What is Method Resolution Order?
In object-oriented programming, method resolution order (MRO) refers to the order in which methods are searched for and executed in a class hierarchy. When a method is called on an object, Python needs to determine which implementation of the method to use. The method resolution order defines the sequence in which Python looks for the method in the class hierarchy.
The method resolution order is important because it determines the behavior of method inheritance and method overriding in Python. It ensures that the correct method is called when there are multiple methods with the same name in different classes.
How is Method Resolution Order determined?
In Python, the method resolution order is determined using the C3 linearization algorithm. This algorithm takes into account the order in which the base classes are defined and the order in which they are inherited.
The C3 linearization algorithm follows three main rules:
- Children precede their parents: If a class C is a subclass of classes A and B, then the methods of C should be searched before the methods of A and B.
- Parents are searched in the order of their appearance: If a class C is a subclass of classes A and B, and A is defined before B, then the methods of A should be searched before the methods of B.
- Order of appearance of classes is preserved: If a class C is a subclass of classes A, B, and D, and A is defined before B and D, then the methods of A should be searched before the methods of B and D.
By following these rules, the C3 linearization algorithm creates a linear order of the classes in the inheritance hierarchy, which is used to determine the method resolution order.
Understanding the C3 Linearization Algorithm
The C3 linearization algorithm can be a bit complex to understand, but let’s break it down step by step:
- Start with the class itself: The first step is to include the class itself in the linearization order.
- Add the linearization of the parent classes: Next, add the linearization of the parent classes in the order of their appearance.
- Remove duplicates: If a class appears more than once in the linearization order, remove all but the first occurrence.
- Add the class itself again: Finally, add the class itself again at the end of the linearization order.
By following these steps recursively for each class in the inheritance hierarchy, the C3 linearization algorithm determines the method resolution order.
Example of Method Resolution Order
Let’s consider an example to understand the method resolution order in Python:
«`python
class A:
def method(self):
print(«Method of class A»)
class B(A):
def method(self):
print(«Method of class B»)
class C(A):
def method(self):
print(«Method of class C»)
class D(B, C):
pass
d = D()
d.method()
«`
In this example, we have four classes: A, B, C, and D. Class D is a subclass of classes B and C, which are both subclasses of class A.
When we create an instance of class D and call the `method()` method, Python needs to determine which implementation of the method to use. The method resolution order in this case is D -> B -> C -> A.
Therefore, the output of the above code will be:
«`
Method of class B
«`
Even though class C is a subclass of class A, the method of class B is called because it appears before class C in the method resolution order.
Multiple Inheritance and Method Resolution Order
Method resolution order becomes particularly important when dealing with multiple inheritance in Python. Multiple inheritance allows a class to inherit from multiple parent classes.
When a class has multiple parent classes, the method resolution order determines the order in which the methods of the parent classes are searched for and executed.
It’s important to note that the method resolution order is not always straightforward in cases of multiple inheritance. In some cases, conflicts may arise when two or more parent classes define a method with the same name.
In such cases, the method resolution order determines which implementation of the method is used. The class that appears first in the method resolution order takes precedence over the classes that appear later.
Changing the Method Resolution Order
In some cases, you may want to change the method resolution order in Python. This can be done by explicitly specifying the order of the parent classes when defining a class.
To change the method resolution order, you can use the `super()` function to call the method of a specific parent class. By doing so, you can control the order in which the methods of the parent classes are called.
Here’s an example that demonstrates how to change the method resolution order:
«`python
class A:
def method(self):
print(«Method of class A»)
class B(A):
def method(self):
print(«Method of class B»)
class C(A):
def method(self):
print(«Method of class C»)
class D(B, C):
def method(self):
super(C, self).method()
d = D()
d.method()
«`
In this example, we have the same class hierarchy as before: A, B, C, and D. However, in class D, we explicitly call the `method()` method of class C using the `super()` function.
By doing so, we change the method resolution order to D -> B -> C -> A. Therefore, the output of the above code will be:
«`
Method of class C
«`
By explicitly specifying the order of the parent classes, we can control the method resolution order and determine which implementation of the method to use.
Conclusion
Method resolution order is an important concept in Python that determines the order in which methods are searched for and executed in a class hierarchy. It ensures that the correct method is called when there are multiple methods with the same name in different classes.
The method resolution order is determined using the C3 linearization algorithm, which takes into account the order in which the base classes are defined and the order in which they are inherited.
Understanding the method resolution order is crucial when dealing with multiple inheritance in Python. It allows you to control the order in which the methods of the parent classes are called and resolve any conflicts that may arise.
By changing the method resolution order using the `super()` function, you can explicitly specify the order of the parent classes and control the behavior of method inheritance and method overriding.
