Backport PR #38330: REGR: Groupby first/last/nth treats None as an observation (#38333)

Co-authored-by: Richard Shadrach <[email protected]>

Author: simonjayhawkins

Diff for: doc/source/whatsnew/v1.1.5.rst

@@ -27,6 +27,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.fillna` not filling ``NaN`` after other operations such as :meth:`DataFrame.pivot` (:issue:`36495`).
 - Fixed performance regression in ``df.groupby(..).rolling(..)`` (:issue:`38038`)
 - Fixed regression in :meth:`MultiIndex.intersection` returning duplicates when at least one of the indexes had duplicates (:issue:`36915`)
+- Fixed regression in :meth:`.GroupBy.first` and :meth:`.GroupBy.last` where ``None`` was considered a non-NA value (:issue:`38286`)
 
 .. ---------------------------------------------------------------------------
 

Diff for: pandas/_libs/groupby.pyx

@@ -926,9 +926,7 @@ def group_last(rank_t[:, :] out,
             for j in range(K):
                 val = values[i, j]
 
-                # None should not be treated like other NA-like
-                # so that it won't be converted to nan
-                if not checknull(val) or val is None:
+                if not checknull(val):
                     # NB: use _treat_as_na here once
                     #  conditional-nogil is available.
                     nobs[lab, j] += 1
@@ -937,7 +935,7 @@ def group_last(rank_t[:, :] out,
         for i in range(ncounts):
             for j in range(K):
                 if nobs[i, j] == 0:
-                    out[i, j] = NAN
+                    out[i, j] = None
                 else:
                     out[i, j] = resx[i, j]
     else:
@@ -1021,9 +1019,7 @@ def group_nth(rank_t[:, :] out,
             for j in range(K):
                 val = values[i, j]
 
-                # None should not be treated like other NA-like
-                # so that it won't be converted to nan
-                if not checknull(val) or val is None:
+                if not checknull(val):
                     # NB: use _treat_as_na here once
                     #  conditional-nogil is available.
                     nobs[lab, j] += 1
@@ -1033,7 +1029,7 @@ def group_nth(rank_t[:, :] out,
         for i in range(ncounts):
             for j in range(K):
                 if nobs[i, j] == 0:
-                    out[i, j] = NAN
+                    out[i, j] = None
                 else:
                     out[i, j] = resx[i, j]
 

Diff for: pandas/tests/groupby/test_nth.py

@@ -105,6 +105,26 @@ def test_first_last_with_None(method):
     tm.assert_frame_equal(result, df)
 
 
+@pytest.mark.parametrize("method", ["first", "last"])
+@pytest.mark.parametrize(
+    "df, expected",
+    [
+        (
+            DataFrame({"id": "a", "value": [None, "foo", np.nan]}),
+            DataFrame({"value": ["foo"]}, index=Index(["a"], name="id")),
+        ),
+        (
+            DataFrame({"id": "a", "value": [np.nan]}, dtype=object),
+            DataFrame({"value": [None]}, index=Index(["a"], name="id")),
+        ),
+    ],
+)
+def test_first_last_with_None_expanded(method, df, expected):
+    # GH 32800, 38286
+    result = getattr(df.groupby("id"), method)()
+    tm.assert_frame_equal(result, expected)
+
+
 def test_first_last_nth_dtypes(df_mixed_floats):
 
     df = df_mixed_floats.copy()