TST: add test case of ngroup with NaN value (pandas-dev#54966)

* add test case of ngroup with nan value

Co-authored-by: José Lucas Silva Mayer <[email protected]>
Co-authored-by: Willian Wang <[email protected]>

* fix linter issues

Co-authored-by: José Lucas Silva Mayer <[email protected]>
Co-authored-by: Willian Wang <[email protected]>

* use Categorical object instead of pd.Categorical

Co-authored-by: José Lucas Silva Mayer <[email protected]>
Co-authored-by: Willian Wang <[email protected]>

* use native assert function

Co-authored-by: Matthew Roeschke <[email protected]>

* test full result of ngroup method

Signed-off-by: José Lucas Silva Mayer <[email protected]>

---------

Signed-off-by: José Lucas Silva Mayer <[email protected]>
Co-authored-by: Willian Wang <[email protected]>
Co-authored-by: Matthew Roeschke <[email protected]>

Author: 3 people

pandas/tests/groupby/test_groupby.py

@@ -3189,6 +3189,14 @@ def test_depr_get_group_len_1_list_likes(test_series, kwarg, value, name, warn):
     tm.assert_equal(result, expected)
 
 
+def test_groupby_ngroup_with_nan():
+    # GH#50100
+    df = DataFrame({"a": Categorical([np.nan]), "b": [1]})
+    result = df.groupby(["a", "b"], dropna=False, observed=False).ngroup()
+    expected = Series([0])
+    tm.assert_series_equal(result, expected)
+
+
 def test_get_group_axis_1():
     # GH#54858
     df = DataFrame(