OOP Internals

Preview — 3 of 10 questions

What happens when the following code runs?

javascript

class MetaA(type):
    pass

class MetaB(type):
    pass

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaB):
    pass

class C(A, B):
    pass

AC is created successfully, using type as its metaclass, since neither MetaA nor MetaB was explicitly requested for C.

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

CSyntaxError

DC is created successfully, using MetaA (the first base's metaclass) automatically.

What is the print order when the following code runs?

javascript

class OrderTrackingMeta(type):
    @classmethod
    def __prepare__(mcs, name, bases):
        print("preparing namespace")
        return {}

    def __new__(mcs, name, bases, namespace):
        print("creating class")
        return super().__new__(mcs, name, bases, namespace)

class Foo(metaclass=OrderTrackingMeta):
    x = 1

Acreating class then preparing namespace

BOnly preparing namespace is printed — __new__ is skipped since __prepare__ already returned the namespace.

CNeither is printed — both hooks require an explicit call.

Dpreparing namespace then creating class

What is printed when the following code runs?

javascript

class Base:
    def __init__(self, **kwargs):
        print("Base kwargs:", kwargs)

class LoggingMixin:
    def __init__(self, verbose=False, **kwargs):
        self.verbose = verbose
        super().__init__(**kwargs)

class Widget(LoggingMixin, Base):
    def __init__(self, name, **kwargs):
        self.name = name
        super().__init__(**kwargs)

w = Widget(name="Button", verbose=True, color="red")
print(w.verbose, w.name)

ABase kwargs: {'color': 'red'} then True Button

BBase kwargs: {'color': 'red'} then False Button

CTypeError: __init__() got an unexpected keyword argument 'color'

DBase kwargs: {'verbose': True, 'color': 'red'} then True Button

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