BUG: get_indexer rountripping through string dtype (#56013)

Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

Author: phofl

doc/source/whatsnew/v2.3.0.rst

@@ -118,7 +118,7 @@ Interval
 
 Indexing
 ^^^^^^^^
--
+- Fixed bug in :meth:`Index.get_indexer` round-tripping through string dtype when ``infer_string`` is enabled (:issue:`55834`)
 -
 
 Missing

pandas/core/indexes/base.py

@@ -6556,7 +6556,16 @@ def _maybe_cast_listlike_indexer(self, target) -> Index:
  """
  Analogue to maybe_cast_indexer for get_indexer instead of get_loc.
  """
- return ensure_index(target)
+ target_index = ensure_index(target)
+ if (
+ not hasattr(target, "dtype")
+ and self.dtype == object
+ and target_index.dtype == "string"
+ ):
+ # If we started with a list-like, avoid inference to string dtype if self
+ # is object dtype (coercing to string dtype will alter the missing values)
+ target_index = Index(target, dtype=self.dtype)
+ return target_index
 
  @final
  def _validate_indexer(

pandas/tests/indexes/object/test_indexing.py

@@ -62,6 +62,15 @@ def test_get_indexer_with_NA_values(
  expected = np.array([0, 1, -1], dtype=np.intp)
  tm.assert_numpy_array_equal(result, expected)
 
+ def test_get_indexer_infer_string_missing_values(self):
+ # ensure the passed list is not cast to string but to object so that
+ # the None value is matched in the index
+ # https://github.com/pandas-dev/pandas/issues/55834
+ idx = Index(["a", "b", None], dtype="object")
+ result = idx.get_indexer([None, "x"])
+ expected = np.array([2, -1], dtype=np.intp)
+ tm.assert_numpy_array_equal(result, expected)
+
 
 class TestGetIndexerNonUnique:
  def test_get_indexer_non_unique_nas(self, nulls_fixture):