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Chapter 14 — Interview Questions: Inheritance and MRO

🟢 L3

Q1: Why is subclassing dict or list directly considered dangerous in Python?

Because dict and list are implemented in C. To optimize performance, their internal C-level methods often call other C-level methods directly, completely bypassing any overrides you write in Python. For example, if you override __setitem__ on a dict subclass, dict.update() will ignore your override.

Q2: How do you safely subclass built-in collections?

Instead of subclassing dict, list, or str directly, subclass UserDict, UserList, or UserString from the collections module. These classes are written in pure Python, meaning they are guaranteed to route all internal operations through the standard dunder methods you can override.

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Q3: What is a Mixin class? Give an example.

A Mixin class is a class designed to provide specific functionality (like logging, serialization, or validation) to other classes through multiple inheritance. It is not meant to be instantiated on its own. Example: Django's LoginRequiredMixin. You apply it to a View class (class DashboardView(LoginRequiredMixin, TemplateView):) to mix in authentication checks without writing a rigid inheritance hierarchy.

Q4: In class C(A, B):, if both A and B implement a process() method, which one gets called by C().process()?

Python resolves this using the Method Resolution Order (MRO). Because A is listed before B in the class signature, A will precede B in the __mro__. Therefore, A.process() will be called.

🔴 L5

Q5: Does super() always call the parent class?

No! This is a very common misconception. In multiple inheritance, super() delegates to the next class in the MRO. For example, in a diamond graph where Leaf inherits from A and B (which both inherit from Root), calling super().ping() from inside A.ping() will actually call B.ping(), not Root.ping(). It acts cooperatively, passing control sideways across the inheritance graph to ensure every class gets called exactly once.

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Q6: What is the C3 Linearization Algorithm? What problem does it solve?

C3 Linearization is the algorithm Python uses to compute the __mro__. It guarantees three properties:

  1. Local precedence order: If class C(A, B) is declared, A will always come before B in the MRO.
  2. Monotonicity: A subclass always comes before its base classes.
  3. Consistency: If a class appears before another class in one MRO, it must appear before that class in all other MROs in the hierarchy.

If these constraints are impossible to satisfy (e.g. A inherits from B, but a subclass inherits from B then A), the algorithm fails and Python raises a TypeError at class compilation time, preventing impossible diamond problems.