From a50bfe94bcc7e366cea7d01a25dc47fe968056b4 Mon Sep 17 00:00:00 2001
From: nrebena <49879400+nrebena@users.noreply.github.com>
Date: Sun, 15 Mar 2020 21:25:43 +0100
Subject: [PATCH 1/3] TST: Add test for #26558

---
 pandas/tests/indexes/multi/test_join.py | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/pandas/tests/indexes/multi/test_join.py b/pandas/tests/indexes/multi/test_join.py
index 062fb92c44552..b0e684cb8b04a 100644
--- a/pandas/tests/indexes/multi/test_join.py
+++ b/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

From 311ad1ca9c622299213ea97800a06f6e260d0ca7 Mon Sep 17 00:00:00 2001
From: nrebena <49879400+nrebena@users.noreply.github.com>
Date: Sun, 15 Mar 2020 20:00:06 +0100
Subject: [PATCH 2/3] BUG: #26558 Fix join on MultiIndex with datetime and
 string

---
 pandas/core/dtypes/generic.py       |  3 +++
 pandas/core/indexes/base.py         | 12 +++++++++++-
 pandas/core/indexes/datetimelike.py | 18 +++++++++++-------
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/pandas/core/dtypes/generic.py b/pandas/core/dtypes/generic.py
index 435d80b2c4dfb..caa7a9f127aa3 100644
--- a/pandas/core/dtypes/generic.py
+++ b/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",)
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 31966489403f4..4432a09b074f8 100644
--- a/pandas/core/indexes/base.py
+++ b/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)
 
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 054a64bf3f990..288c0650104b5 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/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"):

From a59fcc9b73868215678724d4107ce970ace92beb Mon Sep 17 00:00:00 2001
From: nrebena <49879400+nrebena@users.noreply.github.com>
Date: Sun, 15 Mar 2020 23:16:58 +0100
Subject: [PATCH 3/3] DOC: Bugfix for MultiIndex in whatsnew

---
 doc/source/whatsnew/v1.1.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.1.0.rst b/doc/source/whatsnew/v1.1.0.rst
index 4e7bd5a2032a7..e03d2d4376e8a 100644
--- a/doc/source/whatsnew/v1.1.0.rst
+++ b/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
 ^^^