How to check whether a variable is a class or not?

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Question :

How to check whether a variable is a class or not?

I was wondering how to check whether a variable is a class (not an instance!) or not.

I’ve tried to use the function isinstance(object, class_or_type_or_tuple) to do this, but I don’t know what type would a class will have.

For example, in the following code

class Foo: pass  
isinstance(Foo, **???**) # i want to make this return True.

I tried to substitute “class” with ???, but I realized that class is a keyword in python.

Answer #1:

Even better: use the inspect.isclass function.

>>> import inspect
>>> class X(object):
...     pass
... 
>>> inspect.isclass(X)
True

>>> x = X()
>>> isinstance(x, X)
True
>>> y = 25
>>> isinstance(y, X)
False
Answered By: Benjamin Peterson

Answer #2:

The inspect.isclass is probably the best solution, and it’s really easy to see how it’s actually implemented

def isclass(object):
    """Return true if the object is a class.

    Class objects provide these attributes:
        __doc__         documentation string
        __module__      name of module in which this class was defined"""
    return isinstance(object, (type, types.ClassType))
Answered By: andrea_crotti

Answer #3:

>>> class X(object):
...     pass
... 
>>> type(X)
<type 'type'>
>>> isinstance(X,type)
True
Answered By: S.Lott

Answer #4:

isinstance(X, type)

Return True if X is class and False if not.

Answered By: qnub

Answer #5:

This check is compatible with both Python 2.x and Python 3.x.

import six
isinstance(obj, six.class_types)

This is basically a wrapper function that performs the same check as in andrea_crotti answer.

Example:

>>> import datetime
>>> isinstance(datetime.date, six.class_types)
>>> True
>>> isinstance(datetime.date.min, six.class_types)
>>> False
Answered By: Sergey

Answer #6:

Benjamin Peterson is correct about the use of inspect.isclass() for this job.
But note that you can test if a Class object is a specific Class, and therefore implicitly a Class, using the built-in function issubclass.
Depending on your use-case this can be more pythonic.

from typing import Type, Any
def isclass(cl: Type[Any]):
    try:
        return issubclass(cl, cl)
    except TypeError:
        return False

Can then be used like this:

>>> class X():
...     pass
... 
>>> isclass(X)
True
>>> isclass(X())
False
Answered By: omni

Answer #7:

class Foo: is called old style class and class X(object): is called new style class.

Check this What is the difference between old style and new style classes in Python? . New style is recommended. Read about “unifying types and classes

Answered By: JV.

Answer #8:

simplest way is to use inspect.isclass as posted in the most-voted answer.
the implementation details could be found at python2 inspect and python3 inspect.
for new-style class: isinstance(object, type)
for old-style class: isinstance(object, types.ClassType)
em, for old-style class, it is using types.ClassType, here is the code from types.py:

class _C:
    def _m(self): pass
ClassType = type(_C)
Answered By: lyu.l

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