ObjC doesn't support Python-style monkeypatching… but it does support some similar features like method swizzling, class posing, category interposition, etc. And PyObjC can do all of those things.
Off the top of my head, swizzling from PyObjC should look something like this:
def swizzle(cls, sel, func):
oldimp = cls.instanceMethodForSelector_(sel)
def wrapper(self, *args, **kwargs):
return func(self, oldimp, *args, **kwargs)
newmethod = objc.selector(wrapper,
selector=oldimp.selector, signature=oldimp.signature)
objc.classAddMethod(cls, sel, newmethod)
return wrapper