Bug in DataFrame.replace casts columns to object dtype if items in to_replace not in values (#34048)

Author: simonjayhawkins

doc/source/whatsnew/v1.1.0.rst

@@ -751,6 +751,7 @@ Reshaping
 - Bug in :meth:`DataFrame.unstack` when MultiIndexed columns and MultiIndexed rows were used (:issue:`32624`, :issue:`24729` and :issue:`28306`)
 - Bug in :func:`concat` was not allowing for concatenation of ``DataFrame`` and ``Series`` with duplicate keys (:issue:`33654`)
 - Bug in :func:`cut` raised an error when non-unique labels (:issue:`33141`)
+- Bug in :meth:`DataFrame.replace` casts columns to ``object`` dtype if items in ``to_replace`` not in values (:issue:`32988`)
 
 
 Sparse

pandas/core/internals/blocks.py

@@ -728,11 +728,6 @@ def replace(
 
         mask = missing.mask_missing(values, to_replace)
 
-        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

@@ -1380,3 +1380,11 @@ def test_replace_invalid_to_replace(self):
         )
         with pytest.raises(TypeError, match=msg):
             df.replace(lambda x: x.strip())
+
+    @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)