I want to modify a module xyz and its functions like that:

def modify(fun):
    modulename = fun.__module__ # this is string. ok, but not enough

import xyz
modify(xzy.test)

My problem is how to access the namespace of xzy inside modify. Sometimes

globals()[fun.__module__]

works. But then I get problems if the definition modify is in a different file than the rest of
the code.

Use the inspect module:

import inspect

def modify(fun):
    module = inspect.getmodule(fun)

This is the same as polling the module from sys.modules using fun.__module__. Although getmodule tries harder even if fun does not have a __module__ attribute.

You want to get the module object from its name? Look it up in the sys.modules dictionary that contains all currently loaded modules:

import sys

def modify(func):
    module = sys.modules[func.__module__]

You could try

modulename = fun.__module__
module = __import__(modulename)

You do not want to do this.

  1. It does not work. Imagine you defined a function in module abc and then imported it in xyz. test.__module__ would be 'abc' when you called modify(xyz.test). You would then know to change abc.test and you would not end up modifying xyz.test at all!

  2. Monkey patching should be avoided. Fooling with the global state of your program is ill-advised. Instead of doing this, why cannot you just make the new, modified thing and use it?