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Python Multiple Inheritance: Which Parent Method Runs When Names Collide

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EasySoftware EngineerOnsiteSoftware Engineering Fundamentals

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A Python fundamentals question about multiple inheritance in which two parent classes define a method with the same name and a child class inherits from both. It tests the method resolution order, C3 linearization, how super() follows that order, and when composition is the better design.

Two classes each define a method with the same name. A third class inherits from both of them. When that method is called on an instance of the third class, which class's version runs, and why?

class A:
 def greet(self):
 return "A"

class B:
 def greet(self):
 return "B"

class C(A, B):
 pass

print(C().greet())

What does this program print? What changes if the third class is declared as class C(B, A) instead? Explain the rule Python uses to decide.

Clarifying Questions

  • Does the third class inherit from both classes, as in class C(A, B), or does it receive instances of them and delegate to them?
  • Does the third class define the method itself, or call super() anywhere?
  • Do the two parent classes share a common base class?

Hints

Hint 1

Order of the bases: Look at the order in which the parent classes are listed in the class statement, and predict what happens when you swap them.

Hint 2

Ask the class: Every class can tell you the exact order in which Python searches classes for an attribute. Find out how to print that order.

Follow-up questions

  • Both parents define `__init__` and set `self.kw` to different values. What is `C().kw`, and how would you make both initializers run?
  • Show a class declaration for which Python refuses to create the class. What error do you get?
  • `A.greet` and `B.greet` each call `super().greet()`, and both inherit from a common `Base`. What does `C().greet()` return now?
  • When would you prefer composition over multiple inheritance in this situation?