[cf499]

> That’s actually roughly len(arr) times, not three, since two of the calls are inside the loop.

You're right! I noticed it a few minutes ago and changed the wording accordingly. Thanks for pointing out how shockingly bad that algorithm actually is! :D

> (...) the reverse function mutates its argument and returns it.

Yeah, that mutate + return is confusing. It's also worth noting that, as a result of the mutation, the function doesn't work on immutable types like strings and tuples.

[kragen]

It's not shockingly bad to call len(arr) each time through the loop. It would be if we were talking about something like strlen() in C, but in Python it's just one more constant-time† operation each time through the loop, and not a very expensive one at that. Caching it in a local variable would still be better.

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† Nothing is really constant-time in CPython, but it's pretty close.