Hi all, In NumPy 1.21, the output of `np.unique` changed in the presence of multiple NaNs. Previously, all NaNs were returned when we now only return one (all NaNs were considered unique): a = np.array([1, 1, np.nan, np.nan, np.nan]) Before 1.21: >>> np.unique(a) array([ 1., nan, nan, nan]) After 1.21: array([ 1., nan]) This change was requested in an old issue: https://github.com/numpy/numpy/issues/2111 And happened here: https://github.com/numpy/numpy/pull/18070 While, it has a release note. I am not sure the change got the attention it deserved. This would be especially worrying if it is a regression for anyone? Cheers, Sebastian PS: One additional note, is that this does not work for object arrays (it cannot reasonable): >>> np.unique(a.astype(object)) array([1.0, nan, nan, nan], dtype=object)