Question

Code:

import types


class C(object):
    pass


c = C()
print(isinstance(c, types.InstanceType))

Output:

False

What correct way to check if object is instance of user-defined class for new-style classes?

UPD:

I want put additional emphasize on if checking if type of object is user-defined. According to docs:

types.InstanceType
The type of instances of user-defined classes.

UPD2:

Alright – not “correct” ways are OK too.

UPD3:

Also noticed that there is no type for set in module types

Answer 1

You can combine the x.__class__ check with the presence (or not) of either '__dict__' in dir(x) or hasattr(x, '__slots__'), as a hacky way to distinguish between both new/old-style class and user/builtin object.

Actually, this exact same suggestions appears in https://stackoverflow.com/a/2654806/1832154

def is_instance_userdefined_and_newclass(inst):
    cls = inst.__class__
    if hasattr(cls, '__class__'):
        return ('__dict__' in dir(cls) or hasattr(cls, '__slots__'))
    return False

---

>>> class A: pass
... 
>>> class B(object): pass
... 
>>> a = A()
>>> b = B()
>>> is_instance_userdefined_and_newclass(1)
False
>>> is_instance_userdefined_and_newclass(a)
False
>>> is_instance_userdefined_and_newclass(b)
True