Streams¶
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())
    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, *, loop=None, limit=None, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None, ssl_handshake_timeout=None)¶
- Establish a network connection and return a pair of - (reader, writer)objects.- The returned reader and writer objects are instances of - StreamReaderand- StreamWriterclasses.- The loop argument is optional and can always be determined automatically when this function is awaited from a coroutine. - limit determines the buffer size limit used by the returned - StreamReaderinstance. By default the limit is set to 64 KiB.- The rest of the arguments are passed directly to - loop.create_connection().- New in version 3.7: The ssl_handshake_timeout parameter. 
- 
coroutine asyncio.start_server(client_connected_cb, host=None, port=None, *, loop=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, 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 the- StreamReaderand- StreamWriterclasses.- 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.- The loop argument is optional and can always be determined automatically when this method is awaited from a coroutine. - limit determines the buffer size limit used by the returned - StreamReaderinstance. By default the limit is set to 64 KiB.- The rest of the arguments are passed directly to - loop.create_server().- New in version 3.7: The ssl_handshake_timeout and start_serving parameters. 
Unix Sockets
- 
coroutine asyncio.open_unix_connection(path=None, *, loop=None, limit=None, ssl=None, sock=None, server_hostname=None, ssl_handshake_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().- Availability: Unix. - New in version 3.7: The ssl_handshake_timeout parameter. - Changed in version 3.7: The path parameter can now be a path-like object 
- 
coroutine asyncio.start_unix_server(client_connected_cb, path=None, *, loop=None, limit=None, sock=None, backlog=100, ssl=None, ssl_handshake_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().- Availability: Unix. - New in version 3.7: The ssl_handshake_timeout and start_serving parameters. - Changed in version 3.7: The path parameter can now be a path-like object. 
StreamReader¶
- 
class asyncio.StreamReader¶
- Represents a reader object that provides APIs to read data from the IO stream. - It is not recommended to instantiate StreamReader objects directly; use - open_connection()and- start_server()instead.- 
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 EOF was received and the internal buffer is empty, return an empty - bytesobject.
 - 
coroutine readline()¶
- Read one line, where “line” is a sequence of bytes ending with - \n.- If EOF is received and - \nwas not found, the method returns partially read data.- If EOF is received and the internal buffer is empty, return an empty - bytesobject.
 - 
coroutine readexactly(n)¶
- Read exactly n bytes. - Raise an - IncompleteReadErrorif EOF is reached before n can be read. Use the- IncompleteReadError.partialattribute 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 - LimitOverrunErrorexception 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 - IncompleteReadErrorexception is raised, and the internal buffer is reset. The- IncompleteReadError.partialattribute may contain a portion of the separator.- New in version 3.5.2. 
 - 
at_eof()¶
- Return - Trueif the buffer is empty and- feed_eof()was called.
 
- 
coroutine 
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()and- start_server()instead.- 
can_write_eof()¶
- Return - Trueif the underlying transport supports the- write_eof()method,- Falseotherwise.
 - 
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.
 - 
write(data)¶
- Write data to the stream. - This method is not subject to flow control. Calls to - write()should be followed by- drain().
 - 
writelines(data)¶
- Write a list (or any iterable) of bytes to the stream. - This method is not subject to flow control. Calls to - writelines()should be followed by- drain().
 - 
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.
 - 
close()¶
- Close the stream. 
 - 
is_closing()¶
- Return - Trueif 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())
    data = await reader.read(100)
    print(f'Received: {data.decode()!r}')
    print('Close the connection')
    writer.close()
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()
async def main():
    server = await asyncio.start_server(
        handle_echo, '127.0.0.1', 8888)
    addr = server.sockets[0].getsockname()
    print(f'Serving on {addr}')
    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()
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()
    # 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.
