summaryrefslogtreecommitdiffstats
path: root/Doc/library/asyncio-eventloop.rst
blob: 78ae18391f73cd0baeb1c42f6dece876b7be8946 (plain)
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
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
.. currentmodule:: asyncio

.. _asyncio-event-loop:

Base Event Loop
===============

**Source code:** :source:`Lib/asyncio/events.py`

The event loop is the central execution device provided by :mod:`asyncio`.
It provides multiple facilities, including:

* Registering, executing and cancelling delayed calls (timeouts).

* Creating client and server :ref:`transports <asyncio-transport>` for various
  kinds of communication.

* Launching subprocesses and the associated :ref:`transports
  <asyncio-transport>` for communication with an external program.

* Delegating costly function calls to a pool of threads.

.. class:: BaseEventLoop

   This class is an implementation detail.  It is a subclass of
   :class:`AbstractEventLoop` and may be a base class of concrete
   event loop implementations found in :mod:`asyncio`.  It should not
   be used directly; use :class:`AbstractEventLoop` instead.
   ``BaseEventLoop`` should not be subclassed by third-party code; the
   internal interface is not stable.

.. class:: AbstractEventLoop

   Abstract base class of event loops.

   This class is :ref:`not thread safe <asyncio-multithreading>`.

Run an event loop
-----------------

.. method:: AbstractEventLoop.run_forever()

   Run until :meth:`stop` is called.  If :meth:`stop` is called before
   :meth:`run_forever()` is called, this polls the I/O selector once
   with a timeout of zero, runs all callbacks scheduled in response to
   I/O events (and those that were already scheduled), and then exits.
   If :meth:`stop` is called while :meth:`run_forever` is running,
   this will run the current batch of callbacks and then exit.  Note
   that callbacks scheduled by callbacks will not run in that case;
   they will run the next time :meth:`run_forever` is called.

   .. versionchanged:: 3.5.1

.. method:: AbstractEventLoop.run_until_complete(future)

   Run until the :class:`Future` is done.

   If the argument is a :ref:`coroutine object <coroutine>`, it is wrapped by
   :func:`ensure_future`.

   Return the Future's result, or raise its exception.

.. method:: AbstractEventLoop.is_running()

   Returns running status of event loop.

.. method:: AbstractEventLoop.stop()

   Stop running the event loop.

   This causes :meth:`run_forever` to exit at the next suitable
   opportunity (see there for more details).

   .. versionchanged:: 3.5.1

.. method:: AbstractEventLoop.is_closed()

   Returns ``True`` if the event loop was closed.

   .. versionadded:: 3.4.2

.. method:: AbstractEventLoop.close()

   Close the event loop. The loop must not be running.  Pending
   callbacks will be lost.

   This clears the queues and shuts down the executor, but does not wait for
   the executor to finish.

   This is idempotent and irreversible. No other methods should be called after
   this one.


.. coroutinemethod:: AbstractEventLoop.shutdown_asyncgens()

   Schedule all currently open :term:`asynchronous generator` objects to
   close with an :meth:`~agen.aclose()` call.  After calling this method,
   the event loop will issue a warning whenever a new asynchronous generator
   is iterated.  Should be used to finalize all scheduled asynchronous
   generators reliably.  Example::

    try:
        loop.run_forever()
    finally:
        loop.run_until_complete(loop.shutdown_asyncgens())
        loop.close()

   .. versionadded:: 3.6


.. _asyncio-pass-keywords:

Calls
-----

Most :mod:`asyncio` functions don't accept keywords. If you want to pass
keywords to your callback, use :func:`functools.partial`. For example,
``loop.call_soon(functools.partial(print, "Hello", flush=True))`` will call
``print("Hello", flush=True)``.

.. note::
   :func:`functools.partial` is better than ``lambda`` functions, because
   :mod:`asyncio` can inspect :func:`functools.partial` object to display
   parameters in debug mode, whereas ``lambda`` functions have a poor
   representation.

.. method:: AbstractEventLoop.call_soon(callback, \*args)

   Arrange for a callback to be called as soon as possible.  The callback is
   called after :meth:`call_soon` returns, when control returns to the event
   loop.

   This operates as a :abbr:`FIFO (first-in, first-out)` queue, callbacks
   are called in the order in which they are registered.  Each callback
   will be called exactly once.

   Any positional arguments after the callback will be passed to the
   callback when it is called.

   An instance of :class:`asyncio.Handle` is returned, which can be
   used to cancel the callback.

   :ref:`Use functools.partial to pass keywords to the callback
   <asyncio-pass-keywords>`.

.. method:: AbstractEventLoop.call_soon_threadsafe(callback, \*args)

   Like :meth:`call_soon`, but thread safe.

   See the :ref:`concurrency and multithreading <asyncio-multithreading>`
   section of the documentation.


.. _asyncio-delayed-calls:

Delayed calls
-------------

The event loop has its own internal clock for computing timeouts.
Which clock is used depends on the (platform-specific) event loop
implementation; ideally it is a monotonic clock.  This will generally be
a different clock than :func:`time.time`.

.. note::

   Timeouts (relative *delay* or absolute *when*) should not exceed one day.


.. method:: AbstractEventLoop.call_later(delay, callback, *args)

   Arrange for the *callback* to be called after the given *delay*
   seconds (either an int or float).

   An instance of :class:`asyncio.Handle` is returned, which can be
   used to cancel the callback.

   *callback* will be called exactly once per call to :meth:`call_later`.
   If two callbacks are scheduled for exactly the same time, it is
   undefined which will be called first.

   The optional positional *args* will be passed to the callback when it
   is called. If you want the callback to be called with some named
   arguments, use a closure or :func:`functools.partial`.

   :ref:`Use functools.partial to pass keywords to the callback
   <asyncio-pass-keywords>`.

.. method:: AbstractEventLoop.call_at(when, callback, *args)

   Arrange for the *callback* to be called at the given absolute timestamp
   *when* (an int or float), using the same time reference as
   :meth:`AbstractEventLoop.time`.

   This method's behavior is the same as :meth:`call_later`.

   An instance of :class:`asyncio.Handle` is returned, which can be
   used to cancel the callback.

   :ref:`Use functools.partial to pass keywords to the callback
   <asyncio-pass-keywords>`.

.. method:: AbstractEventLoop.time()

   Return the current time, as a :class:`float` value, according to the
   event loop's internal clock.

.. seealso::

   The :func:`asyncio.sleep` function.


Futures
-------

.. method:: AbstractEventLoop.create_future()

   Create an :class:`asyncio.Future` object attached to the loop.

   This is a preferred way to create futures in asyncio, as event
   loop implementations can provide alternative implementations
   of the Future class (with better performance or instrumentation).

   .. versionadded:: 3.5.2


Tasks
-----

.. method:: AbstractEventLoop.create_task(coro)

   Schedule the execution of a :ref:`coroutine object <coroutine>`: wrap it in
   a future. Return a :class:`Task` object.

   Third-party event loops can use their own subclass of :class:`Task` for
   interoperability. In this case, the result type is a subclass of
   :class:`Task`.

   .. versionadded:: 3.4.2

.. method:: AbstractEventLoop.set_task_factory(factory)

   Set a task factory that will be used by
   :meth:`AbstractEventLoop.create_task`.

   If *factory* is ``None`` the default task factory will be set.

   If *factory* is a *callable*, it should have a signature matching
   ``(loop, coro)``, where *loop* will be a reference to the active
   event loop, *coro* will be a coroutine object.  The callable
   must return an :class:`asyncio.Future` compatible object.

   .. versionadded:: 3.4.4

.. method:: AbstractEventLoop.get_task_factory()

   Return a task factory, or ``None`` if the default one is in use.

   .. versionadded:: 3.4.4


Creating connections
--------------------

.. coroutinemethod:: AbstractEventLoop.create_connection(protocol_factory, host=None, port=None, \*, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None, ssl_handshake_timeout=None)

   Create a streaming transport connection to a given Internet *host* and
   *port*: socket family :py:data:`~socket.AF_INET` or
   :py:data:`~socket.AF_INET6` depending on *host* (or *family* if specified),
   socket type :py:data:`~socket.SOCK_STREAM`.  *protocol_factory* must be a
   callable returning a :ref:`protocol <asyncio-protocol>` instance.

   This method will try to establish the connection in the background.
   When successful, it returns a ``(transport, protocol)`` pair.

   The chronological synopsis of the underlying operation is as follows:

   #. The connection is established, and a :ref:`transport <asyncio-transport>`
      is created to represent it.

   #. *protocol_factory* is called without arguments and must return a
      :ref:`protocol <asyncio-protocol>` instance.

   #. The protocol instance is tied to the transport, and its
      :meth:`connection_made` method is called.

   #. The coroutine returns successfully with the ``(transport, protocol)``
      pair.

   The created transport is an implementation-dependent bidirectional stream.

   .. note::
      *protocol_factory* can be any kind of callable, not necessarily
      a class.  For example, if you want to use a pre-created
      protocol instance, you can pass ``lambda: my_protocol``.

   Options that change how the connection is created:

   * *ssl*: if given and not false, a SSL/TLS transport is created
     (by default a plain TCP transport is created).  If *ssl* is
     a :class:`ssl.SSLContext` object, this context is used to create
     the transport; if *ssl* is :const:`True`, a context with some
     unspecified default settings is used.

     .. seealso:: :ref:`SSL/TLS security considerations <ssl-security>`

   * *server_hostname*, is only for use together with *ssl*,
     and sets or overrides the hostname that the target server's certificate
     will be matched against.  By default the value of the *host* argument
     is used.  If *host* is empty, there is no default and you must pass a
     value for *server_hostname*.  If *server_hostname* is an empty
     string, hostname matching is disabled (which is a serious security
     risk, allowing for man-in-the-middle-attacks).

   * *family*, *proto*, *flags* are the optional address family, protocol
     and flags to be passed through to getaddrinfo() for *host* resolution.
     If given, these should all be integers from the corresponding
     :mod:`socket` module constants.

   * *sock*, if given, should be an existing, already connected
     :class:`socket.socket` object to be used by the transport.
     If *sock* is given, none of *host*, *port*, *family*, *proto*, *flags*
     and *local_addr* should be specified.

   * *local_addr*, if given, is a ``(local_host, local_port)`` tuple used
     to bind the socket to locally.  The *local_host* and *local_port*
     are looked up using getaddrinfo(), similarly to *host* and *port*.

   * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds
     to wait for the SSL handshake to complete before aborting the connection.
     ``10.0`` seconds if ``None`` (default).

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionchanged:: 3.5

      On Windows with :class:`ProactorEventLoop`, SSL/TLS is now supported.

   .. seealso::

      The :func:`open_connection` function can be used to get a pair of
      (:class:`StreamReader`, :class:`StreamWriter`) instead of a protocol.


.. coroutinemethod:: AbstractEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, \*, family=0, proto=0, flags=0, reuse_address=None, reuse_port=None, allow_broadcast=None, sock=None)

   Create datagram connection: socket family :py:data:`~socket.AF_INET`,
   :py:data:`~socket.AF_INET6` or :py:data:`~socket.AF_UNIX` depending on
   *host* (or *family* if specified), socket type
   :py:data:`~socket.SOCK_DGRAM`. *protocol_factory* must be a
   callable returning a :ref:`protocol <asyncio-protocol>` instance.

   This method will try to establish the connection in the background.
   When successful, the it returns a ``(transport, protocol)`` pair.

   Options changing how the connection is created:

   * *local_addr*, if given, is a ``(local_host, local_port)`` tuple used
     to bind the socket to locally.  The *local_host* and *local_port*
     are looked up using :meth:`getaddrinfo`.

   * *remote_addr*, if given, is a ``(remote_host, remote_port)`` tuple used
     to connect the socket to a remote address.  The *remote_host* and
     *remote_port* are looked up using :meth:`getaddrinfo`.

   * *family*, *proto*, *flags* are the optional address family, protocol
     and flags to be passed through to :meth:`getaddrinfo` for *host*
     resolution. If given, these should all be integers from the
     corresponding :mod:`socket` module constants.

   * *reuse_address* tells the kernel to reuse a local socket in
     TIME_WAIT state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to ``True`` on
     UNIX.

   * *reuse_port* tells the kernel to allow this endpoint to be bound to the
     same port as other existing endpoints are bound to, so long as they all
     set this flag when being created. This option is not supported on Windows
     and some UNIX's. If the :py:data:`~socket.SO_REUSEPORT` constant is not
     defined then this capability is unsupported.

   * *allow_broadcast* tells the kernel to allow this endpoint to send
     messages to the broadcast address.

   * *sock* can optionally be specified in order to use a preexisting,
     already connected, :class:`socket.socket` object to be used by the
     transport. If specified, *local_addr* and *remote_addr* should be omitted
     (must be :const:`None`).

   On Windows with :class:`ProactorEventLoop`, this method is not supported.

   See :ref:`UDP echo client protocol <asyncio-udp-echo-client-protocol>` and
   :ref:`UDP echo server protocol <asyncio-udp-echo-server-protocol>` examples.


.. coroutinemethod:: AbstractEventLoop.create_unix_connection(protocol_factory, path=None, \*, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None)

   Create UNIX connection: socket family :py:data:`~socket.AF_UNIX`, socket
   type :py:data:`~socket.SOCK_STREAM`. The :py:data:`~socket.AF_UNIX` socket
   family is used to communicate between processes on the same machine
   efficiently.

   This method will try to establish the connection in the background.
   When successful, the it returns a ``(transport, protocol)`` pair.

   *path* is the name of a UNIX domain socket, and is required unless a *sock*
   parameter is specified.  Abstract UNIX sockets, :class:`str`,
   :class:`bytes`, and :class:`~pathlib.Path` paths are supported.

   See the :meth:`AbstractEventLoop.create_connection` method for parameters.

   Availability: UNIX.

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionchanged:: 3.7

      The *path* parameter can now be a :class:`~pathlib.Path` object.


Creating listening connections
------------------------------

.. coroutinemethod:: AbstractEventLoop.create_server(protocol_factory, host=None, port=None, \*, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None, reuse_port=None, ssl_handshake_timeout=None, start_serving=True)

   Create a TCP server (socket type :data:`~socket.SOCK_STREAM`) bound to
   *host* and *port*.

   Return a :class:`Server` object, its :attr:`~Server.sockets` attribute
   contains created sockets. Use the :meth:`Server.close` method to stop the
   server: close listening sockets.

   Parameters:

   * The *host* parameter can be a string, in that case the TCP server is
     bound to *host* and *port*. The *host* parameter can also be a sequence
     of strings and in that case the TCP server is bound to all hosts of the
     sequence. If *host* is an empty string or ``None``, all interfaces are
     assumed and a list of multiple sockets will be returned (most likely one
     for IPv4 and another one for IPv6).

   * *family* can be set to either :data:`socket.AF_INET` or
     :data:`~socket.AF_INET6` to force the socket to use IPv4 or IPv6. If not set
     it will be determined from host (defaults to :data:`socket.AF_UNSPEC`).

   * *flags* is a bitmask for :meth:`getaddrinfo`.

   * *sock* can optionally be specified in order to use a preexisting
     socket object. If specified, *host* and *port* should be omitted (must be
     :const:`None`).

   * *backlog* is the maximum number of queued connections passed to
     :meth:`~socket.socket.listen` (defaults to 100).

   * *ssl* can be set to an :class:`~ssl.SSLContext` to enable SSL over the
     accepted connections.

   * *reuse_address* tells the kernel to reuse a local socket in
     TIME_WAIT state, without waiting for its natural timeout to
     expire. If not specified will automatically be set to ``True`` on
     UNIX.

   * *reuse_port* tells the kernel to allow this endpoint to be bound to the
     same port as other existing endpoints are bound to, so long as they all
     set this flag when being created. This option is not supported on
     Windows.

   * *ssl_handshake_timeout* is (for an SSL server) the time in seconds to wait
     for the SSL handshake to complete before aborting the connection.
     ``10.0`` seconds if ``None`` (default).

   * *start_serving* set to ``True`` (the default) causes the created server
     to start accepting connections immediately.  When set to ``False``,
     the user should await on :meth:`Server.start_serving` or
     :meth:`Server.serve_forever` to make the server to start accepting
     connections.

   .. versionadded:: 3.7

      *ssl_handshake_timeout* and *start_serving* parameters.

   .. versionchanged:: 3.5

      On Windows with :class:`ProactorEventLoop`, SSL/TLS is now supported.

   .. seealso::

      The function :func:`start_server` creates a (:class:`StreamReader`,
      :class:`StreamWriter`) pair and calls back a function with this pair.

   .. versionchanged:: 3.5.1

      The *host* parameter can now be a sequence of strings.


.. coroutinemethod:: AbstractEventLoop.create_unix_server(protocol_factory, path=None, \*, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, start_serving=True)

   Similar to :meth:`AbstractEventLoop.create_server`, but specific to the
   socket family :py:data:`~socket.AF_UNIX`.

   *path* is the name of a UNIX domain socket, and is required unless a *sock*
   parameter is specified.  Abstract UNIX sockets, :class:`str`,
   :class:`bytes`, and :class:`~pathlib.Path` paths are supported.

   Availability: UNIX.

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionchanged:: 3.7

      The *path* parameter can now be a :class:`~pathlib.Path` object.

.. coroutinemethod:: BaseEventLoop.connect_accepted_socket(protocol_factory, sock, \*, ssl=None, ssl_handshake_timeout=None)

   Handle an accepted connection.

   This is used by servers that accept connections outside of
   asyncio but that use asyncio to handle them.

   Parameters:

   * *sock* is a preexisting socket object returned from an ``accept``
     call.

   * *ssl* can be set to an :class:`~ssl.SSLContext` to enable SSL over the
     accepted connections.

   * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to
     wait for the SSL handshake to complete before aborting the connection.
     ``10.0`` seconds if ``None`` (default).

   When completed it returns a ``(transport, protocol)`` pair.

   .. versionadded:: 3.7

      The *ssl_handshake_timeout* parameter.

   .. versionadded:: 3.5.3


File Transferring
-----------------

.. coroutinemethod:: AbstractEventLoop.sendfile(transport, file, \
                                                offset=0, count=None, \
                                                *, fallback=True)

   Send a *file* to *transport*, return the total number of bytes
   which were sent.

   The method uses high-performance :meth:`os.sendfile` if available.

   *file* must be a regular file object opened in binary mode.

   *offset* tells from where to start reading the file. If specified,
   *count* is the total number of bytes to transmit as opposed to
   sending the file until EOF is reached. File position is updated on
   return or also in case of error in which case :meth:`file.tell()
   <io.IOBase.tell>` can be used to figure out the number of bytes
   which were sent.

   *fallback* set to ``True`` makes asyncio to manually read and send
   the file when the platform does not support the sendfile syscall
   (e.g. Windows or SSL socket on Unix).

   Raise :exc:`SendfileNotAvailableError` if the system does not support
   *sendfile* syscall and *fallback* is ``False``.

   .. versionadded:: 3.7


TLS Upgrade
-----------

.. coroutinemethod:: AbstractEventLoop.start_tls(transport, protocol, sslcontext, \*, server_side=False, server_hostname=None, ssl_handshake_timeout=None)

   Upgrades an existing connection to TLS.

   Returns a new transport instance, that the *protocol* must start using
   immediately after the *await*.  The *transport* instance passed to
   the *start_tls* method should never be used again.

   Parameters:

   * *transport* and *protocol* instances that methods like
     :meth:`~AbstractEventLoop.create_server` and
     :meth:`~AbstractEventLoop.create_connection` return.

   * *sslcontext*: a configured instance of :class:`~ssl.SSLContext`.

   * *server_side* pass ``True`` when a server-side connection is being
     upgraded (like the one created by :meth:`~AbstractEventLoop.create_server`).

   * *server_hostname*: sets or overrides the host name that the target
     server's certificate will be matched against.

   * *ssl_handshake_timeout* is (for an SSL connection) the time in seconds to
     wait for the SSL handshake to complete before aborting the connection.
     ``10.0`` seconds if ``None`` (default).

   .. versionadded:: 3.7


Watch file descriptors
----------------------

On Windows with :class:`SelectorEventLoop`, only socket handles are supported
(ex: pipe file descriptors are not supported).

On Windows with :class:`ProactorEventLoop`, these methods are not supported.

.. method:: AbstractEventLoop.add_reader(fd, callback, \*args)

   Start watching the file descriptor for read availability and then call the
   *callback* with specified arguments.

   :ref:`Use functools.partial to pass keywords to the callback
   <asyncio-pass-keywords>`.

.. method:: AbstractEventLoop.remove_reader(fd)

   Stop watching the file descriptor for read availability.

.. method:: AbstractEventLoop.add_writer(fd, callback, \*args)

   Start watching the file descriptor for write availability and then call the
   *callback* with specified arguments.

   :ref:`Use functools.partial to pass keywords to the callback
   <asyncio-pass-keywords>`.

.. method:: AbstractEventLoop.remove_writer(fd)

   Stop watching the file descriptor for write availability.

The :ref:`watch a file descriptor for read events <asyncio-watch-read-event>`
example uses the low-level :meth:`AbstractEventLoop.add_reader` method to register
the file descriptor of a socket.


Low-level socket operations
---------------------------

.. coroutinemethod:: AbstractEventLoop.sock_recv(sock, nbytes)

   Receive data from the socket.  Modeled after blocking
   :meth:`socket.socket.recv` method.

   The return value is a bytes object
   representing the data received.  The maximum amount of data to be received
   at once is specified by *nbytes*.

   With :class:`SelectorEventLoop` event loop, the socket *sock* must be
   non-blocking.

   .. versionchanged:: 3.7
      Even though the method was always documented as a coroutine
      method, before Python 3.7 it returned a :class:`Future`.
      Since Python 3.7, this is an ``async def`` method.

.. coroutinemethod:: AbstractEventLoop.sock_recv_into(sock, buf)

   Receive data from the socket.  Modeled after blocking
   :meth:`socket.socket.recv_into` method.

   The received data is written into *buf* (a writable buffer).
   The return value is the number of bytes written.

   With :class:`SelectorEventLoop` event loop, the socket *sock* must be
   non-blocking.

   .. versionadded:: 3.7

.. coroutinemethod:: AbstractEventLoop.sock_sendall(sock, data)

   Send data to the socket.  Modeled after blocking
   :meth:`socket.socket.sendall` method.

   The socket must be connected to a remote socket.
   This method continues to send data from *data* until either all data has
   been sent or an error occurs.  ``None`` is returned on success.  On error,
   an exception is raised, and there is no way to determine how much data, if
   any, was successfully processed by the receiving end of the connection.

   With :class:`SelectorEventLoop` event loop, the socket *sock* must be
   non-blocking.

   .. versionchanged:: 3.7
      Even though the method was always documented as a coroutine
      method, before Python 3.7 it returned an :class:`Future`.
      Since Python 3.7, this is an ``async def`` method.

.. coroutinemethod:: AbstractEventLoop.sock_connect(sock, address)

   Connect to a remote socket at *address*.  Modeled after
   blocking :meth:`socket.socket.connect` method.

   With :class:`SelectorEventLoop` event loop, the socket *sock* must be
   non-blocking.

   .. versionchanged:: 3.5.2
      ``address`` no longer needs to be resolved.  ``sock_connect``
      will try to check if the *address* is already resolved by calling
      :func:`socket.inet_pton`.  If not,
      :meth:`AbstractEventLoop.getaddrinfo` will be used to resolve the
      *address*.

   .. seealso::

      :meth:`AbstractEventLoop.create_connection`
      and  :func:`asyncio.open_connection() <open_connection>`.


.. coroutinemethod:: AbstractEventLoop.sock_accept(sock)

   Accept a connection.  Modeled after blocking
   :meth:`socket.socket.accept`.

   The socket must be bound to an address and listening
   for connections. The return value is a pair ``(conn, address)`` where *conn*
   is a *new* socket object usable to send and receive data on the connection,
   and *address* is the address bound to the socket on the other end of the
   connection.

   The socket *sock* must be non-blocking.

   .. versionchanged:: 3.7
      Even though the method was always documented as a coroutine
      method, before Python 3.7 it returned a :class:`Future`.
      Since Python 3.7, this is an ``async def`` method.

   .. seealso::

      :meth:`AbstractEventLoop.create_server` and :func:`start_server`.

.. coroutinemethod:: AbstractEventLoop.sock_sendfile(sock, file, \
                                                     offset=0, count=None, \
                                                     *, fallback=True)

   Send a file using high-performance :mod:`os.sendfile` if possible
   and return the total number of bytes which were sent.

   Asynchronous version of :meth:`socket.socket.sendfile`.

   *sock* must be non-blocking :class:`~socket.socket` of
   :const:`socket.SOCK_STREAM` type.

   *file* must be a regular file object opened in binary mode.

   *offset* tells from where to start reading the file. If specified,
   *count* is the total number of bytes to transmit as opposed to
   sending the file until EOF is reached. File position is updated on
   return or also in case of error in which case :meth:`file.tell()
   <io.IOBase.tell>` can be used to figure out the number of bytes
   which were sent.

   *fallback* set to ``True`` makes asyncio to manually read and send
   the file when the platform does not support the sendfile syscall
   (e.g. Windows or SSL socket on Unix).

   Raise :exc:`SendfileNotAvailableError` if the system does not support
   *sendfile* syscall and *fallback* is ``False``.

   .. versionadded:: 3.7


Resolve host name
-----------------

.. coroutinemethod:: AbstractEventLoop.getaddrinfo(host, port, \*, family=0, type=0, proto=0, flags=0)

   This method is a :ref:`coroutine <coroutine>`, similar to
   :meth:`socket.getaddrinfo` function but non-blocking.

.. coroutinemethod:: AbstractEventLoop.getnameinfo(sockaddr, flags=0)

   This method is a :ref:`coroutine <coroutine>`, similar to
   :meth:`socket.getnameinfo` function but non-blocking.


Connect pipes
-------------

On Windows with :class:`SelectorEventLoop`, these methods are not supported.
Use :class:`ProactorEventLoop` to support pipes on Windows.

.. coroutinemethod:: AbstractEventLoop.connect_read_pipe(protocol_factory, pipe)

   Register read pipe in eventloop.

   *protocol_factory* should instantiate object with :class:`Protocol`
   interface.  *pipe* is a :term:`file-like object <file object>`.
   Return pair ``(transport, protocol)``, where *transport* supports the
   :class:`ReadTransport` interface.

   With :class:`SelectorEventLoop` event loop, the *pipe* is set to
   non-blocking mode.

.. coroutinemethod:: AbstractEventLoop.connect_write_pipe(protocol_factory, pipe)

   Register write pipe in eventloop.

   *protocol_factory* should instantiate object with :class:`BaseProtocol`
   interface. *pipe* is :term:`file-like object <file object>`.
   Return pair ``(transport, protocol)``, where *transport* supports
   :class:`WriteTransport` interface.

   With :class:`SelectorEventLoop` event loop, the *pipe* is set to
   non-blocking mode.

.. seealso::

   The :meth:`AbstractEventLoop.subprocess_exec` and
   :meth:`AbstractEventLoop.subprocess_shell` methods.


UNIX signals
------------

Availability: UNIX only.

.. method:: AbstractEventLoop.add_signal_handler(signum, callback, \*args)

   Add a handler for a signal.

   Raise :exc:`ValueError` if the signal number is invalid or uncatchable.
   Raise :exc:`RuntimeError` if there is a problem setting up the handler.

   :ref:`Use functools.partial to pass keywords to the callback
   <asyncio-pass-keywords>`.

.. method:: AbstractEventLoop.remove_signal_handler(sig)

   Remove a handler for a signal.

   Return ``True`` if a signal handler was removed, ``False`` if not.

.. seealso::

   The :mod:`signal` module.


Executor
--------

Call a function in an :class:`~concurrent.futures.Executor` (pool of threads or
pool of processes). By default, an event loop uses a thread pool executor
(:class:`~concurrent.futures.ThreadPoolExecutor`).

.. coroutinemethod:: AbstractEventLoop.run_in_executor(executor, func, \*args)

   Arrange for a *func* to be called in the specified executor.

   The *executor* argument should be an :class:`~concurrent.futures.Executor`
   instance. The default executor is used if *executor* is ``None``.

   :ref:`Use functools.partial to pass keywords to the *func*
   <asyncio-pass-keywords>`.

   .. versionchanged:: 3.5.3
      :meth:`BaseEventLoop.run_in_executor` no longer configures the
      ``max_workers`` of the thread pool executor it creates, instead
      leaving it up to the thread pool executor
      (:class:`~concurrent.futures.ThreadPoolExecutor`) to set the
      default.

   .. versionchanged:: 3.7
      Even though the method was always documented as a coroutine
      method, before Python 3.7 it returned a :class:`Future`.
      Since Python 3.7, this is an ``async def`` method.

.. method:: AbstractEventLoop.set_default_executor(executor)

   Set the default executor used by :meth:`run_in_executor`.


Error Handling API
------------------

Allows customizing how exceptions are handled in the event loop.

.. method:: AbstractEventLoop.set_exception_handler(handler)

   Set *handler* as the new event loop exception handler.

   If *handler* is ``None``, the default exception handler will
   be set.

   If *handler* is a callable object, it should have a
   matching signature to ``(loop, context)``, where ``loop``
   will be a reference to the active event loop, ``context``
   will be a ``dict`` object (see :meth:`call_exception_handler`
   documentation for details about context).

.. method:: AbstractEventLoop.get_exception_handler()

   Return the exception handler, or ``None`` if the default one
   is in use.

   .. versionadded:: 3.5.2

.. method:: AbstractEventLoop.default_exception_handler(context)

   Default exception handler.

   This is called when an exception occurs and no exception
   handler is set, and can be called by a custom exception
   handler that wants to defer to the default behavior.

   *context* parameter has the same meaning as in
   :meth:`call_exception_handler`.

.. method:: AbstractEventLoop.call_exception_handler(context)

   Call the current event loop exception handler.

   *context* is a ``dict`` object containing the following keys
   (new keys may be introduced later):

   * 'message': Error message;
   * 'exception' (optional): Exception object;
   * 'future' (optional): :class:`asyncio.Future` instance;
   * 'handle' (optional): :class:`asyncio.Handle` instance;
   * 'protocol' (optional): :ref:`Protocol <asyncio-protocol>` instance;
   * 'transport' (optional): :ref:`Transport <asyncio-transport>` instance;
   * 'socket' (optional): :class:`socket.socket` instance.

   .. note::

       Note: this method should not be overloaded in subclassed
       event loops.  For any custom exception handling, use
       :meth:`set_exception_handler()` method.

Debug mode
----------

.. method:: AbstractEventLoop.get_debug()

   Get the debug mode (:class:`bool`) of the event loop.

   The default value is ``True`` if the environment variable
   :envvar:`PYTHONASYNCIODEBUG` is set to a non-empty string, ``False``
   otherwise.

   .. versionadded:: 3.4.2

.. method:: AbstractEventLoop.set_debug(enabled: bool)

   Set the debug mode of the event loop.

   .. versionadded:: 3.4.2

.. seealso::

   The :ref:`debug mode of asyncio <asyncio-debug-mode>`.

Server
------

.. class:: Server

   Server listening on sockets.

   Object created by :meth:`AbstractEventLoop.create_server`,
   :meth:`AbstractEventLoop.create_unix_server`, :func:`start_server`,
   and :func:`start_unix_server` functions.  Don't instantiate the class
   directly.

   *Server* objects are asynchronous context managers.  When used in an
   ``async with`` statement, it's guaranteed that the Server object is
   closed and not accepting new connections when the ``async with``
   statement is completed::

      srv = await loop.create_server(...)

      async with srv:
          # some code

      # At this point, srv is closed and no longer accepts new connections.


   .. versionchanged:: 3.7
      Server object is an asynchronous context manager since Python 3.7.

   .. method:: close()

      Stop serving: close listening sockets and set the :attr:`sockets`
      attribute to ``None``.

      The sockets that represent existing incoming client connections are left
      open.

      The server is closed asynchronously, use the :meth:`wait_closed`
      coroutine to wait until the server is closed.

   .. method:: get_loop()

      Gives the event loop associated with the server object.

      .. versionadded:: 3.7

   .. coroutinemethod:: start_serving()

      Start accepting connections.

      This method is idempotent, so it can be called when
      the server is already being serving.

      The new *start_serving* keyword-only parameter to
      :meth:`AbstractEventLoop.create_server` and
      :meth:`asyncio.start_server` allows to create a Server object
      that is not accepting connections right away.  In which case
      this method, or :meth:`Server.serve_forever` can be used
      to make the Server object to start accepting connections.

      .. versionadded:: 3.7

   .. coroutinemethod:: serve_forever()

      Start accepting connections until the coroutine is cancelled.
      Cancellation of ``serve_forever`` task causes the server
      to be closed.

      This method can be called if the server is already accepting
      connections.  Only one ``serve_forever`` task can exist per
      one *Server* object.

      Example::

          async def client_connected(reader, writer):
              # Communicate with the client with
              # reader/writer streams.  For example:
              await reader.readline()

          async def main(host, port):
              srv = await asyncio.start_server(
                  client_connected, host, port)
              await loop.serve_forever()

          asyncio.run(main('127.0.0.1', 0))

      .. versionadded:: 3.7

   .. method:: is_serving()

      Return ``True`` if the server is accepting new connections.

      .. versionadded:: 3.7

   .. coroutinemethod:: wait_closed()

      Wait until the :meth:`close` method completes.

   .. attribute:: sockets

      List of :class:`socket.socket` objects the server is listening to, or
      ``None`` if the server is closed.

      .. versionchanged:: 3.7
         Prior to Python 3.7 ``Server.sockets`` used to return the
         internal list of server's sockets directly.  In 3.7 a copy
         of that list is returned.


Handle
------

.. class:: Handle

   A callback wrapper object returned by :func:`AbstractEventLoop.call_soon`,
   :func:`AbstractEventLoop.call_soon_threadsafe`, :func:`AbstractEventLoop.call_later`,
   and :func:`AbstractEventLoop.call_at`.

   .. method:: cancel()

      Cancel the call.  If the callback is already canceled or executed,
      this method has no effect.

   .. method:: cancelled()

      Return ``True`` if the call was cancelled.

      .. versionadded:: 3.7


SendfileNotAvailableError
-------------------------


.. exception:: SendfileNotAvailableError

   Sendfile syscall is not available, subclass of :exc:`RuntimeError`.

   Raised if the OS does not support senfile syscall for
   given socket or file type.


Event loop examples
-------------------

.. _asyncio-hello-world-callback:

Hello World with call_soon()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example using the :meth:`AbstractEventLoop.call_soon` method to schedule a
callback. The callback displays ``"Hello World"`` and then stops the event
loop::

    import asyncio

    def hello_world(loop):
        print('Hello World')
        loop.stop()

    loop = asyncio.get_event_loop()

    # Schedule a call to hello_world()
    loop.call_soon(hello_world, loop)

    # Blocking call interrupted by loop.stop()
    loop.run_forever()
    loop.close()

.. seealso::

   The :ref:`Hello World coroutine <asyncio-hello-world-coroutine>` example
   uses a :ref:`coroutine <coroutine>`.


.. _asyncio-date-callback:

Display the current date with call_later()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Example of callback displaying the current date every second. The callback uses
the :meth:`AbstractEventLoop.call_later` method to reschedule itself during 5
seconds, and then stops the event loop::

    import asyncio
    import datetime

    def display_date(end_time, loop):
        print(datetime.datetime.now())
        if (loop.time() + 1.0) < end_time:
            loop.call_later(1, display_date, end_time, loop)
        else:
            loop.stop()

    loop = asyncio.get_event_loop()

    # Schedule the first call to display_date()
    end_time = loop.time() + 5.0
    loop.call_soon(display_date, end_time, loop)

    # Blocking call interrupted by loop.stop()
    loop.run_forever()
    loop.close()

.. seealso::

   The :ref:`coroutine displaying the current date
   <asyncio-date-coroutine>` example uses a :ref:`coroutine
   <coroutine>`.


.. _asyncio-watch-read-event:

Watch a file descriptor for read events
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Wait until a file descriptor received some data using the
:meth:`AbstractEventLoop.add_reader` method and then close the event loop::

    import asyncio
    from socket import socketpair

    # Create a pair of connected file descriptors
    rsock, wsock = socketpair()
    loop = asyncio.get_event_loop()

    def reader():
        data = rsock.recv(100)
        print("Received:", data.decode())
        # We are done: unregister the file descriptor
        loop.remove_reader(rsock)
        # Stop the event loop
        loop.stop()

    # Register the file descriptor for read event
    loop.add_reader(rsock, reader)

    # Simulate the reception of data from the network
    loop.call_soon(wsock.send, 'abc'.encode())

    # Run the event loop
    loop.run_forever()

    # We are done, close sockets and the event loop
    rsock.close()
    wsock.close()
    loop.close()

.. seealso::

   The :ref:`register an open socket to wait for data using a protocol
   <asyncio-register-socket>` example uses a low-level protocol created by the
   :meth:`AbstractEventLoop.create_connection` method.

   The :ref:`register an open socket to wait for data using streams
   <asyncio-register-socket-streams>` example uses high-level streams
   created by the :func:`open_connection` function in a coroutine.


Set signal handlers for SIGINT and SIGTERM
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Register handlers for signals :py:data:`SIGINT` and :py:data:`SIGTERM` using
the :meth:`AbstractEventLoop.add_signal_handler` method::

    import asyncio
    import functools
    import os
    import signal

    def ask_exit(signame):
        print("got signal %s: exit" % signame)
        loop.stop()

    loop = asyncio.get_event_loop()
    for signame in ('SIGINT', 'SIGTERM'):
        loop.add_signal_handler(getattr(signal, signame),
                                functools.partial(ask_exit, signame))

    print("Event loop running forever, press Ctrl+C to interrupt.")
    print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
    try:
        loop.run_forever()
    finally:
        loop.close()

This example only works on UNIX.