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) 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. *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. Deprecated since version 3.8, removed in version 3.10: The "loop" parameter. This function has been implicitly getting the current running loop since 3.7. See What's New in 3.10's Removed section for more information. 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, 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". *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. Deprecated since version 3.8, removed in version 3.10: The "loop" parameter. This function has been implicitly getting the current running loop since 3.7. See What's New in 3.10's Removed section for more information. -[ Unix Sockets ]- coroutine asyncio.open_unix_connection(path=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* Deprecated since version 3.8, removed in version 3.10: The "loop" parameter. This function has been implicitly getting the current running loop since 3.7. See What's New in 3.10's Removed section for more information. coroutine asyncio.start_unix_server(client_connected_cb, path=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*. Deprecated since version 3.8, removed in version 3.10: The "loop" parameter. This function has been implicitly getting the current running loop since 3.7. See What's New in 3.10's Removed section for more information. 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) 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() 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.