[Collatz Conjecture]: Update recursion approach snippet

[AndrewLawendy]

The current approach to recursion is not ideal, as it utilizes a suboptimal way of returning values.
This approach should not be encouraged or taught to students, as it may lead to the use of tail recursion.

An alternative approach to recursion, which is considered more ideal, can be found at https://exercism.org/tracks/python/exercises/collatz-conjecture/solutions/bobahop.

Reference: https://stackoverflow.com/questions/33923/what-is-tail-recursion

[BethanyG]

@AndrewLawendy -- thanks for brining this up. However, I think you are misunderstanding Python and CPython here.

Python lacks any sort of tail call optimization (which you can also see is noted in the accepted answer of the iSO post you linked to). While the solution you link to has fewer steps due to the use of bit shifting, it is still tail-recursive, because Python is tail-recursive.

For more information, see this rundown by Al Sweigert, as well as this blog post by Guido van Rossum, the creator of Python.

You can also run each solution through python tutor to see the difference in steps, but the similarity in tail-calls.

[AndrewLawendy]

Ah, OK, thanks for sharing that.

I come from a JS background, and this code could lead to stack overflow.
I'll check the references.

[BethanyG]

I come from a JS background, and this code could lead to stack overflow.

Yup. It will in Python as well, without memoization or careful management. If you read to the bottom of the approach, it talks a bit about Python's recursion limit due to this danger.

[BethanyG]

I am going to go ahead and close this. I've alerted the other author of the approach, who is going to review things and maybe add a little more explanation or links about how recursion works in Python. 🙂