object inheritance in python

Question

this morning I come out a weird ideal. as we know, in python everything is objects include class and function.

DONT ask why I want to do it. it is an experiment for playing around python. I knew it is inherited from instance.

let's try:

kk=dict()
print dir(dict)
print dir(kk)
output of them are same

class yy(dict): pass ---ok
class zz(kk)  : fail ---error
TypeError: Error when calling the metaclass bases
dict expected at most 1 arguments, got 3

Can anyone explain me in depth why I got this error message ?

again. if possible, please explain how python output this error message ?

Answer 1

It's because you need to inherit from a type. dict is a type, dict() is not, it's an instance of a dict. types type is type, and all instances of type are of type type - their instances are not.

>>> type(dict)
<class 'type'>
>>> type(dict())
<class 'dict'>
>>> isinstance(dict, type)
True
>>> isinstance(dict(), type)
False

Answer 2

The reason you get that particular error has to do with metaclasses. The metaclass of any class has to match or subclass the metaclass of each if it's base classes (for the 'is-a' rules of inheritance to work). You haven't supplied a metaclass, so python uses the type of the single base class by default. When the base class is dict, the metaclass is type (the default) and everything works well. When the base class is kk, Python tries to use dict as the metaclass - the problem being, dict isn't a metaclass. The error says 'dict does not follow the metaclass api'.