tail(0, seq) returns the whole sequence instead of empty (and is inconsistent across iterable types)

[pramodavansaber]

Summary

tail(n, seq) is implemented as seq[-n:] (with a deque fallback). For n == 0, seq[-0:] is seq[0:] — the entire sequence — so tail(0, seq) returns everything instead of the last zero elements. It is also inconsistent across input types: sliceable inputs return the whole sequence, while non-sliceable iterables hit the deque(seq, 0) fallback and correctly return empty.

Reproduction

from toolz import tail
print(tail(0, [10, 20, 30]))        # [10, 20, 30]   <- expected () / empty
print(tuple(tail(0, iter([10, 20, 30]))))   # ()      (deque fallback: empty)

Expected

tail(0, seq) returns an empty result (the last zero elements), consistently for any iterable.

Actual

Returns the whole sequence for sliceable inputs and empty for non-sliceable ones — inconsistent, and wrong for n == 0.

Fix sketch

Special-case n <= 0 to return empty, or route through the deque path uniformly.

Environment

toolz 1.1.0 (master @ 568c2b8), Python 3.12.

[hunterhogan]

I fixed the issue you described in https://github.com/hunterhogan/Z0Z_tools.

My hope is that enough people will contribute to the improvement of https://github.com/hunterhogan/Z0Z_tools that the maintainers of pytoolz will update toolz and cytoolz, then we would get rid of Z0Z_tools.

Therefore, I very much welcome your participation in the improvement of https://github.com/hunterhogan/Z0Z_tools.