Test equality of two functions in python

equality, function, metaprogramming, python

Solution

It is not possible to override the `__eq__` implementation for functions (tested with Python 2.7)

>>> def f():
...   pass
...
>>> class A(object):
...   pass
...
>>> a = A()
>>> a == f
False
>>> setattr(A, '__eq__', lambda x,y: True)
>>> a == f
True
>>> setattr(f.__class__, '__eq__', lambda x,y: True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'function'

Problem

I want to make two functions equal to each other, like this: ``` def fn_maker(fn_signature): def _fn(): pass _fn.signature = fn_signature return _fn # test equality of two function instances based on the equality of their signature values >>> fa = fn_maker(1) >>> fb = fn_maker(1) >>> fc = fn_maker(2) >>> fa == fb # should be True, same signature values True >>> fa == fc # should be False, different signature values False ``` How should I do it? I know I could probably override `eq` and `ne` if fa, fb, fc are instances of some class. But here eq is not in dir(fa) and adding it the list doesnt work. I figured out some workaround like using a cache, e.g., ``` def fn_maker(fn_signature): if fn_signature in fn_maker.cache: return fn_maker.cache[fn_signature] def _fn(): pass _fn.signature = fn_signature fn_maker.cache[fn_signature] = _fn return _fn fn_maker.cache = {} ``` By this way there is a guarantee that there is only one function for the same signature value (kinda like a singleton). But I am really looking for some neater solutions.

Original source