Streams¶
Source code: Lib/asyncio/streams.py
Streams are high-level async/await-ready primitives to work with network connections. Streams allow sending and receiving data without using callbacks or low-level protocols and transports.
Here is an example of a TCP echo client written using asyncio streams:
import asyncio
async def tcp_echo_client(message):
reader, writer = await asyncio.open_connection(
'127.0.0.1', 8888)
print(f'Send: {message!r}')
writer.write(message.encode())
await writer.drain()
data = await reader.read(100)
print(f'Received: {data.decode()!r}')
print('Close the connection')
writer.close()
await writer.wait_closed()
asyncio.run(tcp_echo_client('Hello World!'))
See also the Examples section below.
Stream Functions
The following top-level asyncio functions can be used to create and work with streams:
- coroutine asyncio.open_connection(host=None, port=None, *, limit=None, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, happy_eyeballs_delay=None, interleave=None)¶
Establish a network connection and return a pair of
(reader, writer)
objects.The returned reader and writer objects are instances of
StreamReader
andStreamWriter
classes.limit determines the buffer size limit used by the returned
StreamReader
instance. By default the limit is set to 64 KiB.The rest of the arguments are passed directly to
loop.create_connection()
.Note
The sock argument transfers ownership of the socket to the
StreamWriter
created. To close the socket, call itsclose()
method.Changed in version 3.7: Added the ssl_handshake_timeout parameter.
New in version 3.8: Added happy_eyeballs_delay and interleave parameters.
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
- coroutine asyncio.start_server(client_connected_cb, host=None, port=None, *, limit=None, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None, reuse_port=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, start_serving=True)¶
Start a socket server.
The client_connected_cb callback is called whenever a new client connection is established. It receives a
(reader, writer)
pair as two arguments, instances of theStreamReader
andStreamWriter
classes.client_connected_cb can be a plain callable or a coroutine function; if it is a coroutine function, it will be automatically scheduled as a
Task
.limit determines the buffer size limit used by the returned
StreamReader
instance. By default the limit is set to 64 KiB.The rest of the arguments are passed directly to
loop.create_server()
.Note
The sock argument transfers ownership of the socket to the server created. To close the socket, call the server’s
close()
method.Changed in version 3.7: Added the ssl_handshake_timeout and start_serving parameters.
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
Unix Sockets
- coroutine asyncio.open_unix_connection(path=None, *, limit=None, ssl=None, sock=None, server_hostname=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None)¶
Establish a Unix socket connection and return a pair of
(reader, writer)
.Similar to
open_connection()
but operates on Unix sockets.See also the documentation of
loop.create_unix_connection()
.Note
The sock argument transfers ownership of the socket to the
StreamWriter
created. To close the socket, call itsclose()
method.Availability: Unix.
Changed in version 3.7: Added the ssl_handshake_timeout parameter. The path parameter can now be a path-like object
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
- coroutine asyncio.start_unix_server(client_connected_cb, path=None, *, limit=None, sock=None, backlog=100, ssl=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None, start_serving=True)¶
Start a Unix socket server.
Similar to
start_server()
but works with Unix sockets.See also the documentation of
loop.create_unix_server()
.Note
The sock argument transfers ownership of the socket to the server created. To close the socket, call the server’s
close()
method.Availability: Unix.
Changed in version 3.7: Added the ssl_handshake_timeout and start_serving parameters. The path parameter can now be a path-like object.
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
StreamReader¶
- class asyncio.StreamReader¶
Represents a reader object that provides APIs to read data from the IO stream. As an asynchronous iterable, the object supports the
async for
statement.It is not recommended to instantiate StreamReader objects directly; use
open_connection()
andstart_server()
instead.- feed_eof()¶
Acknowledge the EOF.
- coroutine read(n=- 1)¶
Read up to n bytes from the stream.
If n is not provided or set to
-1
, read until EOF, then return all readbytes
. If EOF was received and the internal buffer is empty, return an emptybytes
object.If n is
0
, return an emptybytes
object immediately.If n is positive, return at most n available
bytes
as soon as at least 1 byte is available in the internal buffer. If EOF is received before any byte is read, return an emptybytes
object.
- 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 returns partially read data.If EOF is received and the internal buffer is empty, return an empty
bytes
object.
- coroutine readexactly(n)¶
Read exactly n bytes.
Raise an
IncompleteReadError
if EOF is reached before n can be read. Use theIncompleteReadError.partial
attribute to get the partially read data.
- 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.
If the amount of data read exceeds the configured stream limit, a
LimitOverrunError
exception is raised, and the data is left in the internal buffer and can be read again.If EOF is reached before the complete separator is found, an
IncompleteReadError
exception is raised, and the internal buffer is reset. TheIncompleteReadError.partial
attribute may contain a portion of the separator.New in version 3.5.2.
- at_eof()¶
Return
True
if the buffer is empty andfeed_eof()
was called.
StreamWriter¶
- class asyncio.StreamWriter¶
Represents a writer object that provides APIs to write data to the IO stream.
It is not recommended to instantiate StreamWriter objects directly; use
open_connection()
andstart_server()
instead.- write(data)¶
The method attempts to write the data to the underlying socket immediately. If that fails, the data is queued in an internal write buffer until it can be sent.
The method should be used along with the
drain()
method:stream.write(data) await stream.drain()
- writelines(data)¶
The method writes a list (or any iterable) of bytes to the underlying socket immediately. If that fails, the data is queued in an internal write buffer until it can be sent.
The method should be used along with the
drain()
method:stream.writelines(lines) await stream.drain()
- close()¶
The method closes the stream and the underlying socket.
The method should be used, though not mandatory, along with the
wait_closed()
method:stream.close() await stream.wait_closed()
- can_write_eof()¶
Return
True
if the underlying transport supports thewrite_eof()
method,False
otherwise.
- write_eof()¶
Close the write end of the stream after the buffered write data is flushed.
- transport¶
Return the underlying asyncio transport.
- get_extra_info(name, default=None)¶
Access optional transport information; see
BaseTransport.get_extra_info()
for details.
- coroutine drain()¶
Wait until it is appropriate to resume writing to the stream. Example:
writer.write(data) await writer.drain()
This is a flow control method that interacts with the underlying IO write buffer. When the size of the buffer reaches the high watermark, drain() blocks until the size of the buffer is drained down to the low watermark and writing can be resumed. When there is nothing to wait for, the
drain()
returns immediately.
- coroutine start_tls(sslcontext, \*, server_hostname=None, ssl_handshake_timeout=None)¶
Upgrade an existing stream-based connection to TLS.
Parameters:
sslcontext: a configured instance of
SSLContext
.server_hostname: sets or overrides the host name that the target server’s certificate will be matched against.
ssl_handshake_timeout is the time in seconds to wait for the TLS handshake to complete before aborting the connection.
60.0
seconds ifNone
(default).
New in version 3.11.
- is_closing()¶
Return
True
if the stream is closed or in the process of being closed.New in version 3.7.
Examples¶
TCP echo client using streams¶
TCP echo client using the asyncio.open_connection()
function:
import asyncio
async def tcp_echo_client(message):
reader, writer = await asyncio.open_connection(
'127.0.0.1', 8888)
print(f'Send: {message!r}')
writer.write(message.encode())
await writer.drain()
data = await reader.read(100)
print(f'Received: {data.decode()!r}')
print('Close the connection')
writer.close()
await writer.wait_closed()
asyncio.run(tcp_echo_client('Hello World!'))
See also
The TCP echo client protocol
example uses the low-level loop.create_connection()
method.
TCP echo server using streams¶
TCP echo server using the asyncio.start_server()
function:
import asyncio
async def handle_echo(reader, writer):
data = await reader.read(100)
message = data.decode()
addr = writer.get_extra_info('peername')
print(f"Received {message!r} from {addr!r}")
print(f"Send: {message!r}")
writer.write(data)
await writer.drain()
print("Close the connection")
writer.close()
await writer.wait_closed()
async def main():
server = await asyncio.start_server(
handle_echo, '127.0.0.1', 8888)
addrs = ', '.join(str(sock.getsockname()) for sock in server.sockets)
print(f'Serving on {addrs}')
async with server:
await server.serve_forever()
asyncio.run(main())
See also
The TCP echo server protocol
example uses the loop.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
async def print_http_headers(url):
url = urllib.parse.urlsplit(url)
if url.scheme == 'https':
reader, writer = await asyncio.open_connection(
url.hostname, 443, ssl=True)
else:
reader, writer = await asyncio.open_connection(
url.hostname, 80)
query = (
f"HEAD {url.path or '/'} HTTP/1.0\r\n"
f"Host: {url.hostname}\r\n"
f"\r\n"
)
writer.write(query.encode('latin-1'))
while True:
line = await reader.readline()
if not line:
break
line = line.decode('latin1').rstrip()
if line:
print(f'HTTP header> {line}')
# Ignore the body, close the socket
writer.close()
await writer.wait_closed()
url = sys.argv[1]
asyncio.run(print_http_headers(url))
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
import socket
async def wait_for_data():
# Get a reference to the current event loop because
# we want to access low-level APIs.
loop = asyncio.get_running_loop()
# Create a pair of connected sockets.
rsock, wsock = socket.socketpair()
# Register the open socket to wait for data.
reader, writer = await asyncio.open_connection(sock=rsock)
# Simulate the reception of data from the network
loop.call_soon(wsock.send, 'abc'.encode())
# Wait for data
data = await reader.read(100)
# Got data, we are done: close the socket
print("Received:", data.decode())
writer.close()
await writer.wait_closed()
# Close the second socket
wsock.close()
asyncio.run(wait_for_data())
See also
The register an open socket to wait for data using a protocol example uses a low-level protocol and
the loop.create_connection()
method.
The watch a file descriptor for read events example uses the low-level
loop.add_reader()
method to watch a file descriptor.