PERF: assert_index_equal with MultiIndex (#51544)

Author: jbrockmendel

pandas/_testing/asserters.py

@@ -279,8 +279,7 @@ def _get_ilevel_values(index, level):
         right = safe_sort_index(right)
 
     # MultiIndex special comparison for little-friendly error messages
-    if left.nlevels > 1:
-        left = cast(MultiIndex, left)
+    if isinstance(left, MultiIndex):
         right = cast(MultiIndex, right)
 
         for level in range(left.nlevels):
@@ -303,7 +302,7 @@ def _get_ilevel_values(index, level):
             _check_types(left.levels[level], right.levels[level], obj=obj)
 
     # skip exact index checking when `check_categorical` is False
-    if check_exact and check_categorical:
+    elif check_exact and check_categorical:
         if not left.equals(right):
             mismatch = left._values != right._values
 

pandas/tests/indexes/multi/test_setops.py

@@ -558,6 +558,8 @@ def test_union_with_missing_values_on_both_sides(nulls_fixture):
     mi2 = MultiIndex.from_arrays([[1, nulls_fixture, 3]])
     result = mi1.union(mi2)
     expected = MultiIndex.from_arrays([[1, 3, nulls_fixture]])
+    # We don't particularly care about having levels[0] be float64, but it is
+    expected = expected.set_levels([expected.levels[0].astype(np.float64)])
     tm.assert_index_equal(result, expected)