Backport PR #54983 on branch 2.1.x (BUG: pct_change showing unnecessary FutureWarning) (#55040)

Backport PR #54983: BUG: pct_change showing unnecessary FutureWarning

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

doc/source/whatsnew/v2.1.1.rst

@@ -34,6 +34,7 @@ Bug fixes
 ~~~~~~~~~
 - Fixed bug for :class:`ArrowDtype` raising ``NotImplementedError`` for fixed-size list (:issue:`55000`)
 - Fixed bug in :meth:`DataFrame.stack` with ``future_stack=True`` and columns a non-:class:`MultiIndex` consisting of tuples (:issue:`54948`)
+- Fixed bug in :meth:`Series.pct_change` and :meth:`DataFrame.pct_change` showing unnecessary ``FutureWarning`` (:issue:`54981`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_211.other:

pandas/core/generic.py

@@ -11692,15 +11692,21 @@ def pct_change(
                 stacklevel=find_stack_level(),
             )
         if fill_method is lib.no_default:
-            if self.isna().values.any():
-                warnings.warn(
-                    "The default fill_method='pad' in "
-                    f"{type(self).__name__}.pct_change is deprecated and will be "
-                    "removed in a future version. Call ffill before calling "
-                    "pct_change to retain current behavior and silence this warning.",
-                    FutureWarning,
-                    stacklevel=find_stack_level(),
-                )
+            cols = self.items() if self.ndim == 2 else [(None, self)]
+            for _, col in cols:
+                mask = col.isna().values
+                mask = mask[np.argmax(~mask) :]
+                if mask.any():
+                    warnings.warn(
+                        "The default fill_method='pad' in "
+                        f"{type(self).__name__}.pct_change is deprecated and will be "
+                        "removed in a future version. Call ffill before calling "
+                        "pct_change to retain current behavior and silence this "
+                        "warning.",
+                        FutureWarning,
+                        stacklevel=find_stack_level(),
+                    )
+                    break
             fill_method = "pad"
         if limit is lib.no_default:
             limit = None

pandas/tests/frame/methods/test_pct_change.py

@@ -160,3 +160,21 @@ def test_pct_change_with_duplicated_indices(fill_method):
         index=["a", "b"] * 3,
     )
     tm.assert_frame_equal(result, expected)
+
+
+def test_pct_change_none_beginning_no_warning():
+    # GH#54481
+    df = DataFrame(
+        [
+            [1, None],
+            [2, 1],
+            [3, 2],
+            [4, 3],
+            [5, 4],
+        ]
+    )
+    result = df.pct_change()
+    expected = DataFrame(
+        {0: [np.nan, 1, 0.5, 1 / 3, 0.25], 1: [np.nan, np.nan, 1, 0.5, 1 / 3]}
+    )
+    tm.assert_frame_equal(result, expected)

pandas/tests/series/methods/test_pct_change.py

@@ -107,3 +107,11 @@ def test_pct_change_with_duplicated_indices(fill_method):
 
     expected = Series([np.nan, np.nan, 1.0, 0.5, 2.0, 1.0], index=["a", "b"] * 3)
     tm.assert_series_equal(result, expected)
+
+
+def test_pct_change_no_warning_na_beginning():
+    # GH#54981
+    ser = Series([None, None, 1, 2, 3])
+    result = ser.pct_change()
+    expected = Series([np.nan, np.nan, np.nan, 1, 0.5])
+    tm.assert_series_equal(result, expected)