Add issue number to whatsnew, make test more readable

Author: MarcoGorelli

doc/source/whatsnew/v0.25.1.rst

@@ -121,7 +121,7 @@ Groupby/resample/rolling
 - Bug in windowing over read-only arrays (:issue:`27766`)
 - Fixed segfault in `pandas.core.groupby.DataFrameGroupBy.quantile` when an invalid quantile was passed (:issue:`27470`)
 - Bug in :meth:`pandas.core.groupby.SeriesGroupBy.nunique` where ``NaT`` values were interfering
-with the count of unique values
+with the count of unique values (:issue:`27951`)
 -
 
 Reshaping

pandas/core/groupby/generic.py

@@ -1143,8 +1143,9 @@ def nunique(self, dropna=True):
 
         val = self.obj._internal_get_values()
 
+        # GH 27951
         if dropna:
-            mask = np.where(val == val)[0]
+            mask = notna(val)
             ids = ids[mask]
             val = val[mask]
 

pandas/tests/groupby/test_function.py

@@ -1061,7 +1061,9 @@ def test_nunique_with_timegrouper():
     ],
 )
 def test_nunique_with_NaT(data, expected):
-    assert data.groupby(["key"])["data"].nunique()[0] == expected
+    # GH 27951
+    result = data.groupby(["key"])["data"].nunique()[0]
+    assert result == expected
 
 
 def test_nunique_preserves_column_level_names():