Instance is an “object”, but class is not a subcla

问题:

How is it possible to have an instance of a class which is an object, without the class being a subclass of object? here is an example:

>>> class OldStyle(): pass
>>> issubclass(OldStyle, object)
False
>>> old_style = OldStyle()
>>> isinstance(old_style, object)
True

回答1:

In Python 2, type and class are not the same thing, specifically, for old-style classes, type(obj) is not the same object as obj.__class__. So it is possible because instances of old-style classes are actually of a different type (instance) than their class:

>>> class A(): pass
>>> class B(A): pass
>>> b = B()

>>> assert b.__class__ is B
>>> issubclass(b.__class__, A) # same as issubclass(B, A)
True
>>> issubclass(type(b), A)
False

>>> type(b)
<type 'instance'>
>>> b.__class__
<class __main__.B at 0x10043aa10>

This is resolved in new-style classes:

>>> class NA(object): pass
>>> class NB(NA): pass
>>> nb = NB()
>>> issubclass(type(nb), NA)
True
>>> type(nb)
<class '__main__.NB'>
>>> nb.__class__
<class '__main__.NB'>

Old-style class is not a type, new-style class is:

>>> isinstance(A, type)
False
>>> isinstance(NA, type)
True

Old style classes are declared deprecated. In Python 3, there are only new-style classes; class A() is equivalent to class A(object) and your code will yield True in both checks.

Take a look at this question for some more discussion: What is the difference between old style and new style classes in Python?

回答2:

Everything is an object:

isinstance(123, object) # True
isinstance("green cheese", object) # True
isinstance(someOldClassObject, object) # True
isinstance(someNewClassObject, object) # True
isinstance(object, object) # True
isinstance(None, object) # True

Note that this question has essentially nothing to do with old- vs. new-style classes. isinstance(old_style, object) being True is simply a corollary of the fact that every value in python is an instance of object.

回答3:

When you do the expression

   old_style = OldStyle()

It means you are instantiating the object, which old_style is an instance of the class OldStyle.

Also, both evaluates to True in Python 3.2.