Fix issue #29837: added test case for aggregation with isnan

pandas/tests/groupby/aggregate/test_aggregate.py

@@ -994,3 +994,30 @@ def test_groupby_get_by_index():
     res = df.groupby("A").agg({"B": lambda x: x.get(x.index[-1])})
     expected = pd.DataFrame(dict(A=["S", "W"], B=[1.0, 2.0])).set_index("A")
     pd.testing.assert_frame_equal(res, expected)
+
+
+def test_aggregate_categorical_with_isnan():
+    # GH 29837
+    df = pd.DataFrame(
+        {
+            "A": [1, 1, 1, 1],
+            "B": [1, 2, 1, 2],
+            "numerical_col": [0.1, 0.2, np.nan, 0.3],
+            "object_col": ["foo", "bar", "foo", "fee"],
+            "categorical_col": ["foo", "bar", "foo", "fee"],
+        }
+    )
+
+    df = df.astype({"categorical_col": "category"})
+
+    result = df.groupby(["A", "B"]).agg(lambda df: df.isna().sum())
+    index = pd.MultiIndex.from_arrays([[1, 1], [1, 2]], names=("A", "B"))
+    expected = pd.DataFrame(
+        data={
+            "numerical_col": [1.0, 0.0],
+            "object_col": [0, 0],
+            "categorical_col": [0, 0],
+        },
+        index=index,
+    )
+    tm.assert_frame_equal(result, expected)