18.5.3. Transports and protocols (low-level API) Transports

Transports are classes provided by asyncio in order to abstract various kinds of communication channels. You generally won’t instantiate a transport yourself; instead, you will call a 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 protocol instance. The protocol can then call the transport’s methods for various purposes.

asyncio currently implements transports for TCP, UDP, SSL, and subprocess pipes. The methods available on a transport depend on the transport’s kind. BaseTransport

class asyncio.BaseTransport

Base class for transports.


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 connection_lost() method will be called with None as its argument.

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. ReadTransport

class asyncio.ReadTransport

Interface for read-only transports.


Pause the receiving end of the transport. No data will be passed to the protocol’s data_received() method until resume_reading() is called.


Resume the receiving end. The protocol’s data_received() method will be called once again if some data is available for reading. WriteTransport

class asyncio.WriteTransport

Interface for write-only transports.


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 connection_lost() method will eventually be called with None as its argument.


Return True if the transport supports write_eof(), False if not.


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

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 pause_writing() and 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 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.


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.


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


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

This method can raise NotImplementedError if the transport (e.g. SSL) doesn’t support half-closes. DatagramTransport

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.


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 connection_lost() method will eventually be called with None as its argument. BaseSubprocessTransport

class asyncio.BaseSubprocessTransport

Return the subprocess process id as an integer.


Return the transport for the communication pipe corresponding to the integer file descriptor fd. The return value can be a readable or writable streaming transport, depending on the fd. If fd doesn’t correspond to a pipe belonging to this transport, None is returned.


Return the subprocess returncode as an integer or None if it hasn’t returned, similarly to the subprocess.Popen.returncode attribute.


Kill the subprocess, as in subprocess.Popen.kill()

On POSIX systems, the function sends SIGKILL to the subprocess. On Windows, this method is an alias for terminate().


Send the signal number to the subprocess, as in subprocess.Popen.send_signal().


Ask the subprocess to stop, as in subprocess.Popen.terminate(). This method is an alias for the 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. Protocols

asyncio provides base classes that you can subclass to implement your network protocols. Those classes are used in conjunction with 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.


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 asyncio.Protocol

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

class asyncio.DatagramProtocol

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

class asyncio.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 Protocol and SubprocessProtocol instances:


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.


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.

connection_made() and 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 SubprocessProtocol instances:

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.

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.


Called when the child process has exited. Streaming protocols

The following callbacks are called on Protocol instances:


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 enough. However, data is always received in the correct order.


Calls when the other end signals it won’t send any more data (for example by calling 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.


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.

data_received() can be called an arbitrary number of times during a connection. However, eof_received() is called at most once and, if called, data_received() won’t be called after it. Datagram protocols

The following callbacks are called on DatagramProtocol instances.

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.


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 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 Protocol, DatagramProtocol and SubprocessProtocol instances:


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


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

pause_writing() and resume_writing() calls are paired – 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 resume_writing() is called once when the buffer size reaches the low-water mark.


If the buffer size equals the high-water mark, pause_writing() is not called – it must go strictly over. Conversely, 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.


On BSD systems (OS X, FreeBSD, etc.) flow control is not supported for 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 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. Coroutines and protocols

Coroutines can be scheduled in a protocol method using async(), but there is not guarantee on 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 stream objects in a coroutine with yield from. For example, the StreamWriter.drain() coroutine can be used to wait until the write buffer is flushed. Protocol example: TCP echo server and client Echo client

TCP echo client example, send data and wait until the connection is closed:

import asyncio

class EchoClient(asyncio.Protocol):
    message = 'This is the message. It will be echoed.'

    def connection_made(self, transport):
        print('data sent: {}'.format(self.message))

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

    def connection_lost(self, exc):
        print('server closed the connection')

loop = asyncio.get_event_loop()
coro = loop.create_connection(EchoClient, '', 8888)

The event loop is running twice. The 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 run_until_complete() exit, the loop is no more running, so there is no need to stop the loop in case of an error. Echo server

TCP echo server example, send back received data and close the connection:

import asyncio

class EchoServer(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):
        print('data received: {}'.format(data.decode()))

        # close the socket

loop = asyncio.get_event_loop()
coro = loop.create_server(EchoServer, '', 8888)
server = loop.run_until_complete(coro)
print('serving on {}'.format(server.sockets[0].getsockname()))

except KeyboardInterrupt:

Transport.close() can be called immediately after 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.