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What is a metaclass in Python, and what is the default metaclass for all classes? Why do most Python developers rarely need to write a custom metaclass in everyday code? How do you create a class dynamically using type() with three arguments instead of a class statement? How does __init_subclass__ provide a simpler alternative to metaclasses for many customization needs? What role does the class namespace dict play during class creation, and how can __prepare__ customize it? What is the difference between a class decorator and a metaclass for customizing class behavior? How does __set_name__ let a descriptor discover the attribute name it's assigned to within a class? What is the practical difference between isinstance(obj, cls) and type(obj) is cls, particularly regarding metaclasses and inheritance? How do you write a custom metaclass by subclassing type, and when would you do so? How does __class_getitem__ enable generic-looking syntax like MyClass[int]?
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What is the __mro__ attribute and how does it relate to metaclasses?

Beginner

Answer

__mro__ is a tuple on every class showing its Method Resolution Order -- the linear sequence Python searches through when looking up an attribute or method, computed by the C3 linearization algorithm. Since ordinary classes are instances of the `type` metaclass, __mro__ itself is exposed as a class attribute accessible via ClassName.__mro__ or the equivalent ClassName.mro() method.
class A: pass
class B(A): pass
class C(A): pass
class D(B, C): pass

print(D.__mro__)
# (<class D>, <class B>, <class C>, <class A>, <class object>)
Real-world example Debugging a diamond-inheritance bug, a developer prints D.__mro__ to see the exact order Python will search for a method, revealing why C's override of a method is being used instead of B's.

Common follow-ups: How does C3 linearization differ from simple depth-first search?;Can __mro__ be customized via a metaclass?

Multiple Inheritance & MRO;Metaclasses & Class Customization
All 15 Metaclasses & Class Customization questions