What's The Correct Way To Implement A Metaclass With A Different Signature Than `type`?

Say I want to implement a metaclass that should serve as a class factory. But unlike the type constructor, which takes 3 arguments, my metaclass should be callable without any argu

Solution 1:

An interface deviating from the parent signature is a questionable design in regular classes too. You don't need the extra complexity of metaclasses to get into this kind of mess - you can cause the same new/init jumble by subclassing a datetime or whatever.

I want to have a metaclass and an easy way to create instances of that metaclass.

The usual pattern in Python is to write a factory using a from_something classmethod. To take the example of creating datetime instances from a different init signature, there is for example datetime.fromtimestamp, but you have many other examples too (dict.fromkeys, int.from_bytes, bytes.fromhex...)

There is nothing specific to metaclasses here, so use the same pattern:

class MyMeta(type):
    @classmethod
    def from_no_args(cls, name=None):
        if name is None:
            name = cls.__name__ + 'Instance'
        return cls(name, (), {})

Usage:

>>> class A(metaclass=MyMeta):
...     pass
... 
>>> B = MyMeta.from_no_args()
>>> C = MyMeta.from_no_args(name='C')
>>> A.__name__
'A'
>>> B.__name__
'MyMetaInstance'
>>> C.__name__
'C'