How Does Python's `super()` Function Handle Method Resolution Order (MRO) in Multiple Inheritance?

How Python's super() Interacts with Multiple Inheritance

Introduction

Multiple inheritance presents a unique challenge when determining which parent method to invoke using the super() function. This article explores the behavior of super() in such scenarios, focusing on its interaction with Method Resolution Order (MRO).

Super() and Parent Method Invocation

When using super() with multiple inheritance, it references the method of the parent class that is first in the MRO. For example, in the code snippet below:

class First(object):
    def __init__(self):
        print("first")

class Second(object):
    def __init__(self):
        print("second")

class Third(First, Second):
    def __init__(self):
        super(Third, self).__init__()
        print("that's it")

The MRO for Third is [Third, First, Second, object]. Therefore, super().__init__() in Third refers to First.__init__.

Method Resolution Order (MRO)

The MRO determines the order in which Python searches for methods and attributes inherited from parent classes. In general, the MRO lists the child class first, followed by its parents as they are listed in the class definition.

In the above example, the MRO is [Third, First, Second, object]. This order is used to resolve method calls and attribute lookups, starting from Third and moving down the tree of parent classes.

Customization and Ambiguity

While Python typically constructs the MRO automatically, it is possible to customize it using the mro attribute. However, it's important to ensure that the MRO is unambiguous. If it is not, Python will raise an exception.

Consider the following ambiguous MRO:

class First(object):
    def __init__(self):
        print("first")

class Second(First):
    def __init__(self):
        print("second")

class Third(First, Second):
    def __init__(self):
        print("third")

Should Third's MRO be [First, Second] or [Second, First]? There is no clear expectation, so Python raises an error:

TypeError: Error when calling the metaclass bases
    Cannot create a consistent method resolution order (MRO) for bases Second, First

Understanding MRO

To fully comprehend how super() interacts with multiple inheritance, it is crucial to grasp the concept of MRO. By understanding the rules governing the construction and resolution of MROs, developers can effectively use super() and avoid potential conflicts in multiple inheritance scenarios.

The above is the detailed content of How Does Python's `super()` Function Handle Method Resolution Order (MRO) in Multiple Inheritance?. For more information, please follow other related articles on the PHP Chinese website!