Test equality of two functions in python

Question

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.

Please describe the problem you are trying to solve. There might be an easier approach. — schlamar
– schlamar, Commented May 29, 2012 at 7:34
Yes, @ms4py, thank you for pointing that out. There could be easier solutions to the problem I am working on, like using a class instead of a function. But the thing that I really want to learn is how to 'override' == behavior for functions in python, or it is simply impossible. — dolaameng
– dolaameng, Commented May 29, 2012 at 7:47
No, I think there should be a more general solution than comparing functions, no matter how you do it technically. Can you describe why you want to compare them? — schlamar
– schlamar, Commented May 29, 2012 at 12:56
And you cannot override __eq__ for functions, see my answer below. — schlamar
– schlamar, Commented May 29, 2012 at 13:14

NPE · Accepted Answer · 2012-05-29 07:23:49Z

6

If you turned your functions into instances of some class that overrides __call__() as well as the comparison operators, it will be very easy to achieve the semantics you want.

answered May 29, 2012 at 7:23

NPE

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1 Comment

dolaameng Over a year ago

Yes, thanks for the tip. But I want to know how far I can really go without wrapping it within an object, like what we can do with functions in Ruby.

schlamar · Accepted Answer · 2012-05-29 13:14:11Z

2

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'

answered May 29, 2012 at 13:14

schlamar

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Comments

warvariuc · Accepted Answer · 2012-05-29 08:01:48Z

0

I don't think it's possible.

But overriding __call__ seems a nice solution to me.

answered May 29, 2012 at 8:01

warvariuc

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