What is composition, and why is 'favor composition over inheritance' often recommended?
Composition builds behavior by holding collaborators; it avoids fragile base-class coupling
Composition means an object holds references to other objects and delegates work to them. Inheritance models an is-a relationship and hardcodes the implementation choice at class definition time; composition models has-a or uses-a and lets you swap behavior at runtime. The classic failure of inheritance is the fragile base class problem: a change in a parent can break subclasses, and deep hierarchies become hard to reason about. Composition keeps interfaces narrow, makes mocking trivial, and keeps the class open for extension without touching the base.
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Inheritance is appropriate for genuine is-a relationships with stable contracts, or for abstract interfaces.
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Composition is appropriate for behavior reuse, strategy selection, and anything you may want to swap per instance or per test.
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Trade-off: composition adds delegation boilerplate. Mitigate with explicit interfaces, getattr forwarding, or plain attributes.
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Common mistake: using inheritance only to reuse a method, producing a hierarchy that is conceptually wrong and fragile.
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Common mistake: overusing inheritance for cross-cutting concerns like logging or caching; mixins are inheritance too and can make MRO brittle.
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Version note: the principle is language-agnostic; Python's cooperative super() and ABCs make both styles practical, so the choice is a design decision, not a language limitation.
Scenario Questions
0-2 years experience
- 1You need a Logger class that can write to console or file. Inheritance or composition?
- 2What relationship does is-a describe and what relationship does has-a describe?
2-5 years experience
- 1A deep hierarchy breaks when a parent changes a helper method. How do you refactor toward composition?
- 2You need to swap a pricing strategy per order. How do you model it?
5-8 years experience
- 1You are designing a service layer where caching, retries, and metrics can be enabled independently. How do you compose them?
- 2Your mixin-based framework has fragile MRO interactions. What does the composition alternative look like?
8+ years experience
- 1Design a plugin system where behavior is composed at runtime and each plugin can be tested in isolation with fakes.
- 2Compare composition and inheritance on testability, deployment flexibility, and performance in a large service codebase.
Follow-up Questions
- When is inheritance genuinely the better choice than composition?
- How does dependency injection relate to favoring composition?