Gevent

Brandon Rozek

April 9, 2020

In my last post I spoke about concurrency with asyncio. Now what if you don’t want to concern yourself with async/await practices and just want to write synchronous code that executes I/O asynchronously? That’s where the library gevent comes in. It does this by modifying Python’s standard library during runtime to call it’s own asynchronous versions.

Last post code’s example written in gevent.

# The first two lines must be called before
# any other modules are loaded
import gevent
from gevent import monkey; monkey.patch_all()

import time

def think(duration):
    print("Starting to think for " + str(duration) + " seconds...")
    time.sleep(duration)
    print("Finished thinking for " + str(duration) + " seconds...")

gevent.wait([
    gevent.spawn(think, 5),
    gevent.spawn(think, 2)
])

Notice that the function think is written the same as the synchronous version.

gevent is written on top of C libraries libev or libuv . This combined with the monkey patching can make gevent based applications hard to debug if something goes wrong. Otherwise it’s a great tool to quickly take advantage of concurrency.