BUG: Index.reindex raising with level and no MultiIndex

doc/source/whatsnew/v1.5.0.rst

@@ -384,6 +384,7 @@ Indexing
 - Bug in :meth:`Series.__setitem__` when setting incompatible values into a ``PeriodDtype`` or ``IntervalDtype`` :class:`Series` raising when indexing with a boolean mask but coercing when indexing with otherwise-equivalent indexers; these now consistently coerce, along with :meth:`Series.mask` and :meth:`Series.where` (:issue:`45768`)
 - Bug in :meth:`DataFrame.where` with multiple columns with datetime-like dtypes failing to downcast results consistent with other dtypes (:issue:`45837`)
 - Bug in :meth:`Series.loc.__setitem__` and :meth:`Series.loc.__getitem__` not raising when using multiple keys without using a :class:`MultiIndex` (:issue:`13831`)
+- Bug in :meth:`Index.reindex` raising ``AssertionError`` when ``level`` was specified but no :class:`MultiIndex` was given; level is ignored now (:issue:`35132`)
 - Bug when setting a value too large for a :class:`Series` dtype failing to coerce to a common type (:issue:`26049`, :issue:`32878`)
 - Bug in :meth:`loc.__setitem__` treating ``range`` keys as positional instead of label-based (:issue:`45479`)
 - Bug in :meth:`Series.__setitem__` when setting ``boolean`` dtype values containing ``NA`` incorrectly raising instead of casting to ``boolean`` dtype (:issue:`45462`)

pandas/core/indexes/base.py

@@ -4354,7 +4354,9 @@ def reindex(
         else:
             target = ensure_index(target)
 
-        if level is not None:
+        if level is not None and (
+            isinstance(self, ABCMultiIndex) or isinstance(target, ABCMultiIndex)
+        ):
             if method is not None:
                 raise TypeError("Fill method not supported if level passed")
 

pandas/tests/indexes/multi/test_reindex.py

@@ -42,9 +42,6 @@ def test_reindex_level(idx):
     with pytest.raises(TypeError, match="Fill method not supported"):
         idx.reindex(idx, method="pad", level="second")
 
-    with pytest.raises(TypeError, match="Fill method not supported"):
-        index.reindex(index, method="bfill", level="first")
-
 
 def test_reindex_preserves_names_when_target_is_list_or_ndarray(idx):
     # GH6552

pandas/tests/indexes/test_base.py

@@ -1110,6 +1110,14 @@ def test_reindex_no_type_preserve_target_empty_mi(self):
         assert result.levels[0].dtype.type == np.int64
         assert result.levels[1].dtype.type == np.float64
 
+    def test_reindex_ignoring_level(self):
+        # GH#35132
+        idx = Index([1, 2, 3], name="x")
+        idx2 = Index([1, 2, 3, 4], name="x")
+        expected = Index([1, 2, 3, 4], name="x")
+        result, _ = idx.reindex(idx2, level="x")
+        tm.assert_index_equal(result, expected)
+
     def test_groupby(self):
         index = Index(range(5))
         result = index.groupby(np.array([1, 1, 2, 2, 2]))