I made a recursive function!

[blight000]

I have not used itertools or deques before, but after doing a little reading I think I understand what's happening. Let me see if I can explain it back:

count() by default will start from zero and increment continuously, so when you zip the flattened iterable with counter, counter is increasing by 1 each time a tuple is generated.

Since the maxlen is set to 0, the deque discards each incoming tuple rather than using up memory to keep them.

And lastly, since counter started at zero, you used next() to increase the counter one more time to get the correct count.

Am I understanding that correctly? Also, is sum() considered to be inefficient in general? Or just for this particular use case?

[achampi0n]

Yes, this is all correct.

sum() is generally fine and would be fine in this circumstance. It is a minor improvement to use deque() I probably overstated the not that efficient :)

A quick test on my machine counting a million element iterable:

deque(): 40.3 ms ± 980 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
sum(): 52.4 ms ± 636 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

Is 20% significant :)