summaryrefslogtreecommitdiffstats
path: root/Doc/library/asyncio-protocol.rst
blob: 656816f75b020cfa752f280c7b63b4df5aba0d5e (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
.. currentmodule:: asyncio

+++++++++++++++++++++++++++++++++++++++++
Transports  and protocols (low-level API)
+++++++++++++++++++++++++++++++++++++++++

.. _asyncio-transport:

Transports
==========

Transports are classes provided by :mod:`asyncio` in order to abstract
various kinds of communication channels.  You generally won't instantiate
a transport yourself; instead, you will call a :class:`BaseEventLoop` method
which will create the transport and try to initiate the underlying
communication channel, calling you back when it succeeds.

Once the communication channel is established, a transport is always
paired with a :ref:`protocol <asyncio-protocol>` instance.  The protocol can
then call the transport's methods for various purposes.

:mod:`asyncio` currently implements transports for TCP, UDP, SSL, and
subprocess pipes.  The methods available on a transport depend on
the transport's kind.

The transport classes are :ref:`not thread safe <asyncio-multithreading>`.


BaseTransport
-------------

.. class:: BaseTransport

   Base class for transports.

   .. method:: close(self)

      Close the transport.  If the transport has a buffer for outgoing
      data, buffered data will be flushed asynchronously.  No more data
      will be received.  After all buffered data is flushed, the
      protocol's :meth:`connection_lost` method will be called with
      :const:`None` as its argument.


   .. method:: get_extra_info(name, default=None)

      Return optional transport information.  *name* is a string representing
      the piece of transport-specific information to get, *default* is the
      value to return if the information doesn't exist.

      This method allows transport implementations to easily expose
      channel-specific information.

      * socket:

        - ``'peername'``: the remote address to which the socket is connected,
          result of :meth:`socket.socket.getpeername` (``None`` on error)
        - ``'socket'``: :class:`socket.socket` instance
        - ``'sockname'``: the socket's own address,
          result of :meth:`socket.socket.getsockname`

      * SSL socket:

        - ``'compression'``: the compression algorithm being used as a string,
          or ``None`` if the connection isn't compressed; result of
          :meth:`ssl.SSLSocket.compression`
        - ``'cipher'``: a three-value tuple containing the name of the cipher
          being used, the version of the SSL protocol that defines its use, and
          the number of secret bits being used; result of
          :meth:`ssl.SSLSocket.cipher`
        - ``'peercert'``: peer certificate; result of
          :meth:`ssl.SSLSocket.getpeercert`
        - ``'sslcontext'``: :class:`ssl.SSLContext` instance
        - ``'ssl_object'``: :class:`ssl.SSLObject` or :class:`ssl.SSLSocket`
          instance

      * pipe:

        - ``'pipe'``: pipe object

      * subprocess:

        - ``'subprocess'``: :class:`subprocess.Popen` instance

   .. versionchanged:: 3.4.4
      ``'ssl_object'`` info was added to SSL sockets.


ReadTransport
-------------

.. class:: ReadTransport

   Interface for read-only transports.

   .. method:: pause_reading()

      Pause the receiving end of the transport.  No data will be passed to
      the protocol's :meth:`data_received` method until :meth:`resume_reading`
      is called.

   .. method:: resume_reading()

      Resume the receiving end.  The protocol's :meth:`data_received` method
      will be called once again if some data is available for reading.


WriteTransport
--------------

.. class:: WriteTransport

   Interface for write-only transports.

   .. method:: abort()

      Close the transport immediately, without waiting for pending operations
      to complete.  Buffered data will be lost.  No more data will be received.
      The protocol's :meth:`connection_lost` method will eventually be
      called with :const:`None` as its argument.

   .. method:: can_write_eof()

      Return :const:`True` if the transport supports :meth:`write_eof`,
      :const:`False` if not.

   .. method:: get_write_buffer_size()

      Return the current size of the output buffer used by the transport.

   .. method:: get_write_buffer_limits()

      Get the *high*- and *low*-water limits for write flow control. Return a
      tuple ``(low, high)`` where *low* and *high* are positive number of
      bytes.

      Use :meth:`set_write_buffer_limits` to set the limits.

      .. versionadded:: 3.4.2

   .. method:: set_write_buffer_limits(high=None, low=None)

      Set the *high*- and *low*-water limits for write flow control.

      These two values control when call the protocol's
      :meth:`pause_writing` and :meth:`resume_writing` methods are called.
      If specified, the low-water limit must be less than or equal to the
      high-water limit.  Neither *high* nor *low* can be negative.

      The defaults are implementation-specific.  If only the
      high-water limit is given, the low-water limit defaults to a
      implementation-specific value less than or equal to the
      high-water limit.  Setting *high* to zero forces *low* to zero as
      well, and causes :meth:`pause_writing` to be called whenever the
      buffer becomes non-empty.  Setting *low* to zero causes
      :meth:`resume_writing` to be called only once the buffer is empty.
      Use of zero for either limit is generally sub-optimal as it
      reduces opportunities for doing I/O and computation
      concurrently.

      Use :meth:`get_write_buffer_limits` to get the limits.

   .. method:: write(data)

      Write some *data* bytes to the transport.

      This method does not block; it buffers the data and arranges for it
      to be sent out asynchronously.

   .. method:: writelines(list_of_data)

      Write a list (or any iterable) of data bytes to the transport.
      This is functionally equivalent to calling :meth:`write` on each
      element yielded by the iterable, but may be implemented more efficiently.

   .. method:: write_eof()

      Close the write end of the transport after flushing buffered data.
      Data may still be received.

      This method can raise :exc:`NotImplementedError` if the transport
      (e.g. SSL) doesn't support half-closes.


DatagramTransport
-----------------

.. method:: DatagramTransport.sendto(data, addr=None)

   Send the *data* bytes to the remote peer given by *addr* (a
   transport-dependent target address).  If *addr* is :const:`None`, the
   data is sent to the target address given on transport creation.

   This method does not block; it buffers the data and arranges for it
   to be sent out asynchronously.

.. method:: DatagramTransport.abort()

   Close the transport immediately, without waiting for pending operations
   to complete.  Buffered data will be lost.  No more data will be received.
   The protocol's :meth:`connection_lost` method will eventually be
   called with :const:`None` as its argument.


BaseSubprocessTransport
-----------------------

.. class:: BaseSubprocessTransport

   .. method:: get_pid()

      Return the subprocess process id as an integer.

   .. method:: get_pipe_transport(fd)

      Return the transport for the communication pipe corresponding to the
      integer file descriptor *fd*:

      * ``0``: readable streaming transport of the standard input (*stdin*),
        or :const:`None` if the subprocess was not created with ``stdin=PIPE``
      * ``1``: writable streaming transport of the standard output (*stdout*),
        or :const:`None` if the subprocess was not created with ``stdout=PIPE``
      * ``2``: writable streaming transport of the standard error (*stderr*),
        or :const:`None` if the subprocess was not created with ``stderr=PIPE``
      * other *fd*: :const:`None`

   .. method:: get_returncode()

      Return the subprocess returncode as an integer or :const:`None`
      if it hasn't returned, similarly to the
      :attr:`subprocess.Popen.returncode` attribute.

   .. method:: kill(self)

      Kill the subprocess, as in :meth:`subprocess.Popen.kill`

      On POSIX systems, the function sends SIGKILL to the subprocess.
      On Windows, this method is an alias for :meth:`terminate`.

   .. method:: send_signal(signal)

      Send the *signal* number to the subprocess, as in
      :meth:`subprocess.Popen.send_signal`.

   .. method:: terminate()

      Ask the subprocess to stop, as in :meth:`subprocess.Popen.terminate`.
      This method is an alias for the :meth:`close` method.

      On POSIX systems, this method sends SIGTERM to the subprocess.
      On Windows, the Windows API function TerminateProcess() is called to
      stop the subprocess.

   .. method:: close()

      Ask the subprocess to stop by calling the :meth:`terminate` method if the
      subprocess hasn't returned yet, and close transports of all pipes
      (*stdin*, *stdout* and *stderr*).


.. _asyncio-protocol:

Protocols
=========

:mod:`asyncio` provides base classes that you can subclass to implement
your network protocols.  Those classes are used in conjunction with
:ref:`transports <asyncio-transport>` (see below): the protocol parses incoming
data and asks for the writing of outgoing data, while the transport is
responsible for the actual I/O and buffering.

When subclassing a protocol class, it is recommended you override certain
methods.  Those methods are callbacks: they will be called by the transport
on certain events (for example when some data is received); you shouldn't
call them yourself, unless you are implementing a transport.

.. note::
   All callbacks have default implementations, which are empty.  Therefore,
   you only need to implement the callbacks for the events in which you
   are interested.


Protocol classes
----------------

.. class:: Protocol

   The base class for implementing streaming protocols (for use with
   e.g. TCP and SSL transports).

.. class:: DatagramProtocol

   The base class for implementing datagram protocols (for use with
   e.g. UDP transports).

.. class:: SubprocessProtocol

   The base class for implementing protocols communicating with child
   processes (through a set of unidirectional pipes).


Connection callbacks
--------------------

These callbacks may be called on :class:`Protocol`, :class:`DatagramProtocol`
and :class:`SubprocessProtocol` instances:

.. method:: BaseProtocol.connection_made(transport)

   Called when a connection is made.

   The *transport* argument is the transport representing the
   connection.  You are responsible for storing it somewhere
   (e.g. as an attribute) if you need to.

.. method:: BaseProtocol.connection_lost(exc)

   Called when the connection is lost or closed.

   The argument is either an exception object or :const:`None`.
   The latter means a regular EOF is received, or the connection was
   aborted or closed by this side of the connection.

:meth:`~BaseProtocol.connection_made` and :meth:`~BaseProtocol.connection_lost`
are called exactly once per successful connection.  All other callbacks will be
called between those two methods, which allows for easier resource management
in your protocol implementation.

The following callbacks may be called only on :class:`SubprocessProtocol`
instances:

.. method:: SubprocessProtocol.pipe_data_received(fd, data)

   Called when the child process writes data into its stdout or stderr pipe.
   *fd* is the integer file descriptor of the pipe.  *data* is a non-empty
   bytes object containing the data.

.. method:: SubprocessProtocol.pipe_connection_lost(fd, exc)

   Called when one of the pipes communicating with the child process
   is closed.  *fd* is the integer file descriptor that was closed.

.. method:: SubprocessProtocol.process_exited()

   Called when the child process has exited.


Streaming protocols
-------------------

The following callbacks are called on :class:`Protocol` instances:

.. method:: Protocol.data_received(data)

   Called when some data is received.  *data* is a non-empty bytes object
   containing the incoming data.

   .. note::
      Whether the data is buffered, chunked or reassembled depends on
      the transport.  In general, you shouldn't rely on specific semantics
      and instead make your parsing generic and flexible enough.  However,
      data is always received in the correct order.

.. method:: Protocol.eof_received()

   Calls when the other end signals it won't send any more data
   (for example by calling :meth:`write_eof`, if the other end also uses
   asyncio).

   This method may return a false value (including None), in which case
   the transport will close itself.  Conversely, if this method returns a
   true value, closing the transport is up to the protocol.  Since the
   default implementation returns None, it implicitly closes the connection.

   .. note::
      Some transports such as SSL don't support half-closed connections,
      in which case returning true from this method will not prevent closing
      the connection.

:meth:`data_received` can be called an arbitrary number of times during
a connection.  However, :meth:`eof_received` is called at most once
and, if called, :meth:`data_received` won't be called after it.

State machine:

    start -> :meth:`~BaseProtocol.connection_made`
    [-> :meth:`~Protocol.data_received` \*]
    [-> :meth:`~Protocol.eof_received` ?]
    -> :meth:`~BaseProtocol.connection_lost` -> end


Datagram protocols
------------------

The following callbacks are called on :class:`DatagramProtocol` instances.

.. method:: DatagramProtocol.datagram_received(data, addr)

   Called when a datagram is received.  *data* is a bytes object containing
   the incoming data.  *addr* is the address of the peer sending the data;
   the exact format depends on the transport.

.. method:: DatagramProtocol.error_received(exc)

   Called when a previous send or receive operation raises an
   :class:`OSError`.  *exc* is the :class:`OSError` instance.

   This method is called in rare conditions, when the transport (e.g. UDP)
   detects that a datagram couldn't be delivered to its recipient.
   In many conditions though, undeliverable datagrams will be silently
   dropped.


Flow control callbacks
----------------------

These callbacks may be called on :class:`Protocol`,
:class:`DatagramProtocol` and :class:`SubprocessProtocol` instances:

.. method:: BaseProtocol.pause_writing()

   Called when the transport's buffer goes over the high-water mark.

.. method:: BaseProtocol.resume_writing()

   Called when the transport's buffer drains below the low-water mark.


:meth:`pause_writing` and :meth:`resume_writing` calls are paired --
:meth:`pause_writing` is called once when the buffer goes strictly over
the high-water mark (even if subsequent writes increases the buffer size
even more), and eventually :meth:`resume_writing` is called once when the
buffer size reaches the low-water mark.

.. note::
   If the buffer size equals the high-water mark,
   :meth:`pause_writing` is not called -- it must go strictly over.
   Conversely, :meth:`resume_writing` is called when the buffer size is
   equal or lower than the low-water mark.  These end conditions
   are important to ensure that things go as expected when either
   mark is zero.

.. note::
   On BSD systems (OS X, FreeBSD, etc.) flow control is not supported
   for :class:`DatagramProtocol`, because send failures caused by
   writing too many packets cannot be detected easily.  The socket
   always appears 'ready' and excess packets are dropped; an
   :class:`OSError` with errno set to :const:`errno.ENOBUFS` may or
   may not be raised; if it is raised, it will be reported to
   :meth:`DatagramProtocol.error_received` but otherwise ignored.


Coroutines and protocols
------------------------

Coroutines can be scheduled in a protocol method using :func:`async`, but there
is no guarantee made about the execution order.  Protocols are not aware of
coroutines created in protocol methods and so will not wait for them.

To have a reliable execution order, use :ref:`stream objects <asyncio-streams>` in a
coroutine with ``yield from``. For example, the :meth:`StreamWriter.drain`
coroutine can be used to wait until the write buffer is flushed.


Protocol examples
=================

.. _asyncio-tcp-echo-client-protocol:

TCP echo client protocol
------------------------

TCP echo client  using the :meth:`BaseEventLoop.create_connection` method, send
data and wait until the connection is closed::

    import asyncio

    class EchoClientProtocol(asyncio.Protocol):
        def __init__(self, message, loop):
            self.message = message
            self.loop = loop

        def connection_made(self, transport):
            transport.write(self.message.encode())
            print('Data sent: {!r}'.format(self.message))

        def data_received(self, data):
            print('Data received: {!r}'.format(data.decode()))

        def connection_lost(self, exc):
            print('The server closed the connection')
            print('Stop the event lop')
            self.loop.stop()

    loop = asyncio.get_event_loop()
    message = 'Hello World!'
    coro = loop.create_connection(lambda: EchoClientProtocol(message, loop),
                                  '127.0.0.1', 8888)
    loop.run_until_complete(coro)
    loop.run_forever()
    loop.close()

The event loop is running twice. The
:meth:`~BaseEventLoop.run_until_complete` method is preferred in this short
example to raise an exception if the server is not listening, instead of
having to write a short coroutine to handle the exception and stop the
running loop. At :meth:`~BaseEventLoop.run_until_complete` exit, the loop is
no longer running, so there is no need to stop the loop in case of an error.

.. seealso::

   The :ref:`TCP echo client using streams <asyncio-tcp-echo-client-streams>`
   example uses the :func:`asyncio.open_connection` function.


.. _asyncio-tcp-echo-server-protocol:

TCP echo server protocol
------------------------

TCP echo server using the :meth:`BaseEventLoop.create_server` method, send back
received data and close the connection::

    import asyncio

    class EchoServerClientProtocol(asyncio.Protocol):
        def connection_made(self, transport):
            peername = transport.get_extra_info('peername')
            print('Connection from {}'.format(peername))
            self.transport = transport

        def data_received(self, data):
            message = data.decode()
            print('Data received: {!r}'.format(message))

            print('Send: {!r}'.format(message))
            self.transport.write(data)

            print('Close the client socket')
            self.transport.close()

    loop = asyncio.get_event_loop()
    # Each client connection will create a new protocol instance
    coro = loop.create_server(EchoServerClientProtocol, '127.0.0.1', 8888)
    server = loop.run_until_complete(coro)

    # Serve requests until Ctrl+C is pressed
    print('Serving on {}'.format(server.sockets[0].getsockname()))
    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass

    # Close the server
    server.close()
    loop.run_until_complete(server.wait_closed())
    loop.close()

:meth:`Transport.close` can be called immediately after
:meth:`WriteTransport.write` even if data are not sent yet on the socket: both
methods are asynchronous. ``yield from`` is not needed because these transport
methods are not coroutines.

.. seealso::

   The :ref:`TCP echo server using streams <asyncio-tcp-echo-server-streams>`
   example uses the :func:`asyncio.start_server` function.


.. _asyncio-udp-echo-client-protocol:

UDP echo client protocol
------------------------

UDP echo client using the :meth:`BaseEventLoop.create_datagram_endpoint`
method, send data and close the transport when we received the answer::

    import asyncio

    class EchoClientProtocol:
        def __init__(self, message, loop):
            self.message = message
            self.loop = loop
            self.transport = None

        def connection_made(self, transport):
            self.transport = transport
            print('Send:', self.message)
            self.transport.sendto(self.message.encode())

        def datagram_received(self, data, addr):
            print("Received:", data.decode())

            print("Close the socket")
            self.transport.close()

        def error_received(self, exc):
            print('Error received:', exc)

        def connection_lost(self, exc):
            print("Socket closed, stop the event loop")
            loop = asyncio.get_event_loop()
            loop.stop()

    loop = asyncio.get_event_loop()
    message = "Hello World!"
    connect = loop.create_datagram_endpoint(
        lambda: EchoClientProtocol(message, loop),
        remote_addr=('127.0.0.1', 9999))
    transport, protocol = loop.run_until_complete(connect)
    loop.run_forever()
    transport.close()
    loop.close()


.. _asyncio-udp-echo-server-protocol:

UDP echo server protocol
------------------------

UDP echo server using the :meth:`BaseEventLoop.create_datagram_endpoint`
method, send back received data::

    import asyncio

    class EchoServerProtocol:
        def connection_made(self, transport):
            self.transport = transport

        def datagram_received(self, data, addr):
            message = data.decode()
            print('Received %r from %s' % (message, addr))
            print('Send %r to %s' % (message, addr))
            self.transport.sendto(data, addr)

    loop = asyncio.get_event_loop()
    print("Starting UDP server")
    # One protocol instance will be created to serve all client requests
    listen = loop.create_datagram_endpoint(
        EchoServerProtocol, local_addr=('127.0.0.1', 9999))
    transport, protocol = loop.run_until_complete(listen)

    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass

    transport.close()
    loop.close()


.. _asyncio-register-socket:

Register an open socket to wait for data using a protocol
---------------------------------------------------------

Wait until a socket receives data using the
:meth:`BaseEventLoop.create_connection` method with a protocol, and then close
the event loop ::

    import asyncio
    try:
        from socket import socketpair
    except ImportError:
        from asyncio.windows_utils import socketpair

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

    class MyProtocol(asyncio.Protocol):
        transport = None

        def connection_made(self, transport):
            self.transport = transport

        def data_received(self, data):
            print("Received:", data.decode())

            # We are done: close the transport (it will call connection_lost())
            self.transport.close()

        def connection_lost(self, exc):
            # The socket has been closed, stop the event loop
            loop.stop()

    # Register the socket to wait for data
    connect_coro = loop.create_connection(MyProtocol, sock=rsock)
    transport, protocol = loop.run_until_complete(connect_coro)

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

   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.