional but should be set to the loader object that is loading the
795
exception: ImportError
797
If an error occurs during loading then the loader raises :exc:`ImportError` if
798
some other exception is not already being propagated. Otherwise the loader
799
returns the module that was loaded and initialized.
801
When step (1) finishes without raising an exception, step (2) can begin.
803
The first form of :keyword:`import` statement binds the module name in the local
804
namespace to the module object, and then goes on to import the next identifier,
805
if any. If the module name is followed by :keyword:`as`, the name following
806
:keyword:`as` is used as the local name for the module.
810
exception: ImportError
812
The :keyword:`from` form does not bind the module name: it goes through the list
813
of identifiers, looks each one of them up in the module found in step (1), and
814
binds the name in the local namespace to the object thus found. As with the
815
first form of :keyword:`import`, an alternate local name can be supplied by
816
specifying ":keyword:`as` localname". If a name is not found,
817
:exc:`ImportError` is raised. If the list of identifiers is replaced by a star
818
(``'*'``), all public names defined in the module are bound in the local
819
namespace of the :keyword:`import` statement..
821
.. index:: single: __all__ (optional module attribute)
823
The *public names* defined by a module are determined by checking the module's
824
namespace for a variable named ``__all__``; if defined, it must be a sequence of
825
strings which are names defined or imported by that module. The names given in
826
``__all__`` are all considered public and are required to exist. If ``__all__``
827
is not defined, the set of public names includes all names found in the module's
828
namespace which do not begin with an underscore character (``'_'``).
829
``__all__`` should contain the entire public API. It is intended to avoid
830
accidentally exporting items that are not part of the API (such as library
831
modules which were imported and used within the module).
833
The :keyword:`from` form with ``*`` may only occur in a module scope. If the
834
wild card form of import --- ``import *`` --- is used in a function and the
835
function contains or is a nested block with free variables, the compiler will
836
raise a :exc:`SyntaxError`.
839
single: relative; import
841
When specifying what module to import you do not have to specify the absolute
842
name of the module. When a module or package is contained within another
843
package it is possible to make a relative import within the same top package
844
without having to mention the package name. By using leading dots in the
845
specified module or package after :keyword:`from` you can specify how high to
846
traverse up the current package hierarchy without specifying exact names. One
847
leading dot means the current package where the module making the import
848
exists. Two dots means up one package level. Three dots is up two levels, etc.
849
So if you execute ``from . import mod`` from a module in the ``pkg`` package
850
then you will end up importing ``pkg.mod``. If you execute ``from ..subpkg2
851
import mod`` from within ``pkg.subpkg1`` you will import ``pkg.subpkg2.mod``.
852
The specification for relative imports is contained within :pep:`328`.
854
:func:`importlib.import_module` is provided to support applications that
855
determine which modules need to be loaded dynamically.
863
.. index:: pair: future; statement
865
A :dfn:`future statement` is a directive to the compiler that a particular
866
module should be compiled using syntax or semantics that will be available in a
867
specified future release of Python. The future statement is intended to ease
868
migration to future versions of Python that introduce incompatible changes to
869
the language. It allows use of the new features on a per-module basis before
870
the release in which the feature becomes standard.
872
.. productionlist:: *
873
future_statement: "from" "__future__" "import" feature ["as" name]
874
: ("," feature ["as" name])*
875
: | "from" "__future__" "import" "(" feature ["as" name]
876
: ("," feature ["as" name])* [","] ")"
880
A future statement must appear near the top of the module. The only lines that
881
can appear before a future statement are:
883
* the module docstring (if any),
886
* other future statements.
888
The features recognized by Python 2.6 are ``unicode_literals``,
889
``print_function``, ``absolute_import``, ``division``, ``generators``,
890
``nested_scopes`` and ``with_statement``. ``generators``, ``with_statement``,
891
``nested_scopes`` are redundant in Python version 2.6 and above because they are
894
A future statement is recognized and treated specially at compile time: Changes
895
to the semantics of core constructs are often implemented by generating
896
different code. It may even be the case that a new feature introduces new
897
incompatible syntax (such as a new reserved word), in which case the compiler
898
may need to parse the module differently. Such decisions cannot be pushed off
901
For any given release, the compiler knows which feature names have been defined,
902
and raises a compile-time error if a future statement contains a feature not
905
The direct runtime semantics are the same as for any import statement: there is
906
a standard module :mod:`__future__`, described later, and it will be imported in
907
the usual way at the time the future statement is executed.
909
The interesting runtime semantics depend on the specific feature enabled by the
912
Note that there is nothing special about the statement::
914
import __future__ [as name]
916
That is not a future statement; it's an ordinary import statement with no
917
special semantics or syntax restrictions.
919
Code compiled by an :keyword:`exec` statement or calls to the built-in functions
920
:func:`compile` and :func:`execfile` that occur in a module :mod:`M` containing
921
a future statement will, by default, use the new syntax or semantics associated
922
with the future statement. This can, starting with Python 2.2 be controlled by
923
optional arguments to :func:`compile` --- see the documentation of that function
926
A future statement typed at an interactive interpreter prompt will take effect
927
for the rest of the interpreter session. If an interpreter is started with the
928
:option:`-i` option, is passed a script name to execute, and the script includes
929
a future statement, it will be in effect in the interactive session started
930
after the script is executed.
934
:pep:`236` - Back to the __future__
935
The original proposal for the __future__ mechanism.
940
The :keyword:`global` statement
941
===============================
945
triple: global; name; binding
948
global_stmt: "global" `identifier` ("," `identifier`)*
950
The :keyword:`global` statement is a declaration which holds for the entire
951
current code block. It means that the listed identifiers are to be interpreted
952
as globals. It would be impossible to assign to a global variable without
953
:keyword:`global`, although free variables may refer to globals without being
956
Names listed in a :keyword:`global` statement must not be used in the same code
957
block textually preceding that :keyword:`global` statement.
959
Names listed in a :keyword:`global` statement must not be defined as formal
960
parameters or in a :keyword:`for` loop control target, :keyword:`class`
961
definition, function definition, or :keyword:`import` statement.
965
The current implementation does not enforce the latter two restrictions, but
966
programs should not abuse this freedom, as future implementations may enforce
967
them or silently change the meaning of the program.
975
**Programmer's note:** the :keyword:`global` is a directive to the parser. It
976
applies only to code parsed at the same time as the :keyword:`global` statement.
977
In particular, a :keyword:`global` statement contained in an :keyword:`exec`
978
statement does not affect the code block *containing* the :keyword:`exec`
979
statement, and code contained in an :keyword:`exec` statement is unaffected by
980
:keyword:`global` statements in the code containing the :keyword:`exec`
981
statement. The same applies to the :func:`eval`, :func:`execfile` and
982
:func:`compile` functions.
987
The :keyword:`exec` statement
988
=============================
990
.. index:: statement: exec
993
exec_stmt: "exec" `or_expr` ["in" `expression` ["," `expression`]]
995
This statement supports dynamic execution of Python code. The first expression
996
should evaluate to either a Unicode string, a *Latin-1* encoded string, an open
997
file object, a code object, or a tuple. If it is a string, the string is parsed
998
as a suite of Python statements which is then executed (unless a syntax error
999
occurs). [#]_ If it is an open file, the file is parsed until EOF and executed.
1000
If it is a code object, it is simply executed. For the interpretation of a
1001
tuple, see below. In all cases, the code that's executed is expected to be
1002
valid as file input (see section :ref:`file-input`). Be aware that the
1003
:keyword:`return` and :keyword:`yield` statements may not be used outside of
1004
function definitions even within the context of code passed to the
1005
:keyword:`exec` statement.
1007
In all cases, if the optional parts are omitted, the code is executed in the
1008
current scope. If only the first expression after ``in`` is specified,
1009
it should be a dictionary, which will be used for both the global and the local
1010
variables. If two expressions are given, they are used for the global and local
1011
variables, respectively. If provided, *locals* can be any mapping object.
1012
Remember that at module level, globals and locals are the same dictionary. If
1013
two separate objects are given as *globals* and *locals*, the code will be
1014
executed as if it were embedded in a class definition.
1016
The first expression may also be a tuple of length 2 or 3. In this case, the
1017
optional parts must be omitted. The form ``exec(expr, globals)`` is equivalent
1018
to ``exec expr in globals``, while the form ``exec(expr, globals, locals)`` is
1019
equivalent to ``exec expr in globals, locals``. The tuple form of ``exec``
1020
provides compatibility with Python 3, where ``exec`` is a function rather than
1023
.. versionchanged:: 2.4
1024
Formerly, *locals* was required to be a dictionary.
1027
single: __builtins__
1030
As a side effect, an implementation may insert additional keys into the
1031
dictionaries given besides those corresponding to variable names set by the
1032
executed code. For example, the current implementation may add a reference to
1033
the dictionary of the built-in module :mod:`__builtin__` under the key
1034
``__builtins__`` (!).
1041
**Programmer's hints:** dynamic evaluation of expressions is supported by the
1042
built-in function :func:`eval`. The built-in functions :func:`globals` and
1043
:func:`locals` return the current global and local dictionary, respectively,
1044
which may be useful to pass around for use by :keyword:`exec`.
1047
.. rubric:: Footnotes
1049
.. [#] Note that the parser only accepts the Unix-style end of line convention.
1050
If you are reading the code from a file, make sure to use
1051
:term:`universal newlines` mode to convert Windows or Mac-style newlines.
added
removed
python-2.7.12-docs-html/_sources/reference/simple_stmts.txt
