class - How to fake type with Python

Question

I recently developed a class named DocumentWrapper around some ORM document object in Python to transparently add some features to it without changing its interface in any way.

I just have one issue with this. Let's say I have some User object wrapped in it. Calling isinstance(some_var, User) will return False because some_var indeed is an instance of DocumentWrapper.

Is there any way to fake the type of an object in Python to have the same call return True?

Answer 1

You can use the __instancecheck__ magic method to override the default isinstance behaviour:

@classmethod
def __instancecheck__(cls, instance):
    return isinstance(instance, User)

This is only if you want your object to be a transparent wrapper; that is, if you want a DocumentWrapper to behave like a User. Otherwise, just expose the wrapped class as an attribute.

_{This is a Python 3 addition; it came with abstract base classes. You can't do the same in Python 2.}