Add callbacks to any function in python

callbacks.py

from __future__ import print_function

class Watcher(object):
    def __init__(self, func):
        self.func = func
        self.before = []
        self.after = []

    def __call__(self, *args, **kargs):
        for callback in self.before:
            callback(*args, **kargs)

        retval = self.func(*args, **kargs)

        for callback in self.after:
            callback(*args, **kargs)

def foo():
    print("foo")

foo = Watcher(foo)
foo.before.append(lambda: print("before"))
foo.after.append(lambda: print("after"))