:mod:`asyncio` -- Asynchronous I/O, event loop, coroutines and tasks ==================================================================== .. module:: asyncio :synopsis: Asynchronous I/O, event loop, coroutines and tasks. .. versionadded:: 3.4 This module provides infrastructure for writing single-threaded concurrent code using coroutines, multiplexing I/O access over sockets and other resources, running network clients and servers, and other related primitives. Here is a more detailed list of the package contents: * a pluggable :ref:`event loop ` with various system-specific implementations; * :ref:`transport ` and :ref:`protocol ` abstractions (similar to those in `Twisted `_); * concrete support for TCP, UDP, SSL, subprocess pipes, delayed calls, and others (some may be system-dependent); * a Future class that mimicks the one in the :mod:`concurrent.futures` module, but adapted for use with the event loop; * coroutines and tasks based on ``yield from`` (:PEP:`380`), to help write concurrent code in a sequential fashion; * cancellation support for Futures and coroutines; * :ref:`synchronization primitives ` for use between coroutines in a single thread, mimicking those in the :mod:`threading` module; * an interface for passing work off to a threadpool, for times when you absolutely, positively have to use a library that makes blocking I/O calls. Disclaimer ---------- Full documentation is not yet ready; we hope to have it written before Python 3.4 leaves beta. Until then, the best reference is :PEP:`3156`. For a motivational primer on transports and protocols, see :PEP:`3153`. .. XXX should the asyncio documentation come in several pages, as for logging? .. _event-loop: Event loops ----------- .. _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 ` (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` and :class:`SubprocessProtocol` instances: .. method:: 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:: 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:`connection_made` and :meth:`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:: 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:: 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:: process_exited() Called when the child process has exited. Data reception callbacks ^^^^^^^^^^^^^^^^^^^^^^^^ Streaming protocols """"""""""""""""""" The following callbacks are called on :class:`Protocol` instances: .. method:: 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:: 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. Datagram protocols """""""""""""""""" The following callbacks are called on :class:`DatagramProtocol` instances. .. method:: 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:: 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` and :class:`SubprocessProtocol` instances: .. method:: pause_writing() Called when the transport's buffer goes over the high-water mark. .. method:: 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. .. _transport: Transports ---------- .. _sync: Synchronization primitives -------------------------- Examples -------- A :class:`Protocol` implementing an echo server:: class EchoServer(asyncio.Protocol): TIMEOUT = 5.0 def timeout(self): print('connection timeout, closing.') self.transport.close() def connection_made(self, transport): print('connection made') self.transport = transport # start 5 seconds timeout timer self.h_timeout = asyncio.get_event_loop().call_later( self.TIMEOUT, self.timeout) def data_received(self, data): print('data received: ', data.decode()) self.transport.write(b'Re: ' + data) # restart timeout timer self.h_timeout.cancel() self.h_timeout = asyncio.get_event_loop().call_later( self.TIMEOUT, self.timeout) def eof_received(self): pass def connection_lost(self, exc): print('connection lost:', exc) self.h_timeout.cancel()