How to compare two classes/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 module I am getting the attributes of the module:

import classes

Class1 = getattr(classes, 'ClassA')
Class2 = getattr(classes, 'ClassA')
print type(Class1) == type(Class2)

Class3 = getattr(classes, 'ClassA')
Class4 = getattr(classes, 'ClassB')
print type(Class3) == type(Class4)

Both type comparison are returning True and that's not what I was expecting.

How can I compare class types using python's native type values?

vaultah · Accepted Answer · 2015-02-24 17:33:18Z

18

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).

edited Feb 24, 2015 at 17:33

answered Apr 11, 2014 at 12:13

vaultah

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3 Comments

vaultah Over a year ago

@E.Beach I expected that, so I've added the way to compare them without initialization.

vaultah Over a year ago

@E.Beach anyway, added the extra explanation

NoName Over a year ago

Best examples I've seen so far! Thanks for omitting unnecessary stuff like the class body.

Azat Ibrakov · Accepted Answer · 2021-09-08 02:47:00Z

10

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

>>> StupidClass is StupidClass

returns an expected True value.

So as we can see == operator on classes can be overloaded while there is no way (at least simple one) to change is operator's behavior.

edited Sep 8, 2021 at 2:47

answered Mar 17, 2017 at 7:47

Azat Ibrakov

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Comments

Wooble · Accepted Answer · 2014-04-11 12:12:26Z

3

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

answered Apr 11, 2014 at 12:12

Wooble

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Comments

Community · Accepted Answer · 2017-05-23 12:10:05Z

1

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.

edited May 23, 2017 at 12:10

CommunityBot

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answered Apr 11, 2014 at 12:17

Danstahr

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