From 31c372e575802ce794216561344734ac67f6ceb6 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 18 Jun 2021 09:35:45 -0700
Subject: [PATCH] REF: move MultiIndex._get_indexer casting to _maybe_promote

---
 pandas/core/indexes/base.py  | 22 +++++++++++++++++++---
 pandas/core/indexes/multi.py | 31 -------------------------------
 2 files changed, 19 insertions(+), 34 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 88fded29810ff..eca584239b33e 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -3402,6 +3402,9 @@ def get_indexer(
         if not self._index_as_unique:
             raise InvalidIndexError(self._requires_unique_msg)
 
+        if len(target) == 0:
+            return np.array([], dtype=np.intp)
+
         if not self._should_compare(target) and not is_interval_dtype(self.dtype):
             # IntervalIndex get special treatment bc numeric scalars can be
             #  matched to Interval scalars
@@ -3410,9 +3413,14 @@ def get_indexer(
         if is_categorical_dtype(target.dtype):
             # potential fastpath
             # get an indexer for unique categories then propagate to codes via take_nd
-            # Note: calling get_indexer instead of _get_indexer causes
-            #  RecursionError GH#42088
-            categories_indexer = self._get_indexer(target.categories)
+            if is_categorical_dtype(self.dtype):
+                # Avoid RecursionError GH#42088
+                categories_indexer = self._get_indexer(target.categories)
+            else:
+                # get_indexer instead of _get_indexer needed for MultiIndex cases
+                #  e.g. test_append_different_columns_types
+                categories_indexer = self.get_indexer(target.categories)
+
             indexer = algos.take_nd(categories_indexer, target.codes, fill_value=-1)
 
             if (not self._is_multi and self.hasnans) and target.hasnans:
@@ -5343,6 +5351,14 @@ def _maybe_promote(self, other: Index) -> tuple[Index, Index]:
                 # TODO: may need itemsize check if we have non-64-bit Indexes
                 return self, other.astype(self.dtype)
 
+        elif self._is_multi and not other._is_multi:
+            try:
+                # "Type[Index]" has no attribute "from_tuples"
+                other = type(self).from_tuples(other)  # type: ignore[attr-defined]
+            except (TypeError, ValueError):
+                # let's instead try with a straight Index
+                self = Index(self._values)
+
         if not is_object_dtype(self.dtype) and is_object_dtype(other.dtype):
             # Reverse op so we dont need to re-implement on the subclasses
             other, self = other._maybe_promote(self)
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index f1f418453c68b..83a4b6a8d56c2 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -2644,37 +2644,6 @@ def _get_partial_string_timestamp_match_key(self, key):
 
         return key
 
-    def _get_indexer(
-        self,
-        target: Index,
-        method: str | None = None,
-        limit: int | None = None,
-        tolerance=None,
-    ) -> np.ndarray:
-        # returned ndarray is np.intp
-
-        # empty indexer
-        if not len(target):
-            return ensure_platform_int(np.array([]))
-
-        if not isinstance(target, MultiIndex):
-            try:
-                target = MultiIndex.from_tuples(target)
-            except (TypeError, ValueError):
-
-                # let's instead try with a straight Index
-                if method is None:
-                    return Index(self._values).get_indexer(
-                        target, method=method, limit=limit, tolerance=tolerance
-                    )
-
-                # TODO: explicitly raise here?  we only have one test that
-                #  gets here, and it is checking that we raise with method="nearest"
-
-        # Note: we only get here (in extant tests at least) with
-        #  target.nlevels == self.nlevels
-        return super()._get_indexer(target, method, limit, tolerance)
-
     def get_slice_bound(
         self, label: Hashable | Sequence[Hashable], side: str, kind: str | None = None
     ) -> int: