CODEWITHSUNDEEP
pythonpython-3.xclasstypes
Bee
Bee

Reputation: 274

How to tell the difference between a user-defined class and a built-in in Python 3?

I am porting some Python 2 code to 3. There is a section that relies on knowing if an object is an instance of a user-defined class, or a built-in. The Python 2 code used:

isinstance(my_object.__class__, Types.ClassType)

python modernize recommends changing "Types.ClassType" to "type". That doesn't work for me because all objects are returning true, we are a "type". From what I understand about classes and Python 3, there is not a difference anymore between built-ins and user-defined classes. I have done almost a full day of research and am getting nowhere. I can't believe I'm the only person in this situation, but maybe I'm not searching the right phrases....

So my question is, is it possible to tell the difference, or do I need to refactor the code so it won't matter?

Upvotes: 3

Views: 1313

Answers (2)

wim
wim

Reputation: 363063

You've likely misunderstood the context. That check is not for detecting a difference between user-defined class or built-in class. This is detecting a difference between old-style class:

class A:
    pass

And new-style class:

class B(object):
    pass

In Python 3+, there are no longer any old-style classes, everything inherits from object whether you asked for it or not. Therefore, you'll want to rewrite or remove any code that special-cased for old-style classes.

Related reading: What is the difference between old style and new style classes in Python?

Upvotes: 2

Draconis
Draconis

Reputation: 3461

Check the __module__ attribute of the type. That tells you where the type was defined.

>>> class Test: pass
... 
>>> a = 42
>>> b = Test()
>>> type(a).__module__
'builtins'
>>> type(b).__module__
'__main__'

When classes from other modules are involved it's a bit less trivial, but that was true in Python 2 as well.

Upvotes: 3

Related Questions