How to check in Python from which class methods is derived?

Question

I have two classes:

class A(object):
  def a(self):
    pass

class B(A):
  def b(self):
    pass

print dir(A)
print dir(B)

How can I check from which class methods is derived in Python?

For example:

getMethodClass(A.a) == A
getMethodClass(B.a) == A
getMethodClass(B.b) == B

Answer 1

Interesting question. Here is how I'd go about it.

(This works in python2. I haven't tested it in python3, but I won't be surprised if it does not work...)

You can iterate over all the "nominees" using reversed(inspect.getmro(cls)), and returning the first (by fetching the next value of the iterator) which satisfy the condition that it has the relevant attr, and that attr is the same as the method of the relevant cls.

Method identity-comparison is done by comparing the im_func attribute of the unbound method.

import inspect

def getMethodClass(cls, attr):
   return next(
      basecls for basecls in reversed(inspect.getmro(cls))
      if hasattr(basecls, attr)
      and getattr(basecls, attr).im_func is getattr(cls, attr).im_func
   )

getMethodClass(A, 'a')
=> __main__.A
getMethodClass(B, 'a')
=> __main__.A
getMethodClass(B, 'b')
=> __main__.B

# an alternative implementation, suggested by @chameleon
def getAttributeClass(cls, attrName):
  # check first if has attribute
  attr = getattr(cls, attrName)

  mro = inspect.getmro(cls)
  # only one class on list
  if len(mro) == 1:
    return cls

  # many class on list
  for base in reversed(mro[1:]):
    # check if defined in this base
    try:
      baseAttr = getattr(base, attrName)
    except AttributeError:
      continue
    else:
      if baseAttr.im_func is attr.im_func:
        return base
  # define in top class
  return cls

The function can also have the signature you suggest:

def getMethodClass(unbound_method):
    cls = unbound_method.im_class
    attr = unbound_method.__name__
    # rest of implementation is the same as before...

getMethodClass(B.a)
=> __main__.A