18.5.4. Streams (high-level API)

18.5.4.1. Stream functions

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 BaseEventLoop.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).

(If you want to customize the StreamReader and/or StreamReaderProtocol classes, just copy the code – there’s really nothing special here except some convenience.)

This function is a 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 first parameter, client_connected_cb, takes two parameters: client_reader, client_writer. client_reader is a StreamReader object, while client_writer is a StreamWriter object. This 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. The return value is the same as create_server().

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).

The return value is the same as create_server(), i.e. a AbstractServer object which can be used to stop the service.

This function is a 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.

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.

18.5.4.2. StreamReader

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

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.

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.

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.

at_eof()

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

18.5.4.3. 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.

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().

drain()

Wait until the write buffer of the underlying transport is flushed.

This method has an unusual return value. The intended use is to write:

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

When there’s nothing to wait for, drain() returns (), and the yield-from continues immediately. When the transport buffer is full (the protocol is paused), drain() creates and returns a Future and the yield-from will block until that Future is completed, which will happen when the buffer is (partially) drained and the protocol is resumed.

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().

18.5.4.4. StreamReaderProtocol

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

Trivial helper class to adapt between Protocol and StreamReader. Sublclass 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 to accidentally call inappropriate methods of the protocol.)

18.5.4.5. 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).

18.5.4.6. Example

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)
    reader, writer = yield from asyncio.open_connection(url.hostname, 80)
    query = ('HEAD {url.path} HTTP/1.0\r\n'
             'Host: {url.hostname}\r\n'
             '\r\n').format(url=url)
    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)

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

Usage:

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