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

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

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

put it in a function:

def makefunc( docstring ):
    def f():
        pass
    f.__doc__ = docstring
    return f

f = makefunc('I am f')
g = makefunc("I am f too")

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)
    

The others answers do not allow for serialization with pickle. Here a code that I am using to clone a function and allow for serialization for python3:

import pickle
import dill
import types

def foo():
    print ('a')


oldCode=foo.__code__

name='IAmFooCopied'

newCode= types.CodeType(
        oldCode.co_argcount,             #   integer
        oldCode.co_kwonlyargcount,       #   integer
        oldCode.co_nlocals,              #   integer
        oldCode.co_stacksize,            #   integer
        oldCode.co_flags,                #   integer
        oldCode.co_code,                 #   bytes
        oldCode.co_consts,               #   tuple
        oldCode.co_names,                #   tuple
        oldCode.co_varnames,             #   tuple
        oldCode.co_filename,             #   string
        name,                  #   string
        oldCode.co_firstlineno,          #   integer
        oldCode.co_lnotab,               #   bytes
        oldCode.co_freevars,             #   tuple
        oldCode.co_cellvars              #   tuple
        )

IAmFooCopied=types.FunctionType(newCode, foo.__globals__, name,foo.__defaults__ , foo.__closure__)
IAmFooCopied.__qualname__= name
print ( 'printing foo and the copy', IAmFooCopied, foo )
print ( 'dill output: ', dill.dumps(IAmFooCopied ))
print ( 'pickle Output: ', pickle.dumps (IAmFooCopied) )

Output:

printing foo and the copy <function IAmFooCopied at 0x7f8a6a8159d8> <function foo at 0x7f8a6b5f5268>
dill output:  b'x80x03cdill._dilln_create_functionnqx00(cdill._dilln_load_typenqx01Xx08x00x00x00CodeTypeqx02x85qx03Rqx04(Kx00Kx00Kx00Kx02KCCx0ctx00dx01x83x01x01x00dx00Sx00qx05NXx01x00x00x00aqx06x86qx07Xx05x00x00x00printqx08x85qt)Xx10x00x00x00testCloneFunc.pyqnXx0cx00x00x00IAmFooCopiedqx0bKx05Cx02x00x01qx0c))tqrRqx0ec__builtin__n__main__nhx0bNN}qx0ftqx10Rqx11.'
pickle Output:  b'x80x03c__main__nIAmFooCopiednqx00.'

You may encounter problem with the qualname attribute if you try this snippet with class methods (I think pickle should fail to find your function). I never tried it, however it should be easily fixable. Just check the doc about qualname

It’s quite easy to do using lambda and rest parameters:

def my_copy(f): 
        # Create a lambda that mimics f
        g = lambda *args: f(*args)
        # Add any properties of f
        t = list(filter(lambda prop: not ("__" in prop),dir(f)))
        i = 0
        while i < len(t):
            setattr(g,t[i],getattr(f,t[i]))
            i += 1
        return g
        
# Test
def sqr(x): return x*x
sqr.foo = 500

sqr_copy = my_copy(sqr)
print(sqr_copy(5)) # -> 25
print(sqr_copy(6)) # -> 36
print(sqr_copy.foo) # -> 500
print(sqr_copy == sqr) # -> False

Try it online!

Adjusted for python3

import types
def copy_func(f, name=None):
    return types.FunctionType(f.__code__, f.__globals__, name or f.__name__,
        f.__defaults__, f.__closure__)
def func1(x):
  return 2*x
func2=copy_func(func1)
print(func2(7))

I’ve implemented a general-purpose function copy in haggis, a library which I wrote and maintain (available with pip but probably not conda). haggis.objects.copy_func makes a copy on which you can not only reassign the __doc__ attribute, but also modify the module and __globals__ attributes effectively.

from haggis.objects import copy_func

def a(*args, **kwargs):
    """A docstring"""

a2 = copy_func(a)
a2.__doc__ = """Another docstring"""

>>> a == a2
False
>>> a.__code__ == a2.__code__
True
>>> a.__doc__
'A docstring'
>>> a2.__doc__
'Another docstring'