Comparing two class types in python

Question

I have two classes defined in a module classes.py:

class ClassA(object):
    pass

class ClassB(object):
    pass

And in another m

Answer 1

If you want to check if types are equal then you should use is operator.

Example: we can create next stupid metaclass

class StupidMetaClass(type):
    def __eq__(self, other):
        return False

and then class based on it:

• in Python 2

  class StupidClass(object):
      __metaclass__ = StupidMetaClass
  

• in Python 3

  class StupidClass(metaclass=StupidMetaClass):
      pass
  

then a simple check

StupidClass == StupidClass

returns False, while the next check returns an expected True value

StupidClass is StupidClass

So as we can see == operator can be overridden while there is no simple way to change is operator's behavior.

Answer 2

You're comparing the type of the class object, which are all of type 'type'.

If you just want to compare the classes, compare them directly:

print Class3 == Class4

Answer 3

In addition to the other answers :

Python uses the concept of metaclasses, which are basically "classes of classes". That means, even a Class is an object in Python, which has its own class - accessible using the type in-build function.

Because ClassA and ClassB are by default instances of the same metaclass, the comparisons return True.

If you'd like to know more about metaclasses, this SO post is a good start.

Answer 4

Explanation

This is why your comparison doesn't work as expected

>>> class ClassA(object):
...     pass
... 
>>> class ClassB(object):
...     pass
... 
>>> type(ClassB)
<class 'type'> 
>>> type(ClassA)
<class 'type'> 
>>> type(ClassA) == type(ClassB)
True

But why do ClassA and ClassB have the same type type? Quoting the docs:

By default, classes are constructed using type(). The class body is executed in a new namespace and the class name is bound locally to the result of type(name, bases, namespace).

Example:

>>> ClassB
<class '__main__.ClassB'>
>>> type('ClassB', (), {})
<class '__main__.ClassB'>
>>> type(ClassB)
<class 'type'>
>>> type(type('ClassB', (), {}))
<class 'type'>

Getting the type of ClassB is exactly the same as getting the type of type('ClassB', (), {}), which is type.

Solutions

Compare them directly (w/out using the type() function):

>>> ClassA
<class '__main__.ClassA'>
>>> ClassB
<class '__main__.ClassB'>
>>> ClassA == ClassB
False

or initialize them and compare the types of their objects:

>>> a = ClassA()
>>> b = ClassB()
>>> type(a) 
<class '__main__.ClassA'>
>>> type(b) 
<class '__main__.ClassB'>
>>> type(a) == type(b)
False

FWIW you can also use is in place of == (for classes).