python_doc_md/Intermediate/library/asyncio-stream.md

::: currentmodule
asyncio
:::

# Streams {#asyncio-streams}

**Source code:** `Lib/asyncio/streams.py`{.interpreted-text role="source"}

------------------------------------------------------------------------

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.

::: {#asyncio_example_stream}
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](#examples) section below.

**Stream Functions**

The following top-level asyncio functions can be used to create and work with streams:

::::::::: {.function async=""}
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`{.interpreted-text role="class"} and `StreamWriter`{.interpreted-text role="class"} classes.

*limit* determines the buffer size limit used by the returned `StreamReader`{.interpreted-text role="class"} instance. By default the *limit* is set to 64 KiB.

The rest of the arguments are passed directly to `loop.create_connection`{.interpreted-text role="meth"}.

:::: note
::: title
Note
:::

The *sock* argument transfers ownership of the socket to the `StreamWriter`{.interpreted-text role="class"} created. To close the socket, call its `~asyncio.StreamWriter.close`{.interpreted-text role="meth"} method.
::::

::: versionchanged
3.7 Added the *ssl_handshake_timeout* parameter.
:::

::: versionchanged
3.8 Added the *happy_eyeballs_delay* and *interleave* parameters.
:::

::: versionchanged
3.10 Removed the *loop* parameter.
:::

::: versionchanged
3.11 Added the *ssl_shutdown_timeout* parameter.
:::
:::::::::

::::::::: {.function async=""}
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, keep_alive=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 the `StreamReader`{.interpreted-text role="class"} and `StreamWriter`{.interpreted-text role="class"} classes.

*client_connected_cb* can be a plain callable or a `coroutine function <coroutine>`{.interpreted-text role="ref"}; if it is a coroutine function, it will be automatically scheduled as a `Task`{.interpreted-text role="class"}.

*limit* determines the buffer size limit used by the returned `StreamReader`{.interpreted-text role="class"} instance. By default the *limit* is set to 64 KiB.

The rest of the arguments are passed directly to `loop.create_server`{.interpreted-text role="meth"}.

:::: note
::: title
Note
:::

The *sock* argument transfers ownership of the socket to the server created. To close the socket, call the server\'s `~asyncio.Server.close`{.interpreted-text role="meth"} method.
::::

::: versionchanged
3.7 Added the *ssl_handshake_timeout* and *start_serving* parameters.
:::

::: versionchanged
3.10 Removed the *loop* parameter.
:::

::: versionchanged
3.11 Added the *ssl_shutdown_timeout* parameter.
:::

::: versionchanged
3.13 Added the *keep_alive* parameter.
:::
:::::::::

**Unix Sockets**

::::::::: {.function async=""}
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`{.interpreted-text role="func"} but operates on Unix sockets.

See also the documentation of `loop.create_unix_connection`{.interpreted-text role="meth"}.

:::: note
::: title
Note
:::

The *sock* argument transfers ownership of the socket to the `StreamWriter`{.interpreted-text role="class"} created. To close the socket, call its `~asyncio.StreamWriter.close`{.interpreted-text role="meth"} method.
::::

::: availability
Unix.
:::

::: versionchanged
3.7 Added the *ssl_handshake_timeout* parameter. The *path* parameter can now be a `path-like object`{.interpreted-text role="term"}
:::

::: versionchanged
3.10 Removed the *loop* parameter.
:::

::: versionchanged
3.11 Added the *ssl_shutdown_timeout* parameter.
:::
:::::::::

:::::::::: {.function async=""}
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, cleanup_socket=True)

Start a Unix socket server.

Similar to `start_server`{.interpreted-text role="func"} but works with Unix sockets.

If *cleanup_socket* is true then the Unix socket will automatically be removed from the filesystem when the server is closed, unless the socket has been replaced after the server has been created.

See also the documentation of `loop.create_unix_server`{.interpreted-text role="meth"}.

:::: note
::: title
Note
:::

The *sock* argument transfers ownership of the socket to the server created. To close the socket, call the server\'s `~asyncio.Server.close`{.interpreted-text role="meth"} method.
::::

::: availability
Unix.
:::

::: versionchanged
3.7 Added the *ssl_handshake_timeout* and *start_serving* parameters. The *path* parameter can now be a `path-like object`{.interpreted-text role="term"}.
:::

::: versionchanged
3.10 Removed the *loop* parameter.
:::

::: versionchanged
3.11 Added the *ssl_shutdown_timeout* parameter.
:::

::: versionchanged
3.13 Added the *cleanup_socket* parameter.
:::
::::::::::

## StreamReader

::::::::::: StreamReader
Represents a reader object that provides APIs to read data from the IO stream. As an `asynchronous iterable`{.interpreted-text role="term"}, the object supports the `async for`{.interpreted-text role="keyword"} statement.

It is not recommended to instantiate *StreamReader* objects directly; use `open_connection`{.interpreted-text role="func"} and `start_server`{.interpreted-text role="func"} instead.

::: method
feed_eof()

Acknowledge the EOF.
:::

::: {.method async=""}
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 read `bytes`{.interpreted-text role="class"}. If EOF was received and the internal buffer is empty, return an empty `bytes` object.

If *n* is `0`, return an empty `bytes` 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 empty `bytes` object.
:::

::: {.method async=""}
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.
:::

::: {.method async=""}
readexactly(n)

Read exactly *n* bytes.

Raise an `IncompleteReadError`{.interpreted-text role="exc"} if EOF is reached before *n* can be read. Use the `IncompleteReadError.partial`{.interpreted-text role="attr"} attribute to get the partially read data.
:::

::::: {.method async=""}
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`{.interpreted-text role="exc"} 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`{.interpreted-text role="exc"} exception is raised, and the internal buffer is reset. The `IncompleteReadError.partial`{.interpreted-text role="attr"} attribute may contain a portion of the separator.

The *separator* may also be a tuple of separators. In this case the return value will be the shortest possible that has any separator as the suffix. For the purposes of `LimitOverrunError`{.interpreted-text role="exc"}, the shortest possible separator is considered to be the one that matched.

::: versionadded
3.5.2
:::

::: versionchanged
3.13

The *separator* parameter may now be a `tuple`{.interpreted-text role="class"} of separators.
:::
:::::

::: method
at_eof()

Return `True` if the buffer is empty and `feed_eof`{.interpreted-text role="meth"} was called.
:::
:::::::::::

## StreamWriter

:::::::::::::::::: 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`{.interpreted-text role="func"} and `start_server`{.interpreted-text role="func"} instead.

::: method
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 *data* buffer should be a bytes, bytearray, or C-contiguous one-dimensional memoryview object.

The method should be used along with the `drain()` method:

    stream.write(data)
    await stream.drain()
:::

::: method
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()
:::

::: method
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()
:::

::: method
can_write_eof()

Return `True` if the underlying transport supports the `write_eof`{.interpreted-text role="meth"} method, `False` otherwise.
:::

::: method
write_eof()

Close the write end of the stream after the buffered write data is flushed.
:::

::: attribute
transport

Return the underlying asyncio transport.
:::

::: method
get_extra_info(name, default=None)

Access optional transport information; see `BaseTransport.get_extra_info`{.interpreted-text role="meth"} for details.
:::

::: {.method async=""}
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`{.interpreted-text role="meth"} returns immediately.
:::

::::: {.method async=""}
start_tls(sslcontext, \*, server_hostname=None, ssl_handshake_timeout=None, ssl_shutdown_timeout=None)

Upgrade an existing stream-based connection to TLS.

Parameters:

- *sslcontext*: a configured instance of `~ssl.SSLContext`{.interpreted-text role="class"}.
- *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 if `None` (default).
- *ssl_shutdown_timeout* is the time in seconds to wait for the SSL shutdown to complete before aborting the connection. `30.0` seconds if `None` (default).

::: versionadded
3.11
:::

::: versionchanged
3.12 Added the *ssl_shutdown_timeout* parameter.
:::
:::::

:::: method
is_closing()

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

::: versionadded
3.7
:::
::::

:::: {.method async=""}
wait_closed()

Wait until the stream is closed.

Should be called after `close`{.interpreted-text role="meth"} to wait until the underlying connection is closed, ensuring that all data has been flushed before e.g. exiting the program.

::: versionadded
3.7
:::
::::
::::::::::::::::::

## Examples

### TCP echo client using streams {#asyncio-tcp-echo-client-streams}

TCP echo client using the `asyncio.open_connection`{.interpreted-text role="func"} 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!'))

::: seealso
The `TCP echo client protocol <asyncio_example_tcp_echo_client_protocol>`{.interpreted-text role="ref"} example uses the low-level `loop.create_connection`{.interpreted-text role="meth"} method.
:::

### TCP echo server using streams {#asyncio-tcp-echo-server-streams}

TCP echo server using the `asyncio.start_server`{.interpreted-text role="func"} 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())

::: seealso
The `TCP echo server protocol <asyncio_example_tcp_echo_server_protocol>`{.interpreted-text role="ref"} example uses the `loop.create_server`{.interpreted-text role="meth"} 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 {#asyncio_example_create_connection-streams}

Coroutine waiting until a socket receives data using the `open_connection`{.interpreted-text role="func"} 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())

::: seealso
The `register an open socket to wait for data using a protocol
<asyncio_example_create_connection>`{.interpreted-text role="ref"} example uses a low-level protocol and the `loop.create_connection`{.interpreted-text role="meth"} method.

The `watch a file descriptor for read events
<asyncio_example_watch_fd>`{.interpreted-text role="ref"} example uses the low-level `loop.add_reader`{.interpreted-text role="meth"} method to watch a file descriptor.
:::