Transports and Protocols — Python 3.14.3 documentation
Description: 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...

# Transports and Protocols

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:

- The Transports section documents asyncio `BaseTransport`, `ReadTransport`, `WriteTransport`, `Transport`, `DatagramTransport`, and `SubprocessTransport` classes.

- The Protocols section documents asyncio `BaseProtocol`, `Protocol`, `BufferedProtocol`, `DatagramProtocol`, and `SubprocessProtocol` classes.

- The Examples section showcases how to work with transports, protocols, and low-level event loop APIs.

## Transports

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.

### Transports Hierarchy

class asyncio.BaseTransport

  Base class for all transports. Contains methods that all asyncio transports share.

class asyncio.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()`.

class 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()`.

class 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.

class 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.

class 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()`.

### Base Transport

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. The transport should not be used once it is closed.

BaseTransport.is_closing()

  Return `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()

  Return the current protocol.

### Read-only Transports

ReadTransport.is_reading()

  Return `True` if the transport is receiving new data.

  Added 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.

### Write-only Transports

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

WriteTransport.can_write_eof()

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

WriteTransport.get_write_buffer_size()

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

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.

  Added 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 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.

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.

### Datagram Transports

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.

  Changed in version 3.13: This method can be called with an empty bytes object to send a zero-length datagram. The buffer size calculation used for flow control is also updated to account for the datagram header.

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

### Subprocess Transports

SubprocessTransport.get_pid()

  Return the subprocess process id as an integer.

SubprocessTransport.get_pipe_transport(fd)

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

  - `0`: writable streaming transport of the standard input (stdin), or `None` if the subprocess was not created with `stdin=PIPE`

  - `1`: readable streaming transport of the standard output (stdout), or `None` if the subprocess was not created with `stdout=PIPE`

  - `2`: readable streaming transport of the standard error (stderr), or `None` if the subprocess was not created with `stderr=PIPE`

  - other fd: `None`

SubprocessTransport.get_returncode()

  Return the subprocess return code as an integer or `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)

  Send the signal number to the subprocess, as in `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.

## Protocols

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.

### Base Protocols

class asyncio.BaseProtocol

  Base protocol with methods that all protocols share.

class asyncio.Protocol(BaseProtocol)

  The base class for implementing streaming protocols (TCP, Unix sockets, etc).

class asyncio.BufferedProtocol(BaseProtocol)

  A base class for implementing streaming protocols with manual control of the receive buffer.

class asyncio.DatagramProtocol(BaseProtocol)

  The base class for implementing datagram (UDP) protocols.

class asyncio.SubprocessProtocol(BaseProtocol)

  The base class for implementing protocols communicating with child processes (unidirectional pipes).

### Base Protocol

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()

  Called when the transport’s buffer goes over the high watermark.

BaseProtocol.resume_writing()

  Called when the transport’s buffer drains below the low watermark.

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.

### Streaming Protocols

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

### Buffered Streaming Protocols

Added in version 3.7.

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()

  See the documentation of the `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 Protocols

Datagram Protocol instances should be constructed by protocol factories passed to the `loop.create_datagram_endpoint()` 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.

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.

### Subprocess Protocols

Subprocess 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()

  Called when the child process has exited.

  It can be called before `pipe_data_received()` and `pipe_connection_lost()` methods.

## Examples

### TCP Echo Server

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(
            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.

### TCP Echo Client

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.

### UDP Echo Server

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(
            EchoServerProtocol,
            local_addr=('127.0.0.1', 9999))

        try:
            await asyncio.sleep(3600)  # Serve for 1 hour.
        finally:
            transport.close()

    asyncio.run(main())

### UDP Echo Client

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())

### Connecting Existing Sockets

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.

### loop.subprocess_exec() and SubprocessProtocol

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()
            self.pipe_closed = False
            self.exited = False

        def pipe_connection_lost(self, fd, exc):
            self.pipe_closed = True
            self.check_for_exit()

        def pipe_data_received(self, fd, data):
            self.output.extend(data)

        def process_exited(self):
            self.exited = True
            # process_exited() method can be called before
            # pipe_connection_lost() method: wait until both methods are
            # called.
            self.check_for_exit()

        def check_for_exit(self):
            if self.pipe_closed and self.exited:
                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.