BUG: Fixed Series.replace for EA with casting

Closes pandas-dev#34530

Author: TomAugspurger

doc/source/whatsnew/v1.1.0.rst

@@ -1003,6 +1003,7 @@ Reshaping
 - Bug where :meth:`Index.astype` would lose the name attribute when converting from ``Float64Index`` to ``Int64Index``, or when casting to an ``ExtensionArray`` dtype (:issue:`32013`)
 - :meth:`Series.append` will now raise a ``TypeError`` when passed a DataFrame or a sequence containing Dataframe (:issue:`31413`)
 - :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`)
+- Bug in :meth:`~DataFrame.replace` raising an ``AssertionError`` when replacing values in an extension dtype with values of a different dtype (:issue:`34530`)
 - Bug on inplace operation of a Series that was adding a column to the DataFrame from where it was originally dropped from (using inplace=True) (:issue:`30484`)
 - Bug in :meth:`DataFrame.apply` where callback was called with :class:`Series` parameter even though ``raw=True`` requested. (:issue:`32423`)
 - Bug in :meth:`DataFrame.pivot_table` losing timezone information when creating a :class:`MultiIndex` level from a column with timezone-aware dtype (:issue:`32558`)

pandas/core/internals/blocks.py

@@ -745,7 +745,11 @@ def replace(
             if is_object_dtype(self):
                 raise
 
-            assert not self._can_hold_element(value), value
+            if not self.is_extension:
+                # TODO: https://github.com/pandas-dev/pandas/issues/32586
+                # Need an ExtensionArray._can_hold_element to indicate whether
+                # a scalar value can be placed in the array.
+                assert not self._can_hold_element(value), value
 
             # try again with a compatible block
             block = self.astype(object)

pandas/tests/extension/base/methods.py

@@ -452,3 +452,9 @@ def test_equals(self, data, na_value, as_series, box):
         # other types
         assert data.equals(None) is False
         assert data[[0]].equals(data[0]) is False
+
+    def test_replace_nonsense(self, data):
+        # https://github.com/pandas-dev/pandas/issues/34530
+        ser = pd.Series(data)
+        ser.replace("", "")  # no exception
+        ser.to_frame().replace("", "")  # no exception

pandas/tests/series/methods/test_replace.py

@@ -402,3 +402,8 @@ def test_replace_only_one_dictlike_arg(self):
         msg = "Series.replace cannot use dict-value and non-None to_replace"
         with pytest.raises(ValueError, match=msg):
             ser.replace(to_replace, value)
+
+    def test_replace_extension_other(self):
+        # https://github.com/pandas-dev/pandas/issues/34530
+        ser = pd.Series(pd.array([1, 2, 3], dtype="Int64"))
+        ser.replace("", "")  # no exception