What is the mechanism that allows Python monkey patching in this instance?

Question

Can someone explain the logic behind how this works with the Python interpreter? Is this behavior only thread local? Why does the assignment in the first module import persist after the second module import? I just had a long debugging session that came down to this.

external_library.py

def the_best():
    print "The best!"

modify_external_library.py

import external_library

def the_best_2():
    print "The best 2!"

external_library.the_best = the_best_2

main.py

import modify_external_library

import external_library

external_library.the_best()

Test:

$ python main.py
The best 2!

Answer 1

Nothing thread-local about this. somemodule.anattr = avalue is very global behavior! After this assignment the attribute is changed for good (until maybe changed back later) no matter what.

There's no mysterious mechanics at play! Assignment to any attribute of an object that allows such assignment (as module objects do) just work in the obvious way -- no thread-local anything, nothing strange -- and assignment to attribute persists, as long as the object whose attribute you've assigned persists, of course.

The repeated import external_library doesn't reload the module (reload is a totally separate builtin and import does not call it!) -- it just checks sys.modules, finds an external_library key in that dict, and binds the corresponding value (which was previously modified by that assignment) to name external_library in the appropriate namespace (here, globals of module main).