Preface
Transports and Protocols are used by the low-level event loop
APIs such as loop.create_connection(). They use
callback-based programming style and enable high-performance
implementations of network or IPC protocols (e.g. HTTP).
Essentially, transports and protocols should only be used in libraries and frameworks and never in high-level asyncio applications.
This documentation page covers both Transports and Protocols.
Introduction
At the highest level, the transport is concerned with how bytes are transmitted, while the protocol determines which bytes to transmit (and to some extent when).
A different way of saying the same thing: a transport is an abstraction for a socket (or similar I/O endpoint) while a protocol is an abstraction for an application, from the transport’s point of view.
Yet another view is the transport and protocol interfaces together define an abstract interface for using network I/O and interprocess I/O.
There is always a 1:1 relationship between transport and protocol objects: the protocol calls transport methods to send data, while the transport calls protocol methods to pass it data that has been received.
Most of connection oriented event loop methods
(such as loop.create_connection()) usually accept a
protocol_factory argument used to create a Protocol object
for an accepted connection, represented by a Transport object.
Such methods usually return a tuple of (transport, protocol).
Contents
This documentation page contains the following sections:
BaseTransport, ReadTransport, WriteTransport, Transport, DatagramTransport, and SubprocessTransport classes.BaseProtocol, Protocol, BufferedProtocol, DatagramProtocol, and SubprocessProtocol classes.
Source code: Lib/asyncio/transports.py
Transports are classes provided by asyncio in order to abstract
various kinds of communication channels.
Transport objects are always instantiated by an asyncio event loop.
asyncio 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 not thread safe.
asyncio.BaseTransportasyncio.WriteTransport(BaseTransport)A base transport for write-only connections.
Instances of the WriteTransport class are returned from
the loop.connect_write_pipe() event loop method and
are also used by subprocess-related methods like
loop.subprocess_exec().
asyncio.ReadTransport(BaseTransport)A base transport for read-only connections.
Instances of the ReadTransport class are returned from
the loop.connect_read_pipe() event loop method and
are also used by subprocess-related methods like
loop.subprocess_exec().
asyncio.Transport(WriteTransport, ReadTransport)Interface representing a bidirectional transport, such as a TCP connection.
The user does not instantiate a transport directly; they call a utility function, passing it a protocol factory and other information necessary to create the transport and protocol.
Instances of the Transport class are returned from or used by
event loop methods like loop.create_connection(),
loop.create_unix_connection(),
loop.create_server(), loop.sendfile(), etc.
asyncio.DatagramTransport(BaseTransport)A transport for datagram (UDP) connections.
Instances of the DatagramTransport class are returned from
the loop.create_datagram_endpoint() event loop method.
asyncio.SubprocessTransport(BaseTransport)An abstraction to represent a connection between a parent and its child OS process.
Instances of the SubprocessTransport class are returned from
event loop methods loop.subprocess_shell() and
loop.subprocess_exec().
BaseTransport.close()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 protocol.connection_lost() method will be called with
None as its argument.
BaseTransport.is_closing()True if the transport is closing or is closed.BaseTransport.get_extra_info(name, default=None)Return information about the transport or underlying resources it uses.
name is a string representing the piece of transport-specific information to get.
default is the value to return if the information is not available, or if the transport does not support querying it with the given third-party event loop implementation or on the current platform.
For example, the following code attempts to get the underlying socket object of the transport:
sock = transport.get_extra_info('socket')
if sock is not None:
print(sock.getsockopt(...))
Categories of information that can be queried on some transports:
socket:
'peername': the remote address to which the socket is
connected, result of socket.socket.getpeername()
(None on error)
'socket': socket.socket instance
'sockname': the socket’s own address,
result of socket.socket.getsockname()
SSL socket:
'compression': the compression algorithm being used as a
string, or None if the connection isn’t compressed; result
of 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
ssl.SSLSocket.cipher()
'peercert': peer certificate; result of
ssl.SSLSocket.getpeercert()
'sslcontext': ssl.SSLContext instance
'ssl_object': ssl.SSLObject or
ssl.SSLSocket instance
pipe:
'pipe': pipe object
subprocess:
'subprocess': subprocess.Popen instance
BaseTransport.set_protocol(protocol)Set a new protocol.
Switching protocol should only be done when both protocols are documented to support the switch.
BaseTransport.get_protocol()
ReadTransport.is_reading()Return True if the transport is receiving new data.
New in version 3.7.
ReadTransport.pause_reading()Pause the receiving end of the transport. No data will be passed to
the protocol’s protocol.data_received()
method until resume_reading() is called.
Changed in version 3.7: The method is idempotent, i.e. it can be called when the transport is already paused or closed.
ReadTransport.resume_reading()Resume the receiving end. The protocol’s
protocol.data_received() method
will be called once again if some data is available for reading.
Changed in version 3.7: The method is idempotent, i.e. it can be called when the transport is already reading.
WriteTransport.abort()protocol.connection_lost() method will eventually be called with None as its argument.WriteTransport.can_write_eof()True if the transport supports write_eof(), False if not.WriteTransport.get_write_buffer_size()WriteTransport.get_write_buffer_limits()Get the high and low watermarks for write flow control. Return a
tuple (low, high) where low and high are positive number of
bytes.
Use set_write_buffer_limits() to set the limits.
New in version 3.4.2.
WriteTransport.set_write_buffer_limits(high=None, low=None)Set the high and low watermarks for write flow control.
These two values (measured in number of
bytes) control when the protocol’s
protocol.pause_writing()
and protocol.resume_writing()
methods are called. If specified, the low watermark must be less
than or equal to the high watermark. Neither high nor low
can be negative.
pause_writing() is called when the buffer size
becomes greater than or equal to the high value. If writing has
been paused, resume_writing() is called when
the buffer size becomes less than or equal to the low value.
The defaults are implementation-specific. If only the
high watermark is given, the low watermark defaults to an
implementation-specific value less than or equal to the
high watermark. Setting high to zero forces low to zero as
well, and causes pause_writing() to be called
whenever the buffer becomes non-empty. Setting low to zero causes
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 get_write_buffer_limits()
to get the limits.
WriteTransport.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.
WriteTransport.writelines(list_of_data)write() on each element yielded by the iterable, but may be implemented more efficiently.WriteTransport.write_eof()Close the write end of the transport after flushing all buffered data. Data may still be received.
This method can raise NotImplementedError if the transport
(e.g. SSL) doesn’t support half-closed connections.
DatagramTransport.sendto(data, addr=None)Send the data bytes to the remote peer given by addr (a
transport-dependent target address). If addr is 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.
DatagramTransport.abort()protocol.connection_lost() method will eventually be called with None as its argument.
SubprocessTransport.get_pid()SubprocessTransport.get_pipe_transport(fd)0None if the subprocess was not created with stdin=PIPE1: writable streaming transport of the standard output (stdout), or None if the subprocess was not created with stdout=PIPE2: writable streaming transport of the standard error (stderr), or None if the subprocess was not created with stderr=PIPENoneSubprocessTransport.get_returncode()None if it hasn’t returned, which is similar to the subprocess.Popen.returncode attribute.SubprocessTransport.kill()Kill the subprocess.
On POSIX systems, the function sends SIGKILL to the subprocess.
On Windows, this method is an alias for terminate().
See also subprocess.Popen.kill().
SubprocessTransport.send_signal(signal)subprocess.Popen.send_signal().SubprocessTransport.terminate()Stop the subprocess.
On POSIX systems, this method sends SIGTERM to the subprocess. On Windows, the Windows API function TerminateProcess() is called to stop the subprocess.
See also subprocess.Popen.terminate().
SubprocessTransport.close()Kill the subprocess by calling the kill() method.
If the subprocess hasn’t returned yet, and close transports of stdin, stdout, and stderr pipes.
Source code: Lib/asyncio/protocols.py
asyncio provides a set of abstract base classes that should be used to implement network protocols. Those classes are meant to be used together with transports.
Subclasses of abstract base protocol classes may implement some or all methods. All these methods are callbacks: they are called by transports on certain events, for example when some data is received. A base protocol method should be called by the corresponding transport.
asyncio.BaseProtocolasyncio.Protocol(BaseProtocol)asyncio.BufferedProtocol(BaseProtocol)asyncio.DatagramProtocol(BaseProtocol)asyncio.SubprocessProtocol(BaseProtocol)
All asyncio protocols can implement Base Protocol callbacks.
Connection Callbacks
Connection callbacks are called on all protocols, exactly once per a successful connection. All other protocol callbacks can only be called between those two methods.
BaseProtocol.connection_made(transport)Called when a connection is made.
The transport argument is the transport representing the connection. The protocol is responsible for storing the reference to its transport.
BaseProtocol.connection_lost(exc)Called when the connection is lost or closed.
The argument is either an exception object or None.
The latter means a regular EOF is received, or the connection was
aborted or closed by this side of the connection.
Flow Control Callbacks
Flow control callbacks can be called by transports to pause or resume writing performed by the protocol.
See the documentation of the set_write_buffer_limits()
method for more details.
BaseProtocol.pause_writing()BaseProtocol.resume_writing()If the buffer size equals the high watermark,
pause_writing() is not called: the buffer size must
go strictly over.
Conversely, resume_writing() is called when the
buffer size is equal or lower than the low watermark. These end
conditions are important to ensure that things go as expected when
either mark is zero.
Event methods, such as loop.create_server(),
loop.create_unix_server(), loop.create_connection(),
loop.create_unix_connection(), loop.connect_accepted_socket(),
loop.connect_read_pipe(), and loop.connect_write_pipe()
accept factories that return streaming protocols.
Protocol.data_received(data)Called when some data is received. data is a non-empty bytes object containing the incoming data.
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. However, data is always received in the correct order.
The method can be called an arbitrary number of times while a connection is open.
However, protocol.eof_received()
is called at most once. Once eof_received() is called,
data_received() is not called anymore.
Protocol.eof_received()Called when the other end signals it won’t send any more data
(for example by calling transport.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, the protocol used determines whether to close the transport.
Since the default implementation returns None, it implicitly closes the
connection.
Some transports, including SSL, don’t support half-closed connections, in which case returning true from this method will result in the connection being closed.
State machine:
start -> connection_made
[-> data_received]*
[-> eof_received]?
-> connection_lost -> end
New in version 3.7: Important: this has been added to asyncio in Python 3.7 on a provisional basis! This is as an experimental API that might be changed or removed completely in Python 3.8.
Buffered Protocols can be used with any event loop method that supports Streaming Protocols.
BufferedProtocol implementations allow explicit manual allocation
and control of the receive buffer. Event loops can then use the buffer
provided by the protocol to avoid unnecessary data copies. This
can result in noticeable performance improvement for protocols that
receive big amounts of data. Sophisticated protocol implementations
can significantly reduce the number of buffer allocations.
The following callbacks are called on BufferedProtocol
instances:
BufferedProtocol.get_buffer(sizehint)Called to allocate a new receive buffer.
sizehint is the recommended minimum size for the returned buffer. It is acceptable to return smaller or larger buffers than what sizehint suggests. When set to -1, the buffer size can be arbitrary. It is an error to return a buffer with a zero size.
get_buffer() must return an object implementing the
buffer protocol.
BufferedProtocol.buffer_updated(nbytes)Called when the buffer was updated with the received data.
nbytes is the total number of bytes that were written to the buffer.
BufferedProtocol.eof_received()protocol.eof_received() method.get_buffer() can be called an arbitrary number
of times during a connection. However, protocol.eof_received() is called at most once
and, if called, get_buffer() and
buffer_updated() won’t be called after it.
State machine:
start -> connection_made
[-> get_buffer
[-> buffer_updated]?
]*
[-> eof_received]?
-> connection_lost -> end
Datagram Protocol instances should be constructed by protocol
factories passed to the loop.create_datagram_endpoint() method.
DatagramProtocol.datagram_received(data, addr)DatagramProtocol.error_received(exc)Called when a previous send or receive operation raises an
OSError. exc is the OSError instance.
This method is called in rare conditions, when the transport (e.g. UDP) detects that a datagram could not be delivered to its recipient. In many conditions though, undeliverable datagrams will be silently dropped.
Note
On BSD systems (macOS, FreeBSD, etc.) flow control is not supported for datagram protocols, because there is no reliable way to detect send failures caused by writing too many packets.
The socket always appears ‘ready’ and excess packets are dropped. An
OSError with errno set to errno.ENOBUFS may
or may not be raised; if it is raised, it will be reported to
DatagramProtocol.error_received() but otherwise ignored.
Datagram Protocol instances should be constructed by protocol
factories passed to the loop.subprocess_exec() and
loop.subprocess_shell() methods.
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 received data.
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.
SubprocessProtocol.process_exited()
Create a TCP echo server using the loop.create_server() method, send back
received data, and close the connection:
import asyncio
class EchoServerProtocol(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()
async def main():
# Get a reference to the event loop as we plan to use
# low-level APIs.
loop = asyncio.get_running_loop()
server = await loop.create_server(
lambda: EchoServerProtocol(),
'127.0.0.1', 8888)
async with server:
await server.serve_forever()
asyncio.run(main())
See also
The TCP echo server using streams
example uses the high-level asyncio.start_server() function.
A TCP echo client using the loop.create_connection() method, sends
data, and waits until the connection is closed:
import asyncio
class EchoClientProtocol(asyncio.Protocol):
def __init__(self, message, on_con_lost):
self.message = message
self.on_con_lost = on_con_lost
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')
self.on_con_lost.set_result(True)
async def main():
# Get a reference to the event loop as we plan to use
# low-level APIs.
loop = asyncio.get_running_loop()
on_con_lost = loop.create_future()
message = 'Hello World!'
transport, protocol = await loop.create_connection(
lambda: EchoClientProtocol(message, on_con_lost),
'127.0.0.1', 8888)
# Wait until the protocol signals that the connection
# is lost and close the transport.
try:
await on_con_lost
finally:
transport.close()
asyncio.run(main())
See also
The TCP echo client using streams
example uses the high-level asyncio.open_connection() function.
A UDP echo server, using the loop.create_datagram_endpoint()
method, sends 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)
async def main():
print("Starting UDP server")
# Get a reference to the event loop as we plan to use
# low-level APIs.
loop = asyncio.get_running_loop()
# One protocol instance will be created to serve all
# client requests.
transport, protocol = await loop.create_datagram_endpoint(
lambda: EchoServerProtocol(),
local_addr=('127.0.0.1', 9999))
try:
await asyncio.sleep(3600) # Serve for 1 hour.
finally:
transport.close()
asyncio.run(main())
A UDP echo client, using the loop.create_datagram_endpoint()
method, sends data and closes the transport when it receives the answer:
import asyncio
class EchoClientProtocol:
def __init__(self, message, on_con_lost):
self.message = message
self.on_con_lost = on_con_lost
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("Connection closed")
self.on_con_lost.set_result(True)
async def main():
# Get a reference to the event loop as we plan to use
# low-level APIs.
loop = asyncio.get_running_loop()
on_con_lost = loop.create_future()
message = "Hello World!"
transport, protocol = await loop.create_datagram_endpoint(
lambda: EchoClientProtocol(message, on_con_lost),
remote_addr=('127.0.0.1', 9999))
try:
await on_con_lost
finally:
transport.close()
asyncio.run(main())
Wait until a socket receives data using the
loop.create_connection() method with a protocol:
import asyncio
import socket
class MyProtocol(asyncio.Protocol):
def __init__(self, on_con_lost):
self.transport = None
self.on_con_lost = on_con_lost
def connection_made(self, transport):
self.transport = transport
def data_received(self, data):
print("Received:", data.decode())
# We are done: close the transport;
# connection_lost() will be called automatically.
self.transport.close()
def connection_lost(self, exc):
# The socket has been closed
self.on_con_lost.set_result(True)
async def main():
# Get a reference to the event loop as we plan to use
# low-level APIs.
loop = asyncio.get_running_loop()
on_con_lost = loop.create_future()
# Create a pair of connected sockets
rsock, wsock = socket.socketpair()
# Register the socket to wait for data.
transport, protocol = await loop.create_connection(
lambda: MyProtocol(on_con_lost), sock=rsock)
# Simulate the reception of data from the network.
loop.call_soon(wsock.send, 'abc'.encode())
try:
await protocol.on_con_lost
finally:
transport.close()
wsock.close()
asyncio.run(main())
See also
The watch a file descriptor for read events example uses the low-level
loop.add_reader() method to register an FD.
The register an open socket to wait for data using streams example uses high-level streams
created by the open_connection() function in a coroutine.
An example of a subprocess protocol used to get the output of a subprocess and to wait for the subprocess exit.
The subprocess is created by the loop.subprocess_exec() method:
import asyncio
import sys
class DateProtocol(asyncio.SubprocessProtocol):
def __init__(self, exit_future):
self.exit_future = exit_future
self.output = bytearray()
def pipe_data_received(self, fd, data):
self.output.extend(data)
def process_exited(self):
self.exit_future.set_result(True)
async def get_date():
# Get a reference to the event loop as we plan to use
# low-level APIs.
loop = asyncio.get_running_loop()
code = 'import datetime; print(datetime.datetime.now())'
exit_future = asyncio.Future(loop=loop)
# Create the subprocess controlled by DateProtocol;
# redirect the standard output into a pipe.
transport, protocol = await loop.subprocess_exec(
lambda: DateProtocol(exit_future),
sys.executable, '-c', code,
stdin=None, stderr=None)
# Wait for the subprocess exit using the process_exited()
# method of the protocol.
await exit_future
# Close the stdout pipe.
transport.close()
# Read the output which was collected by the
# pipe_data_received() method of the protocol.
data = bytes(protocol.output)
return data.decode('ascii').rstrip()
date = asyncio.run(get_date())
print(f"Current date: {date}")
See also the same example written using high-level APIs.