BUG-Fix: AssertionError when slicing MultiIndex and setting value of …

pandas/core/indexes/base.py

@@ -4947,6 +4947,7 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
         start_slice, end_slice = self.slice_locs(start, end, step=step, kind=kind)
 
         # return a slice
+
         if not is_scalar(start_slice):
             raise AssertionError("Start slice bound is non-scalar")
         if not is_scalar(end_slice):

pandas/core/indexes/multi.py

@@ -2928,12 +2928,8 @@ def convert_indexer(start, stop, step, indexer=indexer, codes=level_codes):
                     stop = len(level_index) - 1
                 step = key.step
             except KeyError:
-
-                # we have a partial slice (like looking up a partial date
-                # string)
-                start = stop = level_index.slice_indexer(
-                    key.start, key.stop, key.step, kind="loc"
-                )
+                start = key.start
+                stop = key.stop
                 step = start.step
 
             if isinstance(start, slice) or isinstance(stop, slice):

pandas/tests/indexing/test_loc.py

@@ -1119,3 +1119,22 @@ def test_loc_with_period_index_indexer():
     tm.assert_frame_equal(df, df.loc[list(idx)])
     tm.assert_frame_equal(df.iloc[0:5], df.loc[idx[0:5]])
     tm.assert_frame_equal(df, df.loc[list(idx)])
+
+
+def test_loc_setting_value_at_multiindex():
+    # GH 34870
+    arrays = [
+        ["bar", "bar", "baz", "baz", "foo", "foo", "qux", "qux"],
+        ["one", "two", "one", "two", "one", "two", "one", "two"],
+    ]
+    array = [1, 1, 1, 1, 1, 1, 1, 1]
+    answer = [1, 1, 100, 100, 100, 100, 1, 1]
+
+    tuples = list(zip(*arrays))
+    index = pd.MultiIndex.from_tuples(tuples, names=["first", "second"])
+
+    result = pd.Series(array, index=index)
+    expected = pd.Series(answer, index=index)
+
+    result.loc[("baz", "one"):("foo", "two")] = 100
+    tm.assert_series_equal(result, expected)