dgilland-pydash/PR.md

Fix fill growing the array when start comes after end

Problem

pydash.fill(array, value, start, end) is documented to fill elements "from start up to, but not including, end" and to mutate the array in place. When start is greater than the effective end, it instead grows the array and duplicates elements:

>>> import pydash
>>> pydash.fill([1, 2, 3, 4], "x", 4, 1)
[1, 2, 3, 4, 3, 2, 3]      # expected [1, 2, 3, 4] (empty fill range)
>>> pydash.fill([1, 2, 3], "x", -1, -2)
[1, 2, 3, 3]               # expected [1, 2, 3]

An empty fill range should leave the array unchanged (this also matches lodash's _.fill, the API pydash mirrors).

Root cause

src/pydash/arrays.py, in fill:

if end is None:
    end = len(array)
else:
    end = min(end, len(array))

array[:] = array[:start] + [value] * len(array[start:end]) + array[end:]

The array is rebuilt from three slices: the head array[:start], the filled middle, and the tail array[end:]. This only works when 0 <= start <= end <= len(array). When start > end, array[:start] and array[end:] overlap, so the shared elements appear twice and the array grows. The fill range length is computed from array[start:end], which is empty in that case, so no value is actually written either.

Fix

Normalize start and end into the [0, length] range (supporting negative indices as slicing does) and fill in place with an index loop, keeping the array length fixed:

length = len(array)
if end is None:
    end = length
start = max(length + start, 0) if start < 0 else min(start, length)
end = max(length + end, 0) if end < 0 else min(end, length)
for index in range(start, end):
    array[index] = value
return array

When start >= end the loop simply does nothing, leaving the array untouched.

Test

Extended test_fill in tests/test_arrays.py with reversed and negative spans:

(([1, 2, 3, 4], "x", 4, 1), [1, 2, 3, 4]),
(([1, 2, 3, 4, 5], 0, 3, 2), [1, 2, 3, 4, 5]),
(([1, 2, 3, 4, 5], 0, -2), [1, 2, 3, 0, 0]),
(([1, 2, 3, 4, 5], 0, -2, -1), [1, 2, 3, 0, 5]),
(([1, 2, 3], "x", -1, -2), [1, 2, 3]),

test_fill also asserts the passed array is mutated in place, which the fix preserves.

Gate

• python -m pytest tests/test_arrays.py -k test_fill -> 11 passed
• python -m pytest tests/ --ignore=tests/pytest_mypy_testing -> 1920 passed

Counterfactual (fix reverted, tests kept): the reversed-span cases fail, e.g. assert [1, 2, 3, 4, 3, 2, 3] == [1, 2, 3, 4] (the array was grown).

Flags for the account owner

• The tests/pytest_mypy_testing/ suite fails locally with NameError (the reveal_type typing plugin is not resolvable in a bare pip install -e . environment). Pre-existing and unrelated to this change.
• Related but not fixed here: pad_start pads multi-character chars using the suffix of the repeated pattern (e.g. pad_start("abc", 4, "-.") -> ".abc"), whereas pad_end and lodash use the pattern prefix ("-abc"). Left out to keep this change to a single concern.