object 与类型作为基类有什么区别？

Question

I would usually initialize a class this way,

class C:
  pass

but if I do something like this,

class A(object):
  pass

and if I do,

class B(type):
  pass

then, if I run,

dir(A)

it gives me this list,

['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__',
 '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__',
 '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__',
 '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__',
 '__str__', '__subclasshook__', '__weakref__']

while

dir(B)

gives me this list,

['__abstractmethods__', '__base__', '__bases__', '__basicsize__', '__call__',
 '__class__', '__delattr__', '__dict__', '__dictoffset__', '__dir__', '__doc__',
 '__eq__', '__flags__', '__format__', '__ge__', '__getattribute__', '__gt__',
 '__hash__', '__init__', '__init_subclass__', '__instancecheck__',
 '__itemsize__', '__le__', '__lt__', '__module__', '__mro__', '__name__',
 '__ne__', '__new__', '__prepare__', '__qualname__', '__reduce__',
 '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
 '__subclasscheck__', '__subclasses__', '__subclasshook__',
 '__text_signature__', '__weakrefoffset__', 'mro']

but I can use these on class A also, for example, I can use A.__bases__ , A.mro , although it is not listed in dir(A)

A.mro, A.__bases__

gives,

(<function A.mro>, (object,))

while,

B.mro, B.__bases__

gives,

(<method 'mro' of 'type' objects>, (type,))

Furthermore, when I create instances, then,

# both have object as base class
a = A() 
c = C()

works, but,

# type as base class
b = B()

gives this error,

TypeError: type.__new__() takes exactly 3 arguments (0 given)

It appears that by default object is the base class, but I still am confused when exactly should I specify, type as a base class? And what more differences are there between the two?

Answer 1

it appears that as soon as I specify type as the base class, my class becomes a class creator. so, instances of a class with type as the base class, will be classes. that is,

# with type as base class
b = B()

would give an error,

TypeError: type.__new__() takes exactly 3 arguments (0 given)

but,

b = B('B', (object,), {})

would work, and now, b itself is a class, and I can create instances of it

d = b()
type(d)

gives,

__main__.B

whereas, if I specified,

b = B('B', (type,), {})

then b is again a class creator, and instances of it will be classes themselves.