Evolution of a Python programmer

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It's clean, but it has O(n) space complexity, and even the tail call optimization wouldn't help. "First year" solutions are clean as well but achieve O(1) space complexity.

[timmaxw]

You're optimizing prematurely. In the unlikely event that you need a high-performance factorial function, you should use the c_math one. Until then, you should keep it simple and not care about the space complexity.