BUG: Fix bug when concatenating Index of strings (#33436)

Author: dsaxton

doc/source/whatsnew/v1.1.0.rst

@@ -440,7 +440,7 @@ Strings
 ^^^^^^^
 
 - Bug in the :meth:`~Series.astype` method when converting "string" dtype data to nullable integer dtype (:issue:`32450`).
--
+- Bug in :meth:`Series.str.cat` returning ``NaN`` output when other had :class:`Index` type (:issue:`33425`)
 
 
 Interval

pandas/core/strings.py

@@ -2297,7 +2297,7 @@ def _get_series_list(self, others):
         if isinstance(others, ABCSeries):
             return [others]
         elif isinstance(others, ABCIndexClass):
-            return [Series(others._values, index=others)]
+            return [Series(others._values, index=idx)]
         elif isinstance(others, ABCDataFrame):
             return [others[x] for x in others]
         elif isinstance(others, np.ndarray) and others.ndim == 2:

pandas/tests/test_strings.py

@@ -3624,3 +3624,12 @@ def test_string_array_extract():
 
     result = result.astype(object)
     tm.assert_equal(result, expected)
+
+
+@pytest.mark.parametrize("klass", [tuple, list, np.array, pd.Series, pd.Index])
+def test_cat_different_classes(klass):
+    # https://github.com/pandas-dev/pandas/issues/33425
+    s = pd.Series(["a", "b", "c"])
+    result = s.str.cat(klass(["x", "y", "z"]))
+    expected = pd.Series(["ax", "by", "cz"])
+    tm.assert_series_equal(result, expected)