How does Python\'s Method Resolution Order (MRO) ensure unambiguous method inheritance in new-style classes?

Method Resolution Order (MRO) in New-Style Classes

In the context of Python's class-based programming, Method Resolution Order (MRO) defines the sequence in which method lookups are performed on instances of a class. The implementation of MRO differs between old-style and new-style classes.

Old-Style Class Inheritance

In old-style classes, the MRO follows a depth-first approach. When searching for a method in an instance of a subclass, the interpreter traverses the inheritance hierarchy of the subclass, depth-first. The first occurrence of the method in the base classes is returned.

New-Style Class Inheritance

With the introduction of new-style classes, MRO semantics changed to C3 linearization. This approach eliminates the ambiguity that arose with multiple occurrences of the same base class in the inheritance hierarchy. The MRO for new-style classes is calculated as follows:

1. The leaf class (subclass with no subclasses) is placed in the MRO.


2. All base classes are added to the MRO before their subclasses.


3. For each base class in the MRO, recursively repeat steps 1-3 for its subclasses.


4. If a base class appears multiple times in the hierarchy, it is only included once in the MRO.

Example with Multiple Inheritance

Consider the following example involving multiple inheritance in new-style classes:

<code class="python">class Base1(object):
    def amethod(self):
        print("Base1")

class Base2(Base1):
    pass

class Base3(object):
    def amethod(self):
        print("Base3")

class Derived(Base2, Base3):
    pass

instance = Derived()  
instance.amethod()  
print(Derived.__mro__) </code>

In this example, even though Base1 occurs multiple times in the inheritance hierarchy, the MRO for Derived is:

(<class '__main__.Derived'>, <class '__main__.Base2'>, <class '__main__.Base1'>, <class '__main__.Base3'>, <type 'object'>)

This order corresponds to the C3 linearization algorithm described above. Since Derived is the leaf class, it is placed first in the MRO. Then, its base classes are added: Base2 (a subclass of Base1) and Base3, in that order. Finally, object is added as the ultimate base class.

Conclusion

The MRO for new-style classes provides a deterministic and unambiguous method for resolving method inheritance. It eliminates the potential ambiguities that could arise from multiple occurrences of the same base class, as was the case with old-style class inheritance.

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