The Python 3.7 documentation for asyncio streams includes a TCP echo server example:
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())
This is the fragment that I'm particularly interested in:
async with server:
await server.serve_forever()
So we are doing two things:
We are using asyncio.Server as a context manager, for which (from that page) "it’s guaranteed that the Server object is closed and not accepting new connections when the async with statement is completed".
We are calling Server.serve_forever(). This usually starts listening (if not already started) and ensures the server is closed when the coroutine is cancelled. By the time we reach these lines we have already called start_server (with the default start_serving=True) so the only effect is ensuring the server is closed.
It seems like these are doing essentially the same thing. Why are both lines included in the example? Are most reasonable practical applications likely to include both?
The answer is the unnecessary redundancy can be seen here. The serve_forever method can be called if the server is already accepting connections.
In the Server object methods description, the serve_forever coroutine example is as the follows:
async def client_connected(reader, writer):
# Communicate with the client with
# reader/writer streams. For example:
await reader.readline()
async def main(host, port):
srv = await asyncio.start_server(
client_connected, host, port)
await srv.serve_forever()
asyncio.run(main('127.0.0.1', 0))
More information can be found in the official documentation.
Related
I've adapted this TCP echo server with streams example to my needs so that I can have multiple clients send data to my device:
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())
The issue that I'm having is that the clients are essentially producers for another task, but I'm not sure how to be able to consume the data. I've tried declaring and passing asyncio queues into the server and into the callback function, but I've had no luck, as the handle accepts only 2 arguments and I can't declare a queue that's inside the module as it won't be a part of the event loop that's declared in the main task.
Is there a way to do this without going back to sockets?
First: the fact that no asyncio loop is running when module level code is executed does not block the creation of an asyncio.Queue instance - just do it, if you want to use asyncio.Queue s at all: but any data structure will work for you, and maybe using a collections.deque, which is synchronous, will require less boiler plate when putting/retrieving content for it: since you will already have the data to be put/consumed in the queue, it being synchronous won't make any difference.
Second: if you don't want to have a module-level data structure for that, just create a class wrapping your handler: it will then get the reference to self. The same class can wrap other methods or code that will consume your data - for example, an "awaitable get".
import time
from collections import deque
class Handler:
def __init__(self):
self.queue = deque()
async def get(self, timeout=1):
start = time.time()
while time.time() - start <= timeout:
if not self.queue:
return self.queue.popleft()
await asyncio.sleep(.0001)
async def handle_echo(self, reader, writer):
...
self.queue.append(message)
...
async def main():
handler = Handler()
server = await asyncio.start_server(
handler.handle_echo, '127.0.0.1', 8888)
addrs = ', '.join(str(sock.getsockname()) for sock in server.sockets)
print(f'Serving on {addrs}')
# pass the handler instance into other async code that will
# consume the messages:
...
async with server:
await server.serve_forever()
asyncio.run(main())
I am studying asynchronous sockets in python these days for a bigger project. I just used the asyncio module and I referred the streams official documentation. For test purposes I created a server and a client that the server can handle a single client connected and after client is connected both server and client can chat each other.
server.py
import asyncio
async def handle(reader, writer):
while True:
data = await reader.read(100)
message_recieved = data.decode()
addr = writer.get_extra_info('peername')
print(f'{addr}::::{message_recieved}')
message_toSend = input('>>')
writer.write(message_toSend.encode())
await writer.drain()
async def main():
server = await asyncio.start_server(handle, '127.0.0.1', 10001)
addr = ', '.join(str(sock.getsockname()) for sock in server.sockets)
print(f'Serving on {addr}')
async with server:
await server.serve_forever()
asyncio.run(main())
client.py
import asyncio
async def client():
reader, writer = await asyncio.open_connection('127.0.0.1', 10001)
while True:
message = input('>>')
writer.write(message.encode())
data = await reader.read(100)
print(f'Recieved: {data.decode()}')
asyncio.run(client())
This is working fine. But now I have few questions.
How can I check whether is it working asynchronously?
Is it ok to use while loops like I did? (the reason for this question is I feel like when I used a while loop the loop becomes a synchronous part)?
Is this the correct way to code a simple client and server or are there any better ways of doing it?
I highly appreciate if someone experienced with this can help me.
Hello I am wanting to create a client socket via python, and I found this example (https://stackoverflow.com/a/49918082/12354066). The only problem I am wondering about is, I have a whole other program I want to implement this with, and it seems loop.run_until_complete(asyncio.wait(tasks)) is blocking the whole thread and not allowing me to execute any more functions i.e print(1) after loop.run_until_complete(asyncio.wait(tasks)). I want to be able to listen & send messages, but I also want to be able to execute other after I begin listening, maybe this is better suited for threads and not async (I don't know much async..)
import websockets
import asyncio
class WebSocketClient():
def __init__(self):
pass
async def connect(self):
'''
Connecting to webSocket server
websockets.client.connect returns a WebSocketClientProtocol, which is used to send and receive messages
'''
self.connection = await websockets.client.connect('ws://127.0.0.1:8765')
if self.connection.open:
print('Connection stablished. Client correcly connected')
# Send greeting
await self.sendMessage('Hey server, this is webSocket client')
return self.connection
async def sendMessage(self, message):
'''
Sending message to webSocket server
'''
await self.connection.send(message)
async def receiveMessage(self, connection):
'''
Receiving all server messages and handling them
'''
while True:
try:
message = await connection.recv()
print('Received message from server: ' + str(message))
except websockets.exceptions.ConnectionClosed:
print('Connection with server closed')
break
async def heartbeat(self, connection):
'''
Sending heartbeat to server every 5 seconds
Ping - pong messages to verify connection is alive
'''
while True:
try:
await connection.send('ping')
await asyncio.sleep(5)
except websockets.exceptions.ConnectionClosed:
print('Connection with server closed')
break
main:
import asyncio
from webSocketClient import WebSocketClient
if __name__ == '__main__':
# Creating client object
client = WebSocketClient()
loop = asyncio.get_event_loop()
# Start connection and get client connection protocol
connection = loop.run_until_complete(client.connect())
# Start listener and heartbeat
tasks = [
asyncio.ensure_future(client.heartbeat(connection)),
asyncio.ensure_future(client.receiveMessage(connection)),
]
loop.run_until_complete(asyncio.wait(tasks))
print(1) # never gets executed
I have a seemingly simple task that I can't quite wrap my brains around.
Here is what I need to do. Using socket module, start a server, use a client to start a connection, stop the server, return connection data - all in one script. I can do it when I run the two from two terminals but I need to put both server and client code in one script for automation. My problem is that socket.accept() is a blocking call and the script hangs before I can invoke the client. Tried playing with socket.setblocking(False) but it still blocks. I intuitively feel that I can accomplish this with asyncio module, but I have no experience with it and the examples I've seen don't seem to fit my task. Thanks much.
I need to put both server and client code in one script for automation. My problem is that socket.accept() is a blocking call and the script hangs before I can invoke the client. [...] I intuitively feel that I can accomplish this with asyncio module
Asyncio indeed makes it easy to start several tasks "in the background" (see asyncio.create_task) or "in parallel" (see asyncio.gather).
In fact, since the start_server API runs the server "in the background" to begin with (sort of how a server forks to daemonize itself, and you don't have to add & when starting it from a shell), you don't even need to do anything special to start the client and the server in parallel - just start the server, await the client coroutine, and stop the server.
As an example, starting with the echo client/server examples from the documentation, I've quickly arrived to something like this:
import asyncio
async def connect():
print('connecting...')
reader, writer = await asyncio.open_connection('127.0.0.1', 8888)
writer.write(b'hello world')
data = await reader.read(100)
assert data == b'hello world'
writer.close()
await writer.wait_closed()
print('closed connection')
return data
async def handle_client(reader, writer):
print('incoming connection')
while True:
data = await reader.read(100)
if data == b'':
break
writer.write(data)
await writer.drain()
print('incoming connection closed')
async def main():
server = await asyncio.start_server(handle_client, '127.0.0.1', 8888)
print('server now set up')
await connect()
server.close()
await server.wait_closed()
asyncio.run(main())
I am trying to create a script in python that listens to multiple sockets using websockets and asyncio, the problem is that no matter what I do it only listen to the first socket I call.
I think its the infinite loop, what are my option to solve this? using threads for each sockets?
async def start_socket(self, event):
payload = json.dumps(event)
loop = asyncio.get_event_loop()
self.tasks.append(loop.create_task(
self.subscribe(event)))
# this should not block the rest of the code
await asyncio.gather(*tasks)
def test(self):
# I want to be able to add corotines at a different time
self.start_socket(event1)
# some code
self.start_socket(event2)
this is what I did eventually, that way its not blocking the main thread and all subscriptions are working in parallel.
def subscribe(self, payload):
ws = websocket.WebSocket(sslopt={"cert_reqs": ssl.CERT_NONE})
ws.connect(url)
ws.send(payload)
while True:
result = ws.recv()
print("Received '%s'" % result)
def start_thread(self, loop):
asyncio.set_event_loop(loop)
loop.run_forever()
def start_socket(self, **kwargs):
worker_loop = asyncio.new_event_loop()
worker = Thread(target=self.start_thread, args=(worker_loop,))
worker.start()
worker_loop.call_soon_threadsafe(self.subscribe, payload)
def listen(self):
self.start_socket(payload1)
# code
self.start_socket(payload2)
# code
self.start_socket(payload3)
Your code appears incomplete, but what you've shown has two issues. One is that run_until_complete accepts a coroutine object (or other kind of future), not a coroutine function. So it should be:
# note parentheses after your_async_function()
asyncio.get_event_loop().run_until_complete(your_async_function())
the problem is that no matter what I do it only listen to the first socket I call. I think its the infinite loop, what are my option to solve this? using threads for each sockets?
The infinite loop is not the problem, asyncio is designed to support such "infinite loops". The problem is that you are trying to do everything in one coroutine, whereas you should be creating one coroutine per websocket. This is not a problem, as coroutines are very lightweight.
For example (untested):
async def subscribe_all(self, payload):
loop = asyncio.get_event_loop()
# create a task for each URL
for url in url_list:
tasks.append(loop.create_task(self.subscribe_one(url, payload)))
# run all tasks in parallel
await asyncio.gather(*tasks)
async def subsribe_one(self, url, payload):
async with websockets.connect(url) as websocket:
await websocket.send(payload)
while True:
msg = await websocket.recv()
print(msg)
One way to efficiently listen to multiple websocket connections from a websocket server is to keep a list of connected clients and essentially juggle multiple conversations in parallel.
E.g. A simple server that sends random # to each connected client every few secs:
import os
import asyncio
import websockets
import random
websocket_clients = set()
async def handle_socket_connection(websocket, path):
"""Handles the whole lifecycle of each client's websocket connection."""
websocket_clients.add(websocket)
print(f'New connection from: {websocket.remote_address} ({len(websocket_clients)} total)')
try:
# This loop will keep listening on the socket until its closed.
async for raw_message in websocket:
print(f'Got: [{raw_message}] from socket [{id(websocket)}]')
except websockets.exceptions.ConnectionClosedError as cce:
pass
finally:
print(f'Disconnected from socket [{id(websocket)}]...')
websocket_clients.remove(websocket)
async def broadcast_random_number(loop):
"""Keeps sending a random # to each connected websocket client"""
while True:
for c in websocket_clients:
num = str(random.randint(10, 99))
print(f'Sending [{num}] to socket [{id(c)}]')
await c.send(num)
await asyncio.sleep(2)
if __name__ == "__main__":
loop = asyncio.get_event_loop()
try:
socket_server = websockets.serve(handle_socket_connection, 'localhost', 6789)
print(f'Started socket server: {socket_server} ...')
loop.run_until_complete(socket_server)
loop.run_until_complete(broadcast_random_number(loop))
loop.run_forever()
finally:
loop.close()
print(f"Successfully shutdown [{loop}].")
A simple client that connects to the server and listens for the numbers:
import asyncio
import random
import websockets
async def handle_message():
uri = "ws://localhost:6789"
async with websockets.connect(uri) as websocket:
msg = 'Please send me a number...'
print(f'Sending [{msg}] to [{websocket}]')
await websocket.send(msg)
while True:
got_back = await websocket.recv()
print(f"Got: {got_back}")
asyncio.get_event_loop().run_until_complete(handle_message())
Mixing up threads and asyncio is more trouble than its worth and you still have code that will block on the most wasteful steps like network IO (which is the essential benefit of using asyncio).
You need to run each coroutine asynchronously in an event loop, call any blocking calls with await and define each method that interacts with any awaitable interactions with an async
See a working e.g.: https://github.com/adnantium/websocket_client_server