How to create a copy of a python function

Question

Is there a possibility to create real copies of python functions? The most obvious choice was http://docs.python.org/2/library/copy.html but there I read:

It does “copy” functions and classes (shallow and deeply), by
returning the original object unchanged;

I need a real copy, because I might change some attributes of the function.

Update:

I’m aware of all the possibilities which are mentioned in the comments. My use case is based on meta programming where I construct classes out of some declarative specifications. Complete details would be too long for SO, but basically I have a function like

def do_something_usefull(self,arg):
    self.do_work()

I will add this method to various classes. Thoses classes can be completly unrelated. Using mixin classes is not an option: I will have many such functions and would end up adding a base class for each function. My current “workaround” would be to wrap this function in a “factory” like this:

def create_do_something():
    def do_something_usefull(self,arg):
        self.do_work()

That way I always get a new do_something_useful function, but I have to wrap all my functions like this.

You can trust me, that I’m aware, that this is no “normal” OO programming. I know how to solve something like that “normally”. But this is a dynamic code generator and I would like to keep everything as lightweight and simple as possible. And as python functions are quite normal objects, I don’t think it’s too strange to ask how to copy them!?

Asked By: Achim

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Source

Answer 1

In Python3:

import types
import functools

def copy_func(f):
    """Based on http://stackoverflow.com/a/6528148/190597 (Glenn Maynard)"""
    g = types.FunctionType(f.__code__, f.__globals__, name=f.__name__,
                           argdefs=f.__defaults__,
                           closure=f.__closure__)
    g = functools.update_wrapper(g, f)
    g.__kwdefaults__ = f.__kwdefaults__
    return g

def f(arg1, arg2, arg3, kwarg1="FOO", *args, kwarg2="BAR", kwarg3="BAZ"):
    return (arg1, arg2, arg3, args, kwarg1, kwarg2, kwarg3)
f.cache = [1,2,3]
g = copy_func(f)

print(f(1,2,3,4,5))
print(g(1,2,3,4,5))
print(g.cache)
assert f is not g

yields

(1, 2, 3, (5,), 4, 'BAR', 'BAZ')
(1, 2, 3, (5,), 4, 'BAR', 'BAZ')
[1, 2, 3]

In Python2:

import types
import functools
def copy_func(f):
    """Based on http://stackoverflow.com/a/6528148/190597 (Glenn Maynard)"""
    g = types.FunctionType(f.func_code, f.func_globals, name=f.func_name,
                           argdefs=f.func_defaults,
                           closure=f.func_closure)
    g = functools.update_wrapper(g, f)
    return g

def f(x, y=2):
    return x,y
f.cache = [1,2,3]
g = copy_func(f)

print(f(1))
print(g(1))
print(g.cache)
assert f is not g

yields

(1, 2)
(1, 2)
[1, 2, 3]

Answered By: unutbu

How to create a copy of a python function

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