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.
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 , 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 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() .
The sock argument transfers ownership of the socket to the StreamWriter created. To close the socket, call its close() method.
Changed in version 3.7: Added the ssl_handshake_timeout parameter.
New in version 3.8: Added happy_eyeballs_delay and interleave parameters.
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
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 , ssl_shutdown_timeout = None , start_serving = True ) ¶
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() .
The sock argument transfers ownership of the socket to the server created. To close the socket, call the server’s close() method.
Changed in version 3.7: Added the ssl_handshake_timeout and start_serving parameters.
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
coroutine asyncio. 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() but operates on Unix sockets.
See also the documentation of loop.create_unix_connection() .
Note
The sock argument transfers ownership of the socket to the StreamWriter created. To close the socket, call its close() method.
Changed in version 3.7: Added the ssl_handshake_timeout parameter. The path parameter can now be a path-like object
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
coroutine asyncio. 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 ) ¶
Start a Unix socket server.
Similar to start_server() but works with Unix sockets.
See also the documentation of loop.create_unix_server() .
The sock argument transfers ownership of the socket to the server created. To close the socket, call the server’s close() method.
Changed in version 3.7: Added the ssl_handshake_timeout and start_serving parameters. The path parameter can now be a path-like object .
Changed in version 3.10: Removed the loop parameter.
Changed in version 3.11: Added the ssl_shutdown_timeout parameter.
StreamReader¶
Represents a reader object that provides APIs to read data from the IO stream. As an asynchronous iterable , the object supports the async for statement.
It is not recommended to instantiate StreamReader objects directly; use open_connection() and start_server() instead.
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 . 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.
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.
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.
Return True if the buffer is empty and feed_eof() was called.
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.
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()
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()
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()
Return True if the underlying transport supports the write_eof() method, False otherwise.
Close the write end of the stream after the buffered write data is flushed.
Return the underlying asyncio transport.
get_extra_info ( name , default = None ) ¶
Access optional transport information; see BaseTransport.get_extra_info() for details.
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.
coroutine start_tls ( sslcontext , \* , server_hostname=None , ssl_handshake_timeout=None ) ¶
Upgrade an existing stream-based connection to TLS.
- sslcontext: a configured instance of SSLContext .
- 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).
Return True if the stream is closed or in the process of being closed.
Wait until the stream is closed.
Should be called after close() to wait until the underlying connection is closed, ensuring that all data has been flushed before e.g. exiting the program.
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()) 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!'))
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() 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())
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))
python example.py http://example.com/path/page.html