How Does Method Resolution Order (MRO) Differ Between Legacy and New-Style Classes in Python?

Method Resolution Order (MRO) Differences between Legacy and New-Style Classes

New-style classes in Python introduced a significant change in the method resolution order (MRO) compared to legacy classes. While the provided example demonstrates MRO resolution correctly for new-style classes, let's delve into the subtle differences that distinguish the two approaches.

The key difference emerges when an ancestor class appears multiple times in the "naive" depth-first resolution order. Consider the following "diamond inheritance" case:

class A:
    x = 'a'

class B(A):
    pass

class C(A):
    x = 'c'

class D(B, C):
    pass

Using legacy classes, the resolution order would be: D - B - A - C - A. In this order, A is encountered first, resulting in the definition of 'x' being hidden by the subsequent definition in C.

D.x => 'a'

However, new-style classes adopt a revised MRO:

D.__mro__ = (<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <type 'object'>)

This order ensures that ancestor classes appear only once and after all their subclasses. Consequently, overrides like C's modification of 'x' take precedence.

D.x => 'c'

By avoiding multiple appearances of ancestor classes in the resolution order, this approach eliminates ambiguities and ensures intuitive behavior in complex inheritance hierarchies. This is why old-style classes should generally be avoided in favor of new-style classes, especially when dealing with multiple inheritance and diamond-like patterns.

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