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How does __class_getitem__ enable generic-looking syntax like MyClass[int]? How do abstract base classes (ABCs) use a metaclass to enforce that subclasses implement required methods? Why can't you freely mix two classes with different, incompatible metaclasses in multiple inheritance? How can a metaclass be used to implement the Singleton design pattern more robustly than overriding __new__ alone?
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How does __instancecheck__ and __subclasscheck__ on a metaclass enable 'virtual subclassing' as used by collections.abc?

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Answer

ABCMeta overrides __instancecheck__ and __subclasscheck__ so that `isinstance(obj, SomeABC)` can return True even if the object's class never explicitly inherited from SomeABC, as long as it was registered via `SomeABC.register(OtherClass)` or structurally satisfies the ABC's requirements. This lets unrelated classes (like built-in list) be recognized as implementing collections.abc.Sequence without modifying their actual inheritance chain.
from collections.abc import Sequence

print(isinstance([1,2,3], Sequence))  # True -- list wasn't explicitly declared as Sequence subclass

class MyIterable:
    pass

Sequence.register(MyIterable)
print(isinstance(MyIterable(), Sequence))  # True via virtual registration
Real-world example A validation framework checks `isinstance(value, collections.abc.Mapping)` to accept both built-in dicts and custom dict-like classes from third-party libraries that were registered as virtual subclasses.

Common follow-ups: What's the performance cost of custom __instancecheck__ implementations?;How does register() differ from actual inheritance?

Multiple Inheritance & MRO;Magic Methods & Operator Overloading
All 15 Metaclasses & Class Customization questions