How Python Method Resolution Order (MRO) Works

1. Definition and Purpose:

- MRO defines the order in which Python looks for methods in the class hierarchy.

- It is important to determine which method is called when multiple base classes define a method with the same name.

2. C3 Linearization:

- Python uses an algorithm called C3 Linearization to compute the MRO. This algorithm ensures a consistent and predictable method resolution order.

3. Algorithm Details:

- Base Class Order: The MRO is computed by considering the order of base classes and ensuring that each base class appears before its parents.

- Parents' MRO: The MRO of each parent class is also considered, ensuring that the MRO of parents is respected in the order.

- Resolution: The algorithm merges the MROs of base classes, following a left-to-right, depth-first search approach.

4. Steps in MRO Computation:

- Initialize: Start with the MRO of the class itself and the MROs of its base classes.

- Merge MROs: Merge the MROs of the base classes into the MRO of the class. This involves:

- Picking the first class that hasn’t been included yet and whose MRO is compatible with the current MRO.

- Removing that class from the list of potential candidates and including it in the MRO.

- Repeat: Continue merging until all classes are included in the MRO.

5. Example:

```python

class A:

def method(self):

print("A")

class B(A):

def method(self):

print("B")

class C(A):

def method(self):

print("C")

class D(B, C):

pass

d = D()

d.method() # Output: B

In this example, the MRO for class D is computed as follows:

- D inherits from B and C.

- B is checked before C because B appears first in the inheritance list.

- The MRO for D is [D, B, C, A].

6. Viewing MRO: :

- You can view the MRO of a class using the mro attribute or the mro() method

print(D.__mro__) # (<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

print(D.mro()) # [<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>]