Lazy superclass evaluation

October 23, 2023

I need to define a class MyClass within a developed package. The class MyClass should inherit from another class, from an external library ext_lib.Class. So the situation I want to achieve is:

class MyClass(ext_lib.Class):
    def __init__(self):
        super().__init__()

However, there is a trick to be done here. I want my package to work without ext_lib. It should be required iff a user wants to create a MyClass instance

I tried to use meta class:

class MetaClass(type):
    def __new__(cls, name, bases, attrs):
        import ext_lib
        bases += (ext_lib.Class,)
        new_cls = super().__new__(cls, name, bases, attrs)
        return new_cls


class MyClass(metaclass=MetaClass):
    def __init__(self):
        ...

However, I still have an error that the ext_lib is not installed before initialization.

Do you have any hint how to achieve the desired goal?

>Solution :

The __new__ method of a metaclass is called when a class object using the metaclass is created when the class statement is executed, so the import statement is effectively executed unconditionally with this approach.

A workaround is to use a function that dynamically creates the class only when called:

def MyClass(*args, **kwargs):
    import ext_lib

    class _MyClass(ext_lib.Class):
        def __init__(self, *args, **kwargs):
            pass
    
    return _MyClass(*args, **kwargs)