metaclass conflict upon dynamically creating classes with type (Python)

Question

The aim of the following code is to dynamically create a class based on the user's input.

class BaseClass:
    def __init__(self):
        self.placeholder = ""


if __name__ == '__main__':
    className = input("Please enter the name of new class: ")
    baseClass = input("Please enter name of base class: ")
    try:
        newClass = type(className, tuple(baseClass), {})
        print(f"Class {newClass} Created")
    except TypeError as e:
        print(e)

However, upon entering the input:

NewClass
BaseClass

I receive the follwing exception:

metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

Why the program doesn't recognized the class BaseClass? What would be the proper use of the type function?

Note: I'm running python v3.9.6.

Example program output snippet:

baseClass is a string not a reference to the actual class itself — gold_cy
– gold_cy, Commented Aug 7, 2021 at 11:57

Maurice Meyer · Accepted Answer · 2021-08-07 11:59:40Z

1

Bases have to be classes not strings, so you need to get the class from string name:

class BaseClass:
    def __init__(self):
        self.placeholder = ""


if __name__ == '__main__':
    #className = input("Please enter the name of new class: ")
    #baseClass = input("Please enter name of base class: ")

    className = 'Foo'
    baseClass = 'BaseClass'

    baseClassObj = globals().get(baseClass, None)
    if baseClassObj:
        newClass = type(className, (baseClassObj, ), dict())
        print(f"Class {newClass} Created")
    else:
        print('baseClass not found!')

Out:

Class <class '__main__.Foo'> Created

answered Aug 7, 2021 at 11:59

Maurice Meyer

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