.. currentmodule:: asyncio .. _asyncio-streams: ======= 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. .. _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}') await writer.awrite(message.encode()) data = await reader.read(100) print(f'Received: {data.decode()!r}') print('Close the connection') await writer.aclose() asyncio.run(tcp_echo_client('Hello World!')) See also the `Examples`_ section below. .. rubric:: Stream Functions The following top-level asyncio functions can be used to create and work with streams: .. coroutinefunction:: 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 :class:`StreamReader` and :class:`StreamWriter` classes. 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 :class:`StreamReader` instance. By default the *limit* is set to 64 KiB. The rest of the arguments are passed directly to :meth:`loop.create_connection`. .. versionadded:: 3.7 The *ssl_handshake_timeout* parameter. .. coroutinefunction:: 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 :class:`StreamReader` and :class:`StreamWriter` classes. *client_connected_cb* can be a plain callable or a :ref:`coroutine function `; if it is a coroutine function, it will be automatically wrapped into a :class:`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 :class:`StreamReader` instance. By default the *limit* is set to 64 KiB. The rest of the arguments are passed directly to :meth:`loop.create_server`. .. versionadded:: 3.7 The *ssl_handshake_timeout* and *start_serving* parameters. .. rubric:: Unix Sockets .. coroutinefunction:: 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 :func:`open_connection` but operates on UNIX sockets. See also the documentation of :meth:`loop.create_unix_connection`. Availability: UNIX. .. versionadded:: 3.7 The *ssl_handshake_timeout* parameter. .. versionchanged:: 3.7 The *path* parameter can now be a :term:`path-like object` .. coroutinefunction:: 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 :func:`start_server` but works with UNIX sockets. See also the documentation of :meth:`loop.create_unix_server`. Availability: UNIX. .. versionadded:: 3.7 The *ssl_handshake_timeout* and *start_serving* parameters. .. versionchanged:: 3.7 The *path* parameter can now be a :term:`path-like object`. --------- StreamReader ============ .. class:: 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 :func:`open_connection` and :func:`start_server` instead. .. coroutinemethod:: 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 an EOF was received and the internal buffer is empty, return an empty ``bytes`` object. .. coroutinemethod:: readline() Read one line, where "line" is a sequence of bytes ending with ``\n``. If an EOF is received and ``\n`` was not found, the method returns partially read data. If an EOF is received and the internal buffer is empty, return an empty ``bytes`` object. .. coroutinemethod:: readexactly(n) Read exactly *n* bytes. Raise an :exc:`IncompleteReadError` if an EOF reached before *n* can be read. Use the :attr:`IncompleteReadError.partial` attribute to get the partially read data. .. coroutinemethod:: 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 :exc:`IncompleteReadError` exception will be raised, and the internal buffer will be reset. The :attr:`IncompleteReadError.partial` attribute may contain the separator partially. If the data cannot be read because of over limit, a :exc:`LimitOverrunError` exception will be raised, and the data will be left in the internal buffer, so it can be read again. .. versionadded:: 3.5.2 .. method:: at_eof() Return ``True`` if the buffer is empty and :meth:`feed_eof` was called. StreamWriter ============ .. class:: 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 :func:`open_connection` and :func:`start_server` instead. .. coroutinemethod:: awrite(data) Write *data* to the stream. The method respects control-flow, execution is paused if write buffer reaches high-water limit. .. versionadded:: 3.8 .. coroutinemethod:: aclose() Close the stream. Wait for finishing all closing actions, e.g. SSL shutdown for secure sockets. .. versionadded:: 3.8 .. method:: can_write_eof() Return *True* if the underlying transport supports the :meth:`write_eof` 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 :meth:`BaseTransport.get_extra_info` for details. .. method:: write(data) Write *data* to the stream. This method doesn't apply control-flow. The call should be followed by :meth:`drain`. .. method:: writelines(data) Write a list (or any iterable) of bytes to the stream. This method doesn't apply control-flow. The call should be followed by :meth:`drain`. .. coroutinemethod:: drain() Wait until it is appropriate to resume writing to the stream. E.g.:: 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-water limit, *drain()* blocks until the size of the buffer is drained down to the low-water limit and writing can be resumed. When there is nothing to wait for, the :meth:`drain` returns immediately. .. method:: close() Close the stream. .. method:: is_closing() Return ``True`` if the stream is closed or in the process of being closed. .. versionadded:: 3.7 .. coroutinemethod:: wait_closed() Wait until the stream is closed. Should be called after :meth:`close` to wait until the underlying connection is closed. .. versionadded:: 3.7 Examples ======== .. _asyncio-tcp-echo-client-streams: TCP echo client using streams ----------------------------- TCP echo client using the :func:`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!')) .. seealso:: The :ref:`TCP echo client protocol ` example uses the low-level :meth:`loop.create_connection` method. .. _asyncio-tcp-echo-server-streams: TCP echo server using streams ----------------------------- TCP echo server using the :func:`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()) .. seealso:: The :ref:`TCP echo server protocol ` example uses the :meth:`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 .. _asyncio-register-socket-streams: Register an open socket to wait for data using streams ------------------------------------------------------ Coroutine waiting until a socket receives data using the :func:`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()) .. seealso:: The :ref:`register an open socket to wait for data using a protocol ` example uses a low-level protocol and the :meth:`loop.create_connection` method. The :ref:`watch a file descriptor for read events ` example uses the low-level :meth:`loop.add_reader` method to watch a file descriptor.