Question

I’m new to Python… and coming from a mostly Java background, if that accounts for anything.

I’m trying to understand polymorphism in Python. Maybe the problem is that I’m expecting the concepts I already know to project into Python. But I put together the following test code:

class animal(object):
    "empty animal class"

class dog(animal):
    "empty dog class"

myDog = dog()
print myDog.__class__ is animal
print myDog.__class__ is dog

From the polymorphism I’m used to (e.g. java’s instanceof), I would expect both of these statements to print true, as an instance of dog is an animal and also is a dog. But my output is:

False
True

What am I missing?

Answer 1

The is operator in Python checks that the two arguments refer to the same object in memory; it is not like the is operator in C#.

From the docs:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

What you’re looking for in this case is isinstance.

Return true if the object argument is an instance of the classinfo argument, or of a (direct or indirect) subclass thereof.

>>> class animal(object): pass

>>> class dog(animal): pass

>>> myDog = dog()
>>> isinstance(myDog, dog)
True
>>> isinstance(myDog, animal)
True

However, idiomatic Python dictates that you (almost) never do type-checking, but instead rely on duck-typing for polymorphic behavior. There’s nothing wrong with using isinstance to understand inheritance, but it should generally be avoided in “production” code.