Reject n_samples<=0 in mcda sensitivity (#1371)#1373
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brendancol merged 1 commit intomainfrom Apr 29, 2026
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`sensitivity(..., method="monte_carlo", n_samples=0)` used to silently divide by zero in `_monte_carlo` and return a raster of zeros. The loop body never runs, `running_m2` stays at zero, and `0 / n_samples` is masked back to 0.0 by the trailing `np.where`. Negative `n_samples` fell through the same way (`range(-1)` is empty). Validate `n_samples >= 1` at the public entry point and raise ValueError naming the parameter. Three new tests cover n_samples=0, n_samples=-1, and n_samples=1 (the meaningful minimum). Cat 3 sibling of the NaN-weight bug fixed in #1312.
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Summary
Closes #1371.
sensitivity(..., method="monte_carlo", n_samples=0)used to silently divide by zero inside_monte_carloand hand the caller back a raster of zeros. Withn_samples=0the sample loop never runs,running_m2stays zero, and the trailingnp.where(running_mean != 0, ...)masks the resulting NaN as 0.0. Negative values fell through the same way becauserange(-1)is empty.Cat 3 sibling of the NaN-weight bug fixed in #1312. Both came out of the same mcda audit; this one was deferred per the one-fix-per-PR convention.
Changes
xrspatial/mcda/sensitivity.py: validaten_samples >= 1at the publicsensitivity()entry point whenmethod="monte_carlo", raise ValueError naming the parameter. Docstring updated to note the minimum.xrspatial/tests/test_mcda.py: 3 new tests coveringn_samples=0(raises),n_samples=-1(raises), andn_samples=1(succeeds with no NaNs). One sample is the meaningful minimum;test_single_sample_zero_cvalready exercisedn_samples=1, so this just adds a sanity assertion that the new check doesn't break it.All 169 mcda tests pass.
Test plan
pytest xrspatial/tests/test_mcda.py -k "n_samples_zero or n_samples_negative or n_samples_one"-- 3/3 passpytest xrspatial/tests/test_mcda.py-- 169/169 passn_samples