BUG: Fix join on MultiIndex for mixed Datetimelike and string levels

doc/source/whatsnew/v1.1.0.rst

@@ -324,7 +324,7 @@ MultiIndex
         # Common elements are now guaranteed to be ordered by the left side
         left.intersection(right, sort=False)
 
--
+- Bug in :meth:`Index.join` for MultiIndex when joining level between datetimelike and string(:issue:`26558`)
 
 I/O
 ^^^

pandas/core/dtypes/generic.py

@@ -28,6 +28,9 @@ def _check(cls, inst) -> bool:
 ABCTimedeltaIndex = create_pandas_abc_type(
     "ABCTimedeltaIndex", "_typ", ("timedeltaindex",)
 )
+ABCDatetimeTimedeltaMixin = create_pandas_abc_type(
+    "ABCDatetimeTimedeltaMixin", "_typ", ("datetimeindex", "timedeltaindex",)
+)
 ABCPeriodIndex = create_pandas_abc_type("ABCPeriodIndex", "_typ", ("periodindex",))
 ABCCategoricalIndex = create_pandas_abc_type(
     "ABCCategoricalIndex", "_typ", ("categoricalindex",)

pandas/core/indexes/base.py

@@ -54,6 +54,7 @@
     ABCCategorical,
     ABCDataFrame,
     ABCDatetimeIndex,
+    ABCDatetimeTimedeltaMixin,
     ABCIntervalIndex,
     ABCMultiIndex,
     ABCPandasArray,
@@ -3408,7 +3409,16 @@ def join(self, other, how="left", level=None, return_indexers=False, sort=False)
 
             # have the same levels/names so a simple join
             if self.names == other.names:
-                pass
+                # For datetimelike levels, we maybe convert the corresponding level
+                for i, (left, right) in enumerate(zip(self.levels, other.levels)):
+                    if isinstance(left, ABCDatetimeTimedeltaMixin):
+                        left, right = left._prepare_for_join(right)
+                        self = self.set_levels(left, i)
+                        other = other.set_levels(right, i)
+                    elif isinstance(right, ABCDatetimeTimedeltaMixin):
+                        right, left = right._prepare_for_join(left)
+                        self = self.set_levels(left, i)
+                        other = other.set_levels(right, i)
             else:
                 return self._join_multi(other, how=how, return_indexers=return_indexers)
 

pandas/core/indexes/datetimelike.py

@@ -830,13 +830,7 @@ def join(
         """
         See Index.join
         """
-        if self._is_convertible_to_index_for_join(other):
-            try:
-                other = type(self)(other)
-            except (TypeError, ValueError):
-                pass
-
-        this, other = self._maybe_utc_convert(other)
+        this, other = self._prepare_for_join(other)
         return Index.join(
             this,
             other,
@@ -846,6 +840,16 @@ def join(
             sort=sort,
         )
 
+    def _prepare_for_join(self, other):
+        if self._is_convertible_to_index_for_join(other):
+            try:
+                other = type(self)(other)
+            except (TypeError, ValueError):
+                pass
+
+        this, other = self._maybe_utc_convert(other)
+        return this, other
+
     def _maybe_utc_convert(self, other):
         this = self
         if not hasattr(self, "tz"):

pandas/tests/indexes/multi/test_join.py

@@ -103,3 +103,21 @@ def test_join_multi_wrong_order():
     tm.assert_index_equal(midx1, join_idx)
     assert lidx is None
     tm.assert_numpy_array_equal(ridx, exp_ridx)
+
+
+@pytest.mark.parametrize(
+    ("data", "func"), [("2020-03-15", pd.to_datetime), ("1d", pd.to_timedelta)]
+)
+def test_join_mixed_index_datetime_string(data, func, join_type):
+    # GH 26558
+    # Joining multiindex with mixed datetime and string level
+    mi1 = pd.MultiIndex.from_tuples([(data,)])
+    mi2 = pd.MultiIndex.from_tuples([(func(data),)])
+
+    _, indexer_left, indexer_right = mi1.join(mi2, how=join_type, return_indexers=True)
+    assert indexer_left is None
+    assert indexer_right is None
+
+    _, indexer_left, indexer_right = mi2.join(mi1, how=join_type, return_indexers=True)
+    assert indexer_left is None
+    assert indexer_right is None