gevent patch poc

README.md

When run as

python -m main

the expected output is

[abu] patched dolly
[main] Abu

That's because we are patching the dolly module and the Dolly class is now the Abu class.

When run as

PYHTONPATH=. python -m main

we force the pre-loading of the sitecustomize.py script, which loads dolly and spawns a thread that later on creates an instance of dolly.Dolly. However, at this point the module is patched by main, whence

[sitecustomize] Dolly
[abu] patched dolly
[main] Abu
[thread] Abu

When run as

CLONE=1 PYHTONPATH=. python -m main

we ask abu to first make a "clone" of the dolly module by deleting the "dolly" entry from sys.modules. The call to __import__ will reload the module, effectively creating a clone that will be patched instead of the original one. This leaves the references acquired by the sitecustomize.py script intact, while all the code executed after the patching will make use of the patched cloned module:

[sitecustomize] Dolly
[abu] cloning dolly
[abu] patched dolly
[main] Abu
[thread] Dolly

abu.py

import os
import sys


clone = bool(os.getenv("CLONE", False))

class Abu(object):
    pass


def patch():
    global clone

    if clone:
        print("[abu] cloning dolly")
        del sys.modules["dolly"]

    dolly = __import__("dolly")
    dolly.Dolly = Abu
    print("[abu] patched dolly")

dolly.py

class Dolly(object):
    pass

main.py

import abu
abu.patch()

import dolly

d = dolly.Dolly()

print("[main] %s" % d.__class__.__name__)

sitecustomize.py

import dolly
from threading import Thread
from time import sleep


d = dolly.Dolly()

print("[sitecustomize] %s" % d.__class__.__name__)


def target():
    sleep(1)
    d = dolly.Dolly()
    print("[thread] %s" % d.__class__.__name__)


Thread(target=target).start()