API: replace() should raise an exception if invalid argument is given

doc/source/whatsnew/v1.1.0.rst

@@ -322,6 +322,7 @@ Reshaping
 - Bug in :func:`crosstab` when inputs are two Series and have tuple names, the output will keep dummy MultiIndex as columns. (:issue:`18321`)
 - :meth:`DataFrame.pivot` can now take lists for ``index`` and ``columns`` arguments (:issue:`21425`)
 - Bug in :func:`concat` where the resulting indices are not copied when ``copy=True`` (:issue:`29879`)
+- :meth:`DataFrame.replace` and :meth:`Series.replace` will raise a ``TypeError`` if ``to_replace`` is not an expected type. Previously the ``replace`` would fail silently (:issue:`18634`)
 
 
 Sparse

pandas/core/generic.py

@@ -6172,7 +6172,9 @@ def bfill(
         AssertionError
             * If `regex` is not a ``bool`` and `to_replace` is not
               ``None``.
+
         TypeError
+            * If `to_replace` is not a scalar, array-like, ``dict``, or ``None``
             * If `to_replace` is a ``dict`` and `value` is not a ``list``,
               ``dict``, ``ndarray``, or ``Series``
             * If `to_replace` is ``None`` and `regex` is not compilable
@@ -6181,6 +6183,7 @@ def bfill(
             * When replacing multiple ``bool`` or ``datetime64`` objects and
               the arguments to `to_replace` does not match the type of the
               value being replaced
+
         ValueError
             * If a ``list`` or an ``ndarray`` is passed to `to_replace` and
               `value` but they are not the same length.
@@ -6376,6 +6379,18 @@ def replace(
         regex=False,
         method="pad",
     ):
+        if not (
+            is_scalar(to_replace)
+            or isinstance(to_replace, pd.Series)
+            or is_re_compilable(to_replace)
+            or is_list_like(to_replace)
+        ):
+            raise TypeError(
+                "Expecting 'to_replace' to be either a scalar, array-like, "
+                "dict or None, got invalid type "
+                f"{repr(type(to_replace).__name__)}"
+            )
+
         inplace = validate_bool_kwarg(inplace, "inplace")
         if not is_bool(regex) and to_replace is not None:
             raise AssertionError("'to_replace' must be 'None' if 'regex' is not a bool")

pandas/tests/frame/methods/test_replace.py

@@ -1363,3 +1363,14 @@ 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)
+
+    def test_replace_invalid_to_replace(self):
+        # GH 18634
+        # API: replace() should raise an exception if invalid argument is given
+        df = pd.DataFrame({"one": ["a", "b ", "c"], "two": ["d ", "e ", "f "]})
+        msg = (
+            r"Expecting 'to_replace' to be either a scalar, array-like, "
+            r"dict or None, got invalid type.*"
+        )
+        with pytest.raises(TypeError, match=msg):
+            df.replace(lambda x: x.strip())

pandas/tests/series/methods/test_replace.py

@@ -362,3 +362,14 @@ def test_replace_no_cast(self, ser, exp):
         expected = pd.Series(exp)
 
         tm.assert_series_equal(result, expected)
+
+    def test_replace_invalid_to_replace(self):
+        # GH 18634
+        # API: replace() should raise an exception if invalid argument is given
+        series = pd.Series(["a", "b", "c "])
+        msg = (
+            r"Expecting 'to_replace' to be either a scalar, array-like, "
+            r"dict or None, got invalid type.*"
+        )
+        with pytest.raises(TypeError, match=msg):
+            series.replace(lambda x: x.strip())