How Does Method Resolution Order (MRO) Differ in Python\'s New-Style Classes?

Method Resolution Order (MRO) in New-Style Classes

In Python, method resolution order (MRO) determines the order in which inheritance chains are searched for method attributes. While the MRO for old-style classes follows a simple depth-first approach, new-style classes introduce a modified order that ensures multiple inheritance works more intuitively.

MRO Difference in New-Style Classes

When the same ancestor class appears multiple times in the naive depth-first approach in legacy-style classes, it takes precedence over its subclasses. Consider the following "diamond inheritance" example:

<code class="python">class A:
    x = 'a'

class B(A):
    pass

class C(A):
    x = 'c'

class D(B, C):
    pass</code>

In legacy-style, the resolution order for D.x is: D - B - A - C - A. Thus, A's definition hides the override in C.

In new-style classes, however, the modified MRO ensures that overrides are honored. The MRO for the above example is:

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

Here, A is forced to appear only once after all its subclasses. This allows overrides in subclasses, such as C's override of x, to take effect.

Implications for Inheritance

The MRO difference in new-style classes has important implications for multiple inheritance. Legacy-style classes are prone to conflicts in diamond inheritance patterns. In contrast, new-style classes handle multiple inheritance more flexibly and allow overrides to work as expected.

Therefore, it is recommended to avoid old-style classes and use new-style classes instead. New-style classes ensure that multiple inheritance works intuitively and prevents unexpected conflicts in method resolution.

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