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Discussion on: A Pythonic Guide to SOLID Design Principles

ezzy1337 profile image
Derek D. Author

Great question! I had to look up Liskov's again to make sure this example was still valid. Fortunately for me, and quite by accident, I think it still works. Let me try to explain.

Liskov's as it originally appeared states, "Let Φ(x) be a property provable about objects x of type T. Then Φ(y) should be true for objects y of type S where S is a subtype of T." So I took this as the principle applies to objects, not classes. In general, I think most people agree. So a class has a constructor which returns an object and as long as the object returned by the constructor can replace an object of the parent type you are following Liskov's.

All that being said with higher-level languages like Python it's probably a better practice to follow Liskov's even with the constructor since we can pass a Class name to a function and then construct an object of that class inside the function. In that scenario what I've written in this article would absolutely blow up.

joolius profile image

Thank you for the explanation!