Streams (coroutine based API)

1. Stream functions

coroutine asyncio.open_connection(host=None, port=None, *, loop=None, limit=None, **kwds)

A wrapper for create_connection() returning a (reader, writer) pair.

The reader returned is a StreamReader instance; the writer is a StreamWriter instance.

The arguments are all the usual arguments to AbstractEventLoop.create_connection() except protocol_factory; most common are positional host and port, with various optional keyword arguments following.

Additional optional keyword arguments are loop (to set the event loop instance to use) and limit (to set the buffer limit passed to the StreamReader).

This function is a coroutine.

coroutine asyncio.start_server(client_connected_cb, host=None, port=None, *, loop=None, limit=None, **kwds)

Start a socket server, with a callback for each client connected. The return value is the same as create_server().

The client_connected_cb parameter is called with two parameters: client_reader, client_writer. client_reader is a StreamReader object, while client_writer is a StreamWriter object. The client_connected_cb parameter can either be a plain callback function or a coroutine function; if it is a coroutine function, it will be automatically converted into a Task.

The rest of the arguments are all the usual arguments to create_server() except protocol_factory; most common are positional host and port, with various optional keyword arguments following.

Additional optional keyword arguments are loop (to set the event loop instance to use) and limit (to set the buffer limit passed to the StreamReader).

This function is a coroutine.

coroutine asyncio.open_unix_connection(path=None, *, loop=None, limit=None, **kwds)

A wrapper for create_unix_connection() returning a (reader, writer) pair.

See open_connection() for information about return value and other details.

This function is a coroutine.

Availability: UNIX.

coroutine asyncio.start_unix_server(client_connected_cb, path=None, *, loop=None, limit=None, **kwds)

Start a UNIX Domain Socket server, with a callback for each client connected.

See start_server() for information about return value and other details.

This function is a coroutine.

Availability: UNIX.

2. StreamReader

class asyncio.StreamReader(limit=None, loop=None)

This class is not thread safe.

exception()

Get the exception.

feed_eof()

Acknowledge the EOF.

feed_data(data)

Feed data bytes in the internal buffer. Any operations waiting for the data will be resumed.

set_exception(exc)

Set the exception.

set_transport(transport)

Set the transport.

coroutine read(n=-1)

Read up to n bytes. If n is not provided, or set to -1, read until EOF and return all read bytes.

If the EOF was received and the internal buffer is empty, return an empty bytes object.

This method is a coroutine.

coroutine readline()

Read one line, where “line” is a sequence of bytes ending with \n.

If EOF is received, and \n was not found, the method will return the partial read bytes.

If the EOF was received and the internal buffer is empty, return an empty bytes object.

This method is a coroutine.

coroutine readexactly(n)

Read exactly n bytes. Raise an IncompleteReadError if the end of the stream is reached before n can be read, the IncompleteReadError.partial attribute of the exception contains the partial read bytes.

This method is a coroutine.

coroutine readuntil(separator=b'\n')

Read data from the stream until separator is found.

On success, the data and separator will be removed from the internal buffer (consumed). Returned data will include the separator at the end.

Configured stream limit is used to check result. Limit sets the maximal length of data that can be returned, not counting the separator.

If an EOF occurs and the complete separator is still not found, an IncompleteReadError exception will be raised, and the internal buffer will be reset. The IncompleteReadError.partial attribute may contain the separator partially.

If the data cannot be read because of over limit, a LimitOverrunError exception will be raised, and the data will be left in the internal buffer, so it can be read again.

New in version 3.5.2.

at_eof()

Return True if the buffer is empty and feed_eof() was called.

3. StreamWriter

class asyncio.StreamWriter(transport, protocol, reader, loop)

Wraps a Transport.

This exposes write(), writelines(), can_write_eof(), write_eof(), get_extra_info() and close(). It adds drain() which returns an optional Future on which you can wait for flow control. It also adds a transport attribute which references the Transport directly.

This class is not thread safe.

transport

Transport.

can_write_eof()

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

close()

Close the transport: see BaseTransport.close().

coroutine drain()

Let the write buffer of the underlying transport a chance to be flushed.

The intended use is to write:

w.write(data)
yield from w.drain()

When the size of the transport buffer reaches the high-water limit (the protocol is paused), block until the size of the buffer is drained down to the low-water limit and the protocol is resumed. When there is nothing to wait for, the yield-from continues immediately.

Yielding from drain() gives the opportunity for the loop to schedule the write operation and flush the buffer. It should especially be used when a possibly large amount of data is written to the transport, and the coroutine does not yield-from between calls to write().

This method is a coroutine.

get_extra_info(name, default=None)

Return optional transport information: see BaseTransport.get_extra_info().

write(data)

Write some data bytes to the transport: see WriteTransport.write().

writelines(data)

Write a list (or any iterable) of data bytes to the transport: see WriteTransport.writelines().

write_eof()

Close the write end of the transport after flushing buffered data: see WriteTransport.write_eof().

4. StreamReaderProtocol

class asyncio.StreamReaderProtocol(stream_reader, client_connected_cb=None, loop=None)

Trivial helper class to adapt between Protocol and StreamReader. Subclass of Protocol.

stream_reader is a StreamReader instance, client_connected_cb is an optional function called with (stream_reader, stream_writer) when a connection is made, loop is the event loop instance to use.

(This is a helper class instead of making StreamReader itself a Protocol subclass, because the StreamReader has other potential uses, and to prevent the user of the StreamReader from accidentally calling inappropriate methods of the protocol.)

5. IncompleteReadError

exception asyncio.IncompleteReadError

Incomplete read error, subclass of EOFError.

expected

Total number of expected bytes (int).

partial

Read bytes string before the end of stream was reached (bytes).

6. LimitOverrunError

exception asyncio.LimitOverrunError

Reached the buffer limit while looking for a separator.

consumed

Total number of to be consumed bytes.

7. Stream examples

7.1. TCP echo client using streams

TCP echo client using the asyncio.open_connection() function:

import asyncio

@asyncio.coroutine
def tcp_echo_client(message, loop):
    reader, writer = yield from asyncio.open_connection('127.0.0.1', 8888,
                                                        loop=loop)

    print('Send: %r' % message)
    writer.write(message.encode())

    data = yield from reader.read(100)
    print('Received: %r' % data.decode())

    print('Close the socket')
    writer.close()

message = 'Hello World!'
loop = asyncio.get_event_loop()
loop.run_until_complete(tcp_echo_client(message, loop))
loop.close()

7.2. TCP echo server using streams

TCP echo server using the asyncio.start_server() function:

import asyncio

@asyncio.coroutine
def handle_echo(reader, writer):
    data = yield from reader.read(100)
    message = data.decode()
    addr = writer.get_extra_info('peername')
    print("Received %r from %r" % (message, addr))

    print("Send: %r" % message)
    writer.write(data)
    yield from writer.drain()

    print("Close the client socket")
    writer.close()

loop = asyncio.get_event_loop()
coro = asyncio.start_server(handle_echo, '127.0.0.1', 8888, loop=loop)
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()

7.3. Get HTTP headers

Simple example querying HTTP headers of the URL passed on the command line:

import asyncio
import urllib.parse
import sys

@asyncio.coroutine
def print_http_headers(url):
    url = urllib.parse.urlsplit(url)
    if url.scheme == 'https':
        connect = asyncio.open_connection(url.hostname, 443, ssl=True)
    else:
        connect = asyncio.open_connection(url.hostname, 80)
    reader, writer = yield from connect
    query = ('HEAD {path} HTTP/1.0\r\n'
             'Host: {hostname}\r\n'
             '\r\n').format(path=url.path or '/', hostname=url.hostname)
    writer.write(query.encode('latin-1'))
    while True:
        line = yield from reader.readline()
        if not line:
            break
        line = line.decode('latin1').rstrip()
        if line:
            print('HTTP header> %s' % line)

    # Ignore the body, close the socket
    writer.close()

url = sys.argv[1]
loop = asyncio.get_event_loop()
task = asyncio.ensure_future(print_http_headers(url))
loop.run_until_complete(task)
loop.close()

Usage:

python example.py http://example.com/path/page.html

or with HTTPS:

python example.py https://example.com/path/page.html

7.4. Register an open socket to wait for data using streams

Coroutine waiting until a socket receives data using the open_connection() function:

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

@asyncio.coroutine
def wait_for_data(loop):
    # Create a pair of connected sockets
    rsock, wsock = socketpair()

    # Register the open socket to wait for data
    reader, writer = yield from asyncio.open_connection(sock=rsock, loop=loop)

    # Simulate the reception of data from the network
    loop.call_soon(wsock.send, 'abc'.encode())

    # Wait for data
    data = yield from reader.read(100)

    # Got data, we are done: close the socket
    print("Received:", data.decode())
    writer.close()

    # Close the second socket
    wsock.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(wait_for_data(loop))
loop.close()