| Python 3.6.5 Documentation > Streams (coroutine based API)
Streams (coroutine based API)
*****************************
**Source code:** Lib/asyncio/streams.py
Stream functions
================
Note: The top-level functions in this module are meant as
convenience wrappers only; there’s really nothing special there, and
if they don’t do exactly what you want, feel free to copy their
code.
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.
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.
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()".
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.)
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").
LimitOverrunError
=================
exception asyncio.LimitOverrunError
Reached the buffer limit while looking for a separator.
consumed
Total number of to be consumed bytes.
Stream examples
===============
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()
See also: The TCP echo client protocol example uses the
"AbstractEventLoop.create_connection()" method.
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()
See also: The TCP echo server protocol example uses the
"AbstractEventLoop.create_server()" method.
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
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()
See also: The register an open socket to wait for data using a
protocol example uses a low-level protocol created by the
"AbstractEventLoop.create_connection()" method.
The watch a file descriptor for read events example uses the low-
level "AbstractEventLoop.add_reader()" method to register the file
descriptor of a socket.
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