How to make non-blocking raw_input when using eventlet.monkey_patch() and why it block everything, even when executed on another thread?

Question

I wrote this minimum code to explain my case:

import threading
import time
import eventlet

eventlet.monkey_patch()

def printing_function():
    while True:
        # here i want to do some work
        print "printing"
        time.sleep(1)

if __name__ == '__main__':
    thread = threading.Thread(target=printing_function)
    thread.start()

    while True:
        # here i want to wait for users input
        raw_input("raw input\n")
        print "inside main loop"
        time.sleep(1)

Even i have 2 threads, both of them are blocked when i call raw_input. When i comment out eventlet.monkey_patch(), only one thread is blocked and another keep printing "printing". Why and what should i do?

Answer 1

I'd say that there are a couple of things to note here:

• raw_input isn't patched by eventlet, so its calls are blocking
• threading is patched by eventlet, so threads are acting as coroutines

One way to workaround this would be to avoid patching threading, so that threads are real threads. To do that, you just need to replace:

eventlet.monkey_patch()

with:

eventlet.monkey_patch(os=True,
                     select=True,
                     socket=True,
                     thread=False,
                     time=True)

Note that when thread is True the following modules are patched: thread, threading, Queue.

Edit: If you want to patch threading and have an asynchronous raw_input, then I suggest the following implementation:

def raw_input(message):
    sys.stdout.write(message)

    select.select([sys.stdin], [], [])
    return sys.stdin.readline()

This will poll sys.stdin to check if it's ready for reading. If that's not the case, it will yield control to eventlet to let other coroutine execute.