[chrismorgan]

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

But the far bigger red flag there is that that it doesn’t just use arr.reverse(), which does the same thing and is typically 8–10× as fast in some simple testing (assuming a list), or arr[::-1], which makes a shallow copy rather than modifying the object in-place.

This matches what I’ve been seeing in code examples: Copilot likes to implement things from scratch rather than using libraries or even standard library functionality.

It’s possible that the word “array” tripped it up here and that it would have done something saner had it been told “list”, but I doubt it. (Python’s built-in array module is very seldom used; if you talk of arrays, you’re probably dealing with something like numpy’s arrays instead. But it’s far more likely that the built-in list type was what was desired here.)

There’s also one other significant point of bad and dangerous style in the code generated: the reverse function mutates its argument and returns it. Outside of fluent APIs (an uncommon pattern in Python, and not in use here), this is generally considered a bad idea in most languages, Python certainly included. It should either mutate its argument and return None, or not mutate its argument and return a new list.

[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.