TIL that gevent does not switch when you write to a file, even with monkey patching

gevent_file_io.py

#! encoding: utf-8
"""Apparently gevent doesn't switch when writing to files on disk."""
from __future__ import print_function
import gevent
from gevent import monkey
from gevent.fileobject import FileObject
monkey.patch_all()

import socket
import tempfile


def doWorkWithFile(string):
    print(string + " before")
    with tempfile.NamedTemporaryFile() as f:
        f.write("this is a long sentence maybe a few bytes" * 10)
        f.flush()
        f.write("this is a long sentence maybe a few bytes" * 10)
        f.flush()
    print(string + " after")


def doWorkWithSocket(string):
    print(string + " before")
    socket.gethostbyname('affirm.com')
    print(string + " after")


def doWorkWithGeventFile(string):
    print(string + " before")
    with tempfile.NamedTemporaryFile() as t, \
        FileObject(t, mode='wb', close=False) as f:
            f.write("this is a long sentence maybe a few bytes" * 10)
            f.flush()
            f.write("this is a long sentence maybe a few bytes" * 10)
            f.flush()
    print(string + " after")


def main():
    print('== Control ==')
    print('Notice that when using `socket`, the greenlets switch out on block.')
    jobs = [gevent.spawn(doWorkWithSocket, string) for string in ['A', 'B', 'C']]
    gevent.joinall(jobs, timeout=2)
    print('== Treatment ==')
    print('Notice that when using `file`, the greenlets do not switch out on block.')
    jobs = [gevent.spawn(doWorkWithFile, string) for string in ['A', 'B', 'C']]
    gevent.joinall(jobs, timeout=2)
    print('== Treatment ==')
    print('Notice that even when using `FileObject`, the greenlets do not switch out on block.')
    jobs = [gevent.spawn(doWorkWithGeventFile, string) for string in ['A', 'B', 'C']]
    gevent.joinall(jobs, timeout=2)


if __name__ == '__main__':
    main()