Backport PR #34048 on branch 1.0.x (Bug in DataFrame.replace casts columns to object dtype if items in to_replace not in values) (#34115)

Author: simonjayhawkins

doc/source/whatsnew/v1.0.4.rst

@@ -23,6 +23,7 @@ Fixed regressions
 - Bug where an ordered :class:`Categorical` containing only ``NaN`` values would raise rather than returning ``NaN`` when taking the minimum or maximum  (:issue:`33450`)
 - Bug in :meth:`DataFrameGroupBy.agg` with dictionary input losing ``ExtensionArray`` dtypes (:issue:`32194`)
 - Fix to preserve the ability to index with the "nearest" method with xarray's CFTimeIndex, an :class:`Index` subclass (`pydata/xarray#3751 <https://github.com/pydata/xarray/issues/3751>`_, :issue:`32905`).
+- Bug in :meth:`DataFrame.replace` casts columns to ``object`` dtype if items in ``to_replace`` not in values (:issue:`32988`)
 - 
 
 .. _whatsnew_104.bug_fixes:

pandas/core/internals/blocks.py

@@ -765,11 +765,6 @@ def replace(
             filtered_out = ~self.mgr_locs.isin(filter)
             mask[filtered_out.nonzero()[0]] = False
 
-        if not mask.any():
-            if inplace:
-                return [self]
-            return [self.copy()]
-
         try:
             blocks = self.putmask(mask, value, inplace=inplace)
             # Note: it is _not_ the case that self._can_hold_element(value)

pandas/tests/frame/methods/test_replace.py

@@ -1363,3 +1363,11 @@ def test_replace_after_convert_dtypes(self):
         result = df.replace(1, 10)
         expected = pd.DataFrame({"grp": [10, 2, 3, 4, 5]}, dtype="Int64")
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("dtype", ["float", "float64", "int64", "Int64", "boolean"])
+    @pytest.mark.parametrize("value", [np.nan, pd.NA])
+    def test_replace_no_replacement_dtypes(self, dtype, value):
+        # https://github.com/pandas-dev/pandas/issues/32988
+        df = pd.DataFrame(np.eye(2), dtype=dtype)
+        result = df.replace(to_replace=[None, -np.inf, np.inf], value=value)
+        tm.assert_frame_equal(result, df)