Subclassing type vs object in Python3 [duplicate]

Question

I've been reading about metaclasses and I got lost when it came to type and object classes.

I understand that they are at the top of the hierarchy and they are implemented in C code. I also understand that type inherits from object and that object is an instance of type.

In one of the answers I've found on SO, someone said - in reagards to object-type relationship - that:

This kind of mutual inheritance is not normally possible, but that's the way it is for these fundamental types in Python: they break the rules.

My question is why is it implemented this way, what is purpose of such implementation? What problems does it solve/what are the benefits of this design? Couldn't it be just type or just object class that is at the top of the hierarchy that every class inherits from?

Finally, is there any difference between subclassing from object vs subclassing from type, and when would I want to use one over the other?

class Foo(object):
    pass

vs

class Foo(type):
    pass

Answer 1

There is no cross-inheritance between object and type. In fact, cross-inheritance is impossible.

# A type is an object
isinstance(int, object) # True

# But an object is not necessarily a type
isinstance(object(), type) # False

What is true in Python is that...

Everything is an object

Absolutly everything, object is the only base type.

isinstance(1, object) # True
isinstance('Hello World', object) # True
isinstance(int, object) # True
isinstance(object, object) # True
isinstance(type, object) # True

Everything has a type

Everything has a type, either built-in or user-defined, and this type can be obtained with type.

type(1) # int
type('Hello World') # str
type(object) # type

Not everything is a type

That one is fairly obvious

isinstance(1, type) # False
isinstance(isinstance, type) # False
isinstance(int, type) # True

type is its own type

This is the behaviour which is specific to type and that is not reproducible for any other class.

type(type) # type

In other word, type is the only object X in Python such that type(X) is X

type(type) is type # True

# While...
type(object) is object # False

This is because type is the only built-in metaclass. A metaclass is simply a class, but its instances are also classes themselves. So in your example...

# This defines a class
class Foo(object):
    pass

# Its instances are not types
isinstance(Foo(), type) # False

# While this defines a metaclass
class Bar(type):
    pass

# Its instances are types
MyClass = Bar('MyClass', (), {})

isinstance(MyClass, type) # True

# And it is a class
x = MyClass()

isinstance(x, MyClass) # True