forked from python/cpython
-
Notifications
You must be signed in to change notification settings - Fork 2
Expand file tree
/
Copy pathexpressions.rst
More file actions
2148 lines (1609 loc) · 84 KB
/
expressions.rst
File metadata and controls
2148 lines (1609 loc) · 84 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
.. _expressions:
***********
Expressions
***********
.. index:: expression, BNF
This chapter explains the meaning of the elements of expressions in Python.
**Syntax Notes:** In this and the following chapters, extended BNF notation will
be used to describe syntax, not lexical analysis. When (one alternative of) a
syntax rule has the form
.. productionlist:: python-grammar
name: othername
and no semantics are given, the semantics of this form of ``name`` are the same
as for ``othername``.
.. _conversions:
Arithmetic conversions
======================
.. index:: pair: arithmetic; conversion
When a description of an arithmetic operator below uses the phrase "the numeric
arguments are converted to a common real type", this means that the operator
implementation for built-in types works as follows:
* If both arguments are complex numbers, no conversion is performed;
* if either argument is a complex or a floating-point number, the other is converted to a floating-point number;
* otherwise, both must be integers and no conversion is necessary.
Some additional rules apply for certain operators (e.g., a string as a left
argument to the '%' operator). Extensions must define their own conversion
behavior.
.. _atoms:
Atoms
=====
.. index:: atom
Atoms are the most basic elements of expressions. The simplest atoms are
identifiers or literals. Forms enclosed in parentheses, brackets or braces are
also categorized syntactically as atoms. The syntax for atoms is:
.. productionlist:: python-grammar
atom: `identifier` | `literal` | `enclosure`
enclosure: `parenth_form` | `list_display` | `dict_display` | `set_display`
: | `generator_expression` | `yield_atom`
.. _atom-identifiers:
Identifiers (Names)
-------------------
.. index:: name, identifier
An identifier occurring as an atom is a name. See section :ref:`identifiers`
for lexical definition and section :ref:`naming` for documentation of naming and
binding.
.. index:: pair: exception; NameError
When the name is bound to an object, evaluation of the atom yields that object.
When a name is not bound, an attempt to evaluate it raises a :exc:`NameError`
exception.
.. _private-name-mangling:
.. index::
pair: name; mangling
pair: private; names
Private name mangling
^^^^^^^^^^^^^^^^^^^^^
When an identifier that textually occurs in a class definition begins with two
or more underscore characters and does not end in two or more underscores, it
is considered a :dfn:`private name` of that class.
.. seealso::
The :ref:`class specifications <class>`.
More precisely, private names are transformed to a longer form before code is
generated for them. If the transformed name is longer than 255 characters,
implementation-defined truncation may happen.
The transformation is independent of the syntactical context in which the
identifier is used but only the following private identifiers are mangled:
- Any name used as the name of a variable that is assigned or read or any
name of an attribute being accessed.
The :attr:`~definition.__name__` attribute of nested functions, classes, and
type aliases is however not mangled.
- The name of imported modules, e.g., ``__spam`` in ``import __spam``.
If the module is part of a package (i.e., its name contains a dot),
the name is *not* mangled, e.g., the ``__foo`` in ``import __foo.bar``
is not mangled.
- The name of an imported member, e.g., ``__f`` in ``from spam import __f``.
The transformation rule is defined as follows:
- The class name, with leading underscores removed and a single leading
underscore inserted, is inserted in front of the identifier, e.g., the
identifier ``__spam`` occurring in a class named ``Foo``, ``_Foo`` or
``__Foo`` is transformed to ``_Foo__spam``.
- If the class name consists only of underscores, the transformation is the
identity, e.g., the identifier ``__spam`` occurring in a class named ``_``
or ``__`` is left as is.
.. _atom-literals:
Literals
--------
.. index:: single: literal
Python supports string and bytes literals and various numeric literals:
.. grammar-snippet::
:group: python-grammar
literal: `strings` | `NUMBER`
Evaluation of a literal yields an object of the given type (string, bytes,
integer, floating-point number, complex number) with the given value. The value
may be approximated in the case of floating-point and imaginary (complex)
literals.
See section :ref:`literals` for details.
Seee section :ref:`string-concatenation` for details on ``strings``.
.. index::
triple: immutable; data; type
pair: immutable; object
All literals correspond to immutable data types, and hence the object's identity
is less important than its value. Multiple evaluations of literals with the
same value (either the same occurrence in the program text or a different
occurrence) may obtain the same object or a different object with the same
value.
.. _string-concatenation:
String literal concatenation
............................
Multiple adjacent string or bytes literals (delimited by whitespace), possibly
using different quoting conventions, are allowed, and their meaning is the same
as their concatenation. Thus, ``"hello" 'world'`` is equivalent to
``"helloworld"``.
Formally:
.. grammar-snippet::
:group: python-grammar
strings: ( `STRING` | `fstring` | `tstring`)+
Note that this feature is defined at the syntactical level, so it only works
with literals.
To concatenate string expressions at run time, the '+' operator may be used::
greeting = "Hello"
space = " "
name = "Blaise"
print(greeting + space + name) # not: print(greeting space name)
Also note that literal concatenation can freely mix raw strings,
triple-quoted strings, and formatted or template string literals.
However, bytes literals may not be combined with string literals of any kind.
This feature can be used to reduce the number of backslashes
needed, to split long strings conveniently across long lines, or even to add
comments to parts of strings, for example::
re.compile("[A-Za-z_]" # letter or underscore
"[A-Za-z0-9_]*" # letter, digit or underscore
)
.. _parenthesized:
Parenthesized forms
-------------------
.. index::
single: parenthesized form
single: () (parentheses); tuple display
A parenthesized form is an optional expression list enclosed in parentheses:
.. productionlist:: python-grammar
parenth_form: "(" [`starred_expression`] ")"
A parenthesized expression list yields whatever that expression list yields: if
the list contains at least one comma, it yields a tuple; otherwise, it yields
the single expression that makes up the expression list.
.. index:: pair: empty; tuple
An empty pair of parentheses yields an empty tuple object. Since tuples are
immutable, the same rules as for literals apply (i.e., two occurrences of the empty
tuple may or may not yield the same object).
.. index::
single: comma
single: , (comma)
Note that tuples are not formed by the parentheses, but rather by use of the
comma. The exception is the empty tuple, for which parentheses *are*
required --- allowing unparenthesized "nothing" in expressions would cause
ambiguities and allow common typos to pass uncaught.
.. _comprehensions:
Displays for lists, sets and dictionaries
-----------------------------------------
.. index:: single: comprehensions
For constructing a list, a set or a dictionary Python provides special syntax
called "displays", each of them in two flavors:
* either the container contents are listed explicitly, or
* they are computed via a set of looping and filtering instructions, called a
:dfn:`comprehension`.
.. index::
single: for; in comprehensions
single: if; in comprehensions
single: async for; in comprehensions
Common syntax elements for comprehensions are:
.. productionlist:: python-grammar
comprehension: `assignment_expression` `comp_for`
comp_for: ["async"] "for" `target_list` "in" `or_test` [`comp_iter`]
comp_iter: `comp_for` | `comp_if`
comp_if: "if" `or_test` [`comp_iter`]
The comprehension consists of a single expression followed by at least one
:keyword:`!for` clause and zero or more :keyword:`!for` or :keyword:`!if` clauses.
In this case, the elements of the new container are those that would be produced
by considering each of the :keyword:`!for` or :keyword:`!if` clauses a block,
nesting from left to right, and evaluating the expression to produce an element
each time the innermost block is reached.
However, aside from the iterable expression in the leftmost :keyword:`!for` clause,
the comprehension is executed in a separate implicitly nested scope. This ensures
that names assigned to in the target list don't "leak" into the enclosing scope.
The iterable expression in the leftmost :keyword:`!for` clause is evaluated
directly in the enclosing scope and then passed as an argument to the implicitly
nested scope. Subsequent :keyword:`!for` clauses and any filter condition in the
leftmost :keyword:`!for` clause cannot be evaluated in the enclosing scope as
they may depend on the values obtained from the leftmost iterable. For example:
``[x*y for x in range(10) for y in range(x, x+10)]``.
To ensure the comprehension always results in a container of the appropriate
type, ``yield`` and ``yield from`` expressions are prohibited in the implicitly
nested scope.
.. index::
single: await; in comprehensions
Since Python 3.6, in an :keyword:`async def` function, an :keyword:`!async for`
clause may be used to iterate over a :term:`asynchronous iterator`.
A comprehension in an :keyword:`!async def` function may consist of either a
:keyword:`!for` or :keyword:`!async for` clause following the leading
expression, may contain additional :keyword:`!for` or :keyword:`!async for`
clauses, and may also use :keyword:`await` expressions.
If a comprehension contains :keyword:`!async for` clauses, or if it contains
:keyword:`!await` expressions or other asynchronous comprehensions anywhere except
the iterable expression in the leftmost :keyword:`!for` clause, it is called an
:dfn:`asynchronous comprehension`. An asynchronous comprehension may suspend the
execution of the coroutine function in which it appears.
See also :pep:`530`.
.. versionadded:: 3.6
Asynchronous comprehensions were introduced.
.. versionchanged:: 3.8
``yield`` and ``yield from`` prohibited in the implicitly nested scope.
.. versionchanged:: 3.11
Asynchronous comprehensions are now allowed inside comprehensions in
asynchronous functions. Outer comprehensions implicitly become
asynchronous.
.. _lists:
List displays
-------------
.. index::
pair: list; display
pair: list; comprehensions
pair: empty; list
pair: object; list
single: [] (square brackets); list expression
single: , (comma); expression list
A list display is a possibly empty series of expressions enclosed in square
brackets:
.. productionlist:: python-grammar
list_display: "[" [`flexible_expression_list` | `comprehension`] "]"
A list display yields a new list object, the contents being specified by either
a list of expressions or a comprehension. When a comma-separated list of
expressions is supplied, its elements are evaluated from left to right and
placed into the list object in that order. When a comprehension is supplied,
the list is constructed from the elements resulting from the comprehension.
.. _set:
Set displays
------------
.. index::
pair: set; display
pair: set; comprehensions
pair: object; set
single: {} (curly brackets); set expression
single: , (comma); expression list
A set display is denoted by curly braces and distinguishable from dictionary
displays by the lack of colons separating keys and values:
.. productionlist:: python-grammar
set_display: "{" (`flexible_expression_list` | `comprehension`) "}"
A set display yields a new mutable set object, the contents being specified by
either a sequence of expressions or a comprehension. When a comma-separated
list of expressions is supplied, its elements are evaluated from left to right
and added to the set object. When a comprehension is supplied, the set is
constructed from the elements resulting from the comprehension.
An empty set cannot be constructed with ``{}``; this literal constructs an empty
dictionary.
.. _dict:
Dictionary displays
-------------------
.. index::
pair: dictionary; display
pair: dictionary; comprehensions
key, value, key/value pair
pair: object; dictionary
single: {} (curly brackets); dictionary expression
single: : (colon); in dictionary expressions
single: , (comma); in dictionary displays
A dictionary display is a possibly empty series of dict items (key/value pairs)
enclosed in curly braces:
.. productionlist:: python-grammar
dict_display: "{" [`dict_item_list` | `dict_comprehension`] "}"
dict_item_list: `dict_item` ("," `dict_item`)* [","]
dict_item: `expression` ":" `expression` | "**" `or_expr`
dict_comprehension: `expression` ":" `expression` `comp_for`
A dictionary display yields a new dictionary object.
If a comma-separated sequence of dict items is given, they are evaluated
from left to right to define the entries of the dictionary: each key object is
used as a key into the dictionary to store the corresponding value. This means
that you can specify the same key multiple times in the dict item list, and the
final dictionary's value for that key will be the last one given.
.. index::
unpacking; dictionary
single: **; in dictionary displays
A double asterisk ``**`` denotes :dfn:`dictionary unpacking`.
Its operand must be a :term:`mapping`. Each mapping item is added
to the new dictionary. Later values replace values already set by
earlier dict items and earlier dictionary unpackings.
.. versionadded:: 3.5
Unpacking into dictionary displays, originally proposed by :pep:`448`.
A dict comprehension, in contrast to list and set comprehensions, needs two
expressions separated with a colon followed by the usual "for" and "if" clauses.
When the comprehension is run, the resulting key and value elements are inserted
in the new dictionary in the order they are produced.
.. index:: pair: immutable; object
hashable
Restrictions on the types of the key values are listed earlier in section
:ref:`types`. (To summarize, the key type should be :term:`hashable`, which excludes
all mutable objects.) Clashes between duplicate keys are not detected; the last
value (textually rightmost in the display) stored for a given key value
prevails.
.. versionchanged:: 3.8
Prior to Python 3.8, in dict comprehensions, the evaluation order of key
and value was not well-defined. In CPython, the value was evaluated before
the key. Starting with 3.8, the key is evaluated before the value, as
proposed by :pep:`572`.
.. _genexpr:
Generator expressions
---------------------
.. index::
pair: generator; expression
pair: object; generator
single: () (parentheses); generator expression
A generator expression is a compact generator notation in parentheses:
.. productionlist:: python-grammar
generator_expression: "(" `expression` `comp_for` ")"
A generator expression yields a new generator object. Its syntax is the same as
for comprehensions, except that it is enclosed in parentheses instead of
brackets or curly braces.
Variables used in the generator expression are evaluated lazily when the
:meth:`~generator.__next__` method is called for the generator object (in the same
fashion as normal generators). However, the iterable expression in the
leftmost :keyword:`!for` clause is immediately evaluated, and the
:term:`iterator` is immediately created for that iterable, so that an error
produced while creating the iterator will be emitted at the point where the generator expression
is defined, rather than at the point where the first value is retrieved.
Subsequent :keyword:`!for` clauses and any filter condition in the leftmost
:keyword:`!for` clause cannot be evaluated in the enclosing scope as they may
depend on the values obtained from the leftmost iterable. For example:
``(x*y for x in range(10) for y in range(x, x+10))``.
The parentheses can be omitted on calls with only one argument. See section
:ref:`calls` for details.
To avoid interfering with the expected operation of the generator expression
itself, ``yield`` and ``yield from`` expressions are prohibited in the
implicitly defined generator.
If a generator expression contains either :keyword:`!async for`
clauses or :keyword:`await` expressions it is called an
:dfn:`asynchronous generator expression`. An asynchronous generator
expression returns a new asynchronous generator object,
which is an asynchronous iterator (see :ref:`async-iterators`).
.. versionadded:: 3.6
Asynchronous generator expressions were introduced.
.. versionchanged:: 3.7
Prior to Python 3.7, asynchronous generator expressions could
only appear in :keyword:`async def` coroutines. Starting
with 3.7, any function can use asynchronous generator expressions.
.. versionchanged:: 3.8
``yield`` and ``yield from`` prohibited in the implicitly nested scope.
.. _yieldexpr:
Yield expressions
-----------------
.. index::
pair: keyword; yield
pair: keyword; from
pair: yield; expression
pair: generator; function
.. productionlist:: python-grammar
yield_atom: "(" `yield_expression` ")"
yield_from: "yield" "from" `expression`
yield_expression: "yield" `yield_list` | `yield_from`
The yield expression is used when defining a :term:`generator` function
or an :term:`asynchronous generator` function and
thus can only be used in the body of a function definition. Using a yield
expression in a function's body causes that function to be a generator function,
and using it in an :keyword:`async def` function's body causes that
coroutine function to be an asynchronous generator function. For example::
def gen(): # defines a generator function
yield 123
async def agen(): # defines an asynchronous generator function
yield 123
Due to their side effects on the containing scope, ``yield`` expressions
are not permitted as part of the implicitly defined scopes used to
implement comprehensions and generator expressions.
.. versionchanged:: 3.8
Yield expressions prohibited in the implicitly nested scopes used to
implement comprehensions and generator expressions.
Generator functions are described below, while asynchronous generator
functions are described separately in section
:ref:`asynchronous-generator-functions`.
When a generator function is called, it returns an iterator known as a
generator. That generator then controls the execution of the generator
function. The execution starts when one of the generator's methods is called.
At that time, the execution proceeds to the first yield expression, where it is
suspended again, returning the value of :token:`~python-grammar:yield_list`
to the generator's caller,
or ``None`` if :token:`~python-grammar:yield_list` is omitted.
By suspended, we mean that all local state is
retained, including the current bindings of local variables, the instruction
pointer, the internal evaluation stack, and the state of any exception handling.
When the execution is resumed by calling one of the generator's methods, the
function can proceed exactly as if the yield expression were just another
external call. The value of the yield expression after resuming depends on the
method which resumed the execution. If :meth:`~generator.__next__` is used
(typically via either a :keyword:`for` or the :func:`next` builtin) then the
result is :const:`None`. Otherwise, if :meth:`~generator.send` is used, then
the result will be the value passed in to that method.
.. index:: single: coroutine
All of this makes generator functions quite similar to coroutines; they yield
multiple times, they have more than one entry point and their execution can be
suspended. The only difference is that a generator function cannot control
where the execution should continue after it yields; the control is always
transferred to the generator's caller.
Yield expressions are allowed anywhere in a :keyword:`try` construct. If the
generator is not resumed before it is
finalized (by reaching a zero reference count or by being garbage collected),
the generator-iterator's :meth:`~generator.close` method will be called,
allowing any pending :keyword:`finally` clauses to execute.
.. index::
single: from; yield from expression
When ``yield from <expr>`` is used, the supplied expression must be an
iterable. The values produced by iterating that iterable are passed directly
to the caller of the current generator's methods. Any values passed in with
:meth:`~generator.send` and any exceptions passed in with
:meth:`~generator.throw` are passed to the underlying iterator if it has the
appropriate methods. If this is not the case, then :meth:`~generator.send`
will raise :exc:`AttributeError` or :exc:`TypeError`, while
:meth:`~generator.throw` will just raise the passed in exception immediately.
When the underlying iterator is complete, the :attr:`~StopIteration.value`
attribute of the raised :exc:`StopIteration` instance becomes the value of
the yield expression. It can be either set explicitly when raising
:exc:`StopIteration`, or automatically when the subiterator is a generator
(by returning a value from the subgenerator).
.. versionchanged:: 3.3
Added ``yield from <expr>`` to delegate control flow to a subiterator.
The parentheses may be omitted when the yield expression is the sole expression
on the right hand side of an assignment statement.
.. seealso::
:pep:`255` - Simple Generators
The proposal for adding generators and the :keyword:`yield` statement to Python.
:pep:`342` - Coroutines via Enhanced Generators
The proposal to enhance the API and syntax of generators, making them
usable as simple coroutines.
:pep:`380` - Syntax for Delegating to a Subgenerator
The proposal to introduce the :token:`~python-grammar:yield_from` syntax,
making delegation to subgenerators easy.
:pep:`525` - Asynchronous Generators
The proposal that expanded on :pep:`492` by adding generator capabilities to
coroutine functions.
.. index:: pair: object; generator
.. _generator-methods:
Generator-iterator methods
^^^^^^^^^^^^^^^^^^^^^^^^^^
This subsection describes the methods of a generator iterator. They can
be used to control the execution of a generator function.
Note that calling any of the generator methods below when the generator
is already executing raises a :exc:`ValueError` exception.
.. index:: pair: exception; StopIteration
.. method:: generator.__next__()
Starts the execution of a generator function or resumes it at the last
executed yield expression. When a generator function is resumed with a
:meth:`~generator.__next__` method, the current yield expression always
evaluates to :const:`None`. The execution then continues to the next yield
expression, where the generator is suspended again, and the value of the
:token:`~python-grammar:yield_list` is returned to :meth:`__next__`'s
caller. If the generator exits without yielding another value, a
:exc:`StopIteration` exception is raised.
This method is normally called implicitly, e.g. by a :keyword:`for` loop, or
by the built-in :func:`next` function.
.. method:: generator.send(value)
Resumes the execution and "sends" a value into the generator function. The
*value* argument becomes the result of the current yield expression. The
:meth:`send` method returns the next value yielded by the generator, or
raises :exc:`StopIteration` if the generator exits without yielding another
value. When :meth:`send` is called to start the generator, it must be called
with :const:`None` as the argument, because there is no yield expression that
could receive the value.
.. method:: generator.throw(value)
generator.throw(type[, value[, traceback]])
Raises an exception at the point where the generator was paused,
and returns the next value yielded by the generator function. If the generator
exits without yielding another value, a :exc:`StopIteration` exception is
raised. If the generator function does not catch the passed-in exception, or
raises a different exception, then that exception propagates to the caller.
In typical use, this is called with a single exception instance similar to the
way the :keyword:`raise` keyword is used.
For backwards compatibility, however, the second signature is
supported, following a convention from older versions of Python.
The *type* argument should be an exception class, and *value*
should be an exception instance. If the *value* is not provided, the
*type* constructor is called to get an instance. If *traceback*
is provided, it is set on the exception, otherwise any existing
:attr:`~BaseException.__traceback__` attribute stored in *value* may
be cleared.
.. versionchanged:: 3.12
The second signature \(type\[, value\[, traceback\]\]\) is deprecated and
may be removed in a future version of Python.
.. index:: pair: exception; GeneratorExit
.. method:: generator.close()
Raises a :exc:`GeneratorExit` at the point where the generator function was
paused. If the generator function catches the exception and returns a
value, this value is returned from :meth:`close`. If the generator function
is already closed, or raises :exc:`GeneratorExit` (by not catching the
exception), :meth:`close` returns :const:`None`. If the generator yields a
value, a :exc:`RuntimeError` is raised. If the generator raises any other
exception, it is propagated to the caller. If the generator has already
exited due to an exception or normal exit, :meth:`close` returns
:const:`None` and has no other effect.
.. versionchanged:: 3.13
If a generator returns a value upon being closed, the value is returned
by :meth:`close`.
.. index:: single: yield; examples
Examples
^^^^^^^^
Here is a simple example that demonstrates the behavior of generators and
generator functions::
>>> def echo(value=None):
... print("Execution starts when 'next()' is called for the first time.")
... try:
... while True:
... try:
... value = (yield value)
... except Exception as e:
... value = e
... finally:
... print("Don't forget to clean up when 'close()' is called.")
...
>>> generator = echo(1)
>>> print(next(generator))
Execution starts when 'next()' is called for the first time.
1
>>> print(next(generator))
None
>>> print(generator.send(2))
2
>>> generator.throw(TypeError, "spam")
TypeError('spam',)
>>> generator.close()
Don't forget to clean up when 'close()' is called.
For examples using ``yield from``, see :ref:`pep-380` in "What's New in
Python."
.. _asynchronous-generator-functions:
Asynchronous generator functions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The presence of a yield expression in a function or method defined using
:keyword:`async def` further defines the function as an
:term:`asynchronous generator` function.
When an asynchronous generator function is called, it returns an
asynchronous iterator known as an asynchronous generator object.
That object then controls the execution of the generator function.
An asynchronous generator object is typically used in an
:keyword:`async for` statement in a coroutine function analogously to
how a generator object would be used in a :keyword:`for` statement.
Calling one of the asynchronous generator's methods returns an :term:`awaitable`
object, and the execution starts when this object is awaited on. At that time,
the execution proceeds to the first yield expression, where it is suspended
again, returning the value of :token:`~python-grammar:yield_list` to the
awaiting coroutine. As with a generator, suspension means that all local state
is retained, including the current bindings of local variables, the instruction
pointer, the internal evaluation stack, and the state of any exception handling.
When the execution is resumed by awaiting on the next object returned by the
asynchronous generator's methods, the function can proceed exactly as if the
yield expression were just another external call. The value of the yield
expression after resuming depends on the method which resumed the execution. If
:meth:`~agen.__anext__` is used then the result is :const:`None`. Otherwise, if
:meth:`~agen.asend` is used, then the result will be the value passed in to that
method.
If an asynchronous generator happens to exit early by :keyword:`break`, the caller
task being cancelled, or other exceptions, the generator's async cleanup code
will run and possibly raise exceptions or access context variables in an
unexpected context--perhaps after the lifetime of tasks it depends, or
during the event loop shutdown when the async-generator garbage collection hook
is called.
To prevent this, the caller must explicitly close the async generator by calling
:meth:`~agen.aclose` method to finalize the generator and ultimately detach it
from the event loop.
In an asynchronous generator function, yield expressions are allowed anywhere
in a :keyword:`try` construct. However, if an asynchronous generator is not
resumed before it is finalized (by reaching a zero reference count or by
being garbage collected), then a yield expression within a :keyword:`!try`
construct could result in a failure to execute pending :keyword:`finally`
clauses. In this case, it is the responsibility of the event loop or
scheduler running the asynchronous generator to call the asynchronous
generator-iterator's :meth:`~agen.aclose` method and run the resulting
coroutine object, thus allowing any pending :keyword:`!finally` clauses
to execute.
To take care of finalization upon event loop termination, an event loop should
define a *finalizer* function which takes an asynchronous generator-iterator and
presumably calls :meth:`~agen.aclose` and executes the coroutine.
This *finalizer* may be registered by calling :func:`sys.set_asyncgen_hooks`.
When first iterated over, an asynchronous generator-iterator will store the
registered *finalizer* to be called upon finalization. For a reference example
of a *finalizer* method see the implementation of
``asyncio.Loop.shutdown_asyncgens`` in :source:`Lib/asyncio/base_events.py`.
The expression ``yield from <expr>`` is a syntax error when used in an
asynchronous generator function.
.. index:: pair: object; asynchronous-generator
.. _asynchronous-generator-methods:
Asynchronous generator-iterator methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This subsection describes the methods of an asynchronous generator iterator,
which are used to control the execution of a generator function.
.. index:: pair: exception; StopAsyncIteration
.. method:: agen.__anext__()
:async:
Returns an awaitable which when run starts to execute the asynchronous
generator or resumes it at the last executed yield expression. When an
asynchronous generator function is resumed with an :meth:`~agen.__anext__`
method, the current yield expression always evaluates to :const:`None` in the
returned awaitable, which when run will continue to the next yield
expression. The value of the :token:`~python-grammar:yield_list` of the
yield expression is the value of the :exc:`StopIteration` exception raised by
the completing coroutine. If the asynchronous generator exits without
yielding another value, the awaitable instead raises a
:exc:`StopAsyncIteration` exception, signalling that the asynchronous
iteration has completed.
This method is normally called implicitly by a :keyword:`async for` loop.
.. method:: agen.asend(value)
:async:
Returns an awaitable which when run resumes the execution of the
asynchronous generator. As with the :meth:`~generator.send` method for a
generator, this "sends" a value into the asynchronous generator function,
and the *value* argument becomes the result of the current yield expression.
The awaitable returned by the :meth:`asend` method will return the next
value yielded by the generator as the value of the raised
:exc:`StopIteration`, or raises :exc:`StopAsyncIteration` if the
asynchronous generator exits without yielding another value. When
:meth:`asend` is called to start the asynchronous
generator, it must be called with :const:`None` as the argument,
because there is no yield expression that could receive the value.
.. method:: agen.athrow(value)
agen.athrow(type[, value[, traceback]])
:async:
Returns an awaitable that raises an exception of type ``type`` at the point
where the asynchronous generator was paused, and returns the next value
yielded by the generator function as the value of the raised
:exc:`StopIteration` exception. If the asynchronous generator exits
without yielding another value, a :exc:`StopAsyncIteration` exception is
raised by the awaitable.
If the generator function does not catch the passed-in exception, or
raises a different exception, then when the awaitable is run that exception
propagates to the caller of the awaitable.
.. versionchanged:: 3.12
The second signature \(type\[, value\[, traceback\]\]\) is deprecated and
may be removed in a future version of Python.
.. index:: pair: exception; GeneratorExit
.. method:: agen.aclose()
:async:
Returns an awaitable that when run will throw a :exc:`GeneratorExit` into
the asynchronous generator function at the point where it was paused.
If the asynchronous generator function then exits gracefully, is already
closed, or raises :exc:`GeneratorExit` (by not catching the exception),
then the returned awaitable will raise a :exc:`StopIteration` exception.
Any further awaitables returned by subsequent calls to the asynchronous
generator will raise a :exc:`StopAsyncIteration` exception. If the
asynchronous generator yields a value, a :exc:`RuntimeError` is raised
by the awaitable. If the asynchronous generator raises any other exception,
it is propagated to the caller of the awaitable. If the asynchronous
generator has already exited due to an exception or normal exit, then
further calls to :meth:`aclose` will return an awaitable that does nothing.
.. _primaries:
Primaries
=========
.. index:: single: primary
Primaries represent the most tightly bound operations of the language. Their
syntax is:
.. productionlist:: python-grammar
primary: `atom` | `attributeref` | `subscription` | `slicing` | `call`
.. _attribute-references:
Attribute references
--------------------
.. index::
pair: attribute; reference
single: . (dot); attribute reference
An attribute reference is a primary followed by a period and a name:
.. productionlist:: python-grammar
attributeref: `primary` "." `identifier`
.. index::
pair: exception; AttributeError
pair: object; module
pair: object; list
The primary must evaluate to an object of a type that supports attribute
references, which most objects do. This object is then asked to produce the
attribute whose name is the identifier. The type and value produced is
determined by the object. Multiple evaluations of the same attribute
reference may yield different objects.
This production can be customized by overriding the
:meth:`~object.__getattribute__` method or the :meth:`~object.__getattr__`
method. The :meth:`!__getattribute__` method is called first and either
returns a value or raises :exc:`AttributeError` if the attribute is not
available.
If an :exc:`AttributeError` is raised and the object has a :meth:`!__getattr__`
method, that method is called as a fallback.
.. _subscriptions:
Subscriptions
-------------
.. index::
single: subscription
single: [] (square brackets); subscription
.. index::
pair: object; sequence
pair: object; mapping
pair: object; string
pair: object; tuple
pair: object; list
pair: object; dictionary
pair: sequence; item
The subscription of an instance of a :ref:`container class <sequence-types>`
will generally select an element from the container. The subscription of a
:term:`generic class <generic type>` will generally return a
:ref:`GenericAlias <types-genericalias>` object.
.. productionlist:: python-grammar
subscription: `primary` "[" `flexible_expression_list` "]"
When an object is subscripted, the interpreter will evaluate the primary and
the expression list.
The primary must evaluate to an object that supports subscription. An object
may support subscription through defining one or both of
:meth:`~object.__getitem__` and :meth:`~object.__class_getitem__`. When the
primary is subscripted, the evaluated result of the expression list will be
passed to one of these methods. For more details on when ``__class_getitem__``
is called instead of ``__getitem__``, see :ref:`classgetitem-versus-getitem`.
If the expression list contains at least one comma, or if any of the expressions
are starred, the expression list will evaluate to a :class:`tuple` containing
the items of the expression list. Otherwise, the expression list will evaluate
to the value of the list's sole member.
.. versionchanged:: 3.11
Expressions in an expression list may be starred. See :pep:`646`.
For built-in objects, there are two types of objects that support subscription
via :meth:`~object.__getitem__`:
1. Mappings. If the primary is a :term:`mapping`, the expression list must
evaluate to an object whose value is one of the keys of the mapping, and the
subscription selects the value in the mapping that corresponds to that key.
An example of a builtin mapping class is the :class:`dict` class.
2. Sequences. If the primary is a :term:`sequence`, the expression list must
evaluate to an :class:`int` or a :class:`slice` (as discussed in the
following section). Examples of builtin sequence classes include the
:class:`str`, :class:`list` and :class:`tuple` classes.
The formal syntax makes no special provision for negative indices in
:term:`sequences <sequence>`. However, built-in sequences all provide a :meth:`~object.__getitem__`
method that interprets negative indices by adding the length of the sequence
to the index so that, for example, ``x[-1]`` selects the last item of ``x``. The
resulting value must be a nonnegative integer less than the number of items in
the sequence, and the subscription selects the item whose index is that value
(counting from zero). Since the support for negative indices and slicing
occurs in the object's :meth:`~object.__getitem__` method, subclasses overriding
this method will need to explicitly add that support.
.. index::
single: character
pair: string; item
A :class:`string <str>` is a special kind of sequence whose items are
*characters*. A character is not a separate data type but a
string of exactly one character.
.. _slicings:
Slicings
--------
.. index::
single: slicing
single: slice