BUG: Do not raise deprecation warning for all nans in unique (#19301)

This PR adjusts np.unique for the edge cases where all values are nan.

Fixes gh-19300

2 files changed, 14 insertions, 1 deletions

diff --git a/numpy/lib/arraysetops.py b/numpy/lib/arraysetops.py
index 7600e17be..bd56b6975 100644
--- a/numpy/lib/arraysetops.py
+++ b/numpy/lib/arraysetops.py
@@ -339,7 +339,9 @@ def _unique1d(ar, return_index=False, return_inverse=False,
             aux_firstnan = np.searchsorted(np.isnan(aux), True, side='left')
         else:
             aux_firstnan = np.searchsorted(aux, aux[-1], side='left')
-        mask[1:aux_firstnan] = (aux[1:aux_firstnan] != aux[:aux_firstnan - 1])
+        if aux_firstnan > 0:
+            mask[1:aux_firstnan] = (
+                aux[1:aux_firstnan] != aux[:aux_firstnan - 1])
         mask[aux_firstnan] = True
         mask[aux_firstnan + 1:] = False
     else:
diff --git a/numpy/lib/tests/test_arraysetops.py b/numpy/lib/tests/test_arraysetops.py
index d62da9efb..13385cd24 100644
--- a/numpy/lib/tests/test_arraysetops.py
+++ b/numpy/lib/tests/test_arraysetops.py
@@ -610,6 +610,17 @@ class TestUnique:
         assert_equal(np.unique(a, return_inverse=True), (ua, ua_inv))
         assert_equal(np.unique(a, return_counts=True), (ua, ua_cnt))
 
+        # test for gh-19300
+        all_nans = [np.nan] * 4
+        ua = [np.nan]
+        ua_idx = [0]
+        ua_inv = [0, 0, 0, 0]
+        ua_cnt = [4]
+        assert_equal(np.unique(all_nans), ua)
+        assert_equal(np.unique(all_nans, return_index=True), (ua, ua_idx))
+        assert_equal(np.unique(all_nans, return_inverse=True), (ua, ua_inv))
+        assert_equal(np.unique(all_nans, return_counts=True), (ua, ua_cnt))
+
     def test_unique_axis_errors(self):
         assert_raises(TypeError, self._run_axis_tests, object)
         assert_raises(TypeError, self._run_axis_tests,