# 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())
        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, *, 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 `StreamReader` and `StreamWriter` classes.

  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 `StreamReader` instance. 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 `StreamReader` and `StreamWriter` 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`.

  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 `StreamReader` instance. 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 `bytes` 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 the `IncompleteReadError.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. The `IncompleteReadError.partial` attribute may contain a portion of the separator.

    New in version 3.5.2.

  `at_eof`()

    Return `True` if the buffer is empty and `feed_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()` and `start_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 along with the `wait_closed()` method:

        stream.close()
        await stream.wait_closed()

  `can_write_eof`()

    Return True if the underlying transport supports the `write_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.

  `is_closing`()

    Return `True` if the stream is closed or in the process of being closed.

    New in version 3.7.

  coroutine `wait_closed`()

    Wait until the stream is closed.

    Should be called after `close()` to wait until the underlying connection is 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.