BUG: TypeError in groupby pct_change when fill_method is None

doc/source/whatsnew/v1.0.0.rst

@@ -869,6 +869,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrame.groupby` when using axis=1 and having a single level columns index (:issue:`30208`)
 - Bug in :meth:`DataFrame.groupby` when using nunique on axis=1 (:issue:`30253`)
 - Bug in :meth:`GroupBy.quantile` with multiple list-like q value and integer column names (:issue:`30289`)
+- Bug in :meth:`GroupBy.pct_change` and :meth:`SeriesGroupBy.pct_change` causes ``TypeError`` when ``fill_method`` is ``None`` (:issue:`30463`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/groupby/generic.py

@@ -809,6 +809,9 @@ def pct_change(self, periods=1, fill_method="pad", limit=None, freq=None):
                     periods=periods, fill_method=fill_method, limit=limit, freq=freq
                 )
             )
+        if fill_method is None:  # GH30463
+            fill_method = "pad"
+            limit = 0
         filled = getattr(self, fill_method)(limit=limit)
         fill_grp = filled.groupby(self.grouper.codes)
         shifted = fill_grp.shift(periods=periods, freq=freq)

pandas/core/groupby/groupby.py

@@ -2362,6 +2362,9 @@ def pct_change(self, periods=1, fill_method="pad", limit=None, freq=None, axis=0
                     axis=axis,
                 )
             )
+        if fill_method is None:  # GH30463
+            fill_method = "pad"
+            limit = 0
         filled = getattr(self, fill_method)(limit=limit)
         fill_grp = filled.groupby(self.grouper.codes)
         shifted = fill_grp.shift(periods=periods, freq=freq)

pandas/tests/groupby/test_transform.py

@@ -874,27 +874,19 @@ def test_pad_stable_sorting(fill_method):
         ),
     ],
 )
-@pytest.mark.parametrize(
-    "periods,fill_method,limit",
-    [
-        (1, "ffill", None),
-        (1, "ffill", 1),
-        (1, "bfill", None),
-        (1, "bfill", 1),
-        (-1, "ffill", None),
-        (-1, "ffill", 1),
-        (-1, "bfill", None),
-        (-1, "bfill", 1),
-    ],
-)
+@pytest.mark.parametrize("periods", [1, -1])
+@pytest.mark.parametrize("fill_method", ["ffill", "bfill", None])
+@pytest.mark.parametrize("limit", [None, 1])
 def test_pct_change(test_series, freq, periods, fill_method, limit):
-    # GH  21200, 21621
+    # GH  21200, 21621, 30463
     vals = [3, np.nan, np.nan, np.nan, 1, 2, 4, 10, np.nan, 4]
     keys = ["a", "b"]
     key_v = np.repeat(keys, len(vals))
     df = DataFrame({"key": key_v, "vals": vals * 2})
 
-    df_g = getattr(df.groupby("key"), fill_method)(limit=limit)
+    df_g = df
+    if fill_method is not None:
+        df_g = getattr(df.groupby("key"), fill_method)(limit=limit)
     grp = df_g.groupby(df.key)
 
     expected = grp["vals"].obj / grp["vals"].shift(periods) - 1