95. Method Resolution Order (MRO)

Here are 10 Python snippets to demonstrate Method Resolution Order (MRO) and how Python determines the order of method calls in multi-level and multiple inheritance scenarios:

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1. Basic MRO in Single Inheritance

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class A:
    def method(self):
        return "Method from A"

class B(A):
    pass

b = B()
print(b.method())  # Output: Method from A

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2. Overriding a Method

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class A:
    def method(self):
        return "Method from A"

class B(A):
    def method(self):
        return "Method from B"

b = B()
print(b.method())  # Output: Method from B

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3. Multi-Level Inheritance

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class A:
    def method(self):
        return "Method from A"

class B(A):
    pass

class C(B):
    pass

c = C()
print(c.method())  # Output: Method from A

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4. Diamond Problem in Multiple Inheritance

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class A:
    def method(self):
        return "Method from A"

class B(A):
    def method(self):
        return "Method from B"

class C(A):
    pass

class D(B, C):
    pass

d = D()
print(d.method())  # Output: Method from B

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5. Using super() in MRO

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class A:
    def method(self):
        return "Method from A"

class B(A):
    def method(self):
        return super().method() + " and B"

class C(B):
    def method(self):
        return super().method() + " and C"

c = C()
print(c.method())  # Output: Method from A and B and C

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6. Checking MRO with __mro__

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class A:
    pass

class B(A):
    pass

class C(B):
    pass

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

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7. Complex MRO with Multiple Inheritance

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class A:
    def method(self):
        return "Method from A"

class B:
    def method(self):
        return "Method from B"

class C(A, B):
    pass

c = C()
print(c.method())  # Output: Method from A
print(C.__mro__)  
# Output: (<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class 'object'>)

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8. Reordering Base Classes

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class A:
    def method(self):
        return "Method from A"

class B:
    def method(self):
        return "Method from B"

class C(B, A):  # B is listed before A
    pass

c = C()
print(c.method())  # Output: Method from B
print(C.__mro__)  
# Output: (<class '__main__.C'>, <class '__main__.B'>, <class '__main__.A'>, <class 'object'>)

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9. MRO with super() in Multiple Inheritance

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class A:
    def method(self):
        return "Method from A"

class B(A):
    def method(self):
        return super().method() + " and B"

class C(A):
    def method(self):
        return super().method() + " and C"

class D(B, C):
    def method(self):
        return super().method() + " and D"

d = D()
print(d.method())  # Output: Method from A and C and B and D
print(D.__mro__)  
# Output: (<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

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10. Using inspect.getmro

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import inspect

class A:
    pass

class B(A):
    pass

class C(B):
    pass

print(inspect.getmro(C))  
# Output: (<class '__main__.C'>, <class '__main__.B'>, <class '__main__.A'>, <class 'object'>)

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Key Points About MRO:

1. MRO stands for Method Resolution Order, the order in which Python looks for methods in a hierarchy of classes.

2. Python uses the C3 Linearization Algorithm to determine the MRO in case of multiple inheritance.

3. You can check the MRO of a class using:

   • ClassName.__mro__

   • inspect.getmro(ClassName)

   • help(ClassName)

4. super() is resolved according to the MRO, making it easier to call parent class methods dynamically in a predictable order.