Is it possible to inheret a metaclass.

[Peter Otten]

Antoon Pardon wrote:

> 
> 
> Op 9/04/20 om 18:37 schreef Peter Otten:
>> Antoon Pardon wrote:
>> 
>>> I am experimenting with subclasses that all need the same metaclass as
>>> the base class. Is there a way to make the metaclass be inherited, so
>>> that you don't have to repeat the "metaclass = MetaClass" with every
>>> subclass.
>> 
>> ?
>> 
>> This is not only possible, this is the default:
>> 
>>   >>> class Pardon(type): pass
>> ...
>>>>> class A(metaclass=Pardon): pass
>> ...
>>>>> class B(A): pass
>> ...
>>>>> type(B)
>> <class '__main__.Pardon'>
> 
> Can you explain what is wrong with the code below:
> 
> This produces only: Calling Pardon with A.
> 
> 
> def Pardon(cls, *args):
>      print("Calling Pardon with", cls)
>      return type(cls, *args)
> 
> class A(metaclass=Pardon): pass
> 
> class B(A): pass

As has been pointed out, the problem is the factory function you are using 
instead of a class. One way to look at it:

class Foo: 
    pass

def make_foo(): 
    return Foo()

foo = make_foo()
assert isinstance(foo, make_foo)

Would you expect the assertion to succeed? Would you expect that foo 
remembers that it has been created by make_foo()? 
No? Then replace the instance with the class, the class with the metaclass.