How can I make a deepcopy of a function in Python?

from functools import partial

def a():
    """Returns 1"""
    return 1

b = partial(a)
b.__doc__ = """Returns 1, OR DOES IT!"""

print help(a)
print help(b)

Wrap it as a partial?

My goal is to have two functions with the same implementation but with different docstrings.

Most users will do this, say the original function is in old_module.py:

def implementation(arg1, arg2): 
    """this is a killer function"""

and in new_module.py

from old_module import implementation as _implementation

def implementation(arg1, arg2):
    """a different docstring"""
    return _implementation(arg1, arg2)

This is the most straightforward way to reuse functionality. It is easy to read and understand the intent.

Nevertheless, perhaps you have a good reason for your main question:

How can I make a deepcopy of a function in Python?

To keep this compatible with Python 2 and 3, I recommend using the function's special __dunder__ attributes. For example:

import types

def copy_func(f, name=None):
    '''
    return a function with same code, globals, defaults, closure, and 
    name (or provide a new name)
    '''
    fn = types.FunctionType(f.__code__, f.__globals__, name or f.__name__,
        f.__defaults__, f.__closure__)
    # in case f was given attrs (note this dict is a shallow copy):
    fn.__dict__.update(f.__dict__) 
    return fn

And here's an example usage:

def main():
    from logging import getLogger as _getLogger # pyflakes:ignore, must copy
    getLogger = copy_func(_getLogger)
    getLogger.__doc__ += '\n    This function is from the Std Lib logging module.\n    '
    assert getLogger.__doc__ is not _getLogger.__doc__
    assert getLogger.__doc__ != _getLogger.__doc__

A commenter says:

This can’t work for built‑in functions

Well I wouldn't do this for a built-in function. I have very little reason to do this for functions written in pure Python, and my suspicion is that if you are doing this, you're probably doing something very wrong (though I could be wrong here).

If you want a function that does what a builtin function does, and reuses the implementation, like a copy would, then you should wrap the function with another function, e.g.:

_sum = sum
def sum(iterable, start=0):
    """sum function that works like the regular sum function, but noisy"""
    print('calling the sum function')
    return _sum(iterable, start)
    

def A():
    """A"""
    pass

def B():
    """B"""
    return A()

The FunctionType constructor is used to make a deep copy of a function.

import types
def copy_func(f, name=None):
    return types.FunctionType(f.func_code, f.func_globals, name or f.func_name,
        f.func_defaults, f.func_closure)

def A():
    """A"""
    pass
B = copy_func(A, "B")
B.__doc__ = """B"""