[REVIEW] Fix .str.cat when objects with different index are passed

CHANGELOG.md

@@ -257,6 +257,7 @@
 - PR #5820 Fix ListColumn.to_arrow for all null case
 - PR #5837 Bash syntax error in prebuild.sh preventing `cudf_kafka` and `libcudf_kafka` from being uploaded to Anaconda
 - PR #5841 Added custreamz functions that were missing in interface layer
+- PR #5844 Fix `.str.cat` when objects with different index are passed
 - PR #5849 Modify custreamz api to integrate seamlessly with python streamz
 
 

python/cudf/cudf/core/column/string.py

@@ -414,7 +414,7 @@ def cat(self, others=None, sep=None, na_rep=None, **kwargs):
                 self._column, as_scalar(sep), as_scalar(na_rep, "str")
             )
         else:
-            other_cols = _get_cols_list(others)
+            other_cols = _get_cols_list(self._parent, others)
             all_cols = [self._column] + other_cols
             data = cpp_concatenate(
                 cudf.DataFrame(
@@ -428,10 +428,10 @@ def cat(self, others=None, sep=None, na_rep=None, **kwargs):
             data = [""]
         out = self._return_or_inplace(data, **kwargs)
         if len(out) == 1 and others is None:
-            if isinstance(out, StringColumn):
-                out = out[0]
-            else:
+            if isinstance(out, cudf.Series):
                 out = out.iloc[0]
+            else:
+                out = out[0]
         return out
 
     def join(self, sep):
@@ -4739,7 +4739,11 @@ def _string_column_binop(lhs, rhs, op, out_dtype):
     return out
 
 
-def _get_cols_list(others):
+def _get_cols_list(parent_obj, others):
+
+    parent_index = (
+        parent_obj.index if isinstance(parent_obj, cudf.Series) else parent_obj
+    )
 
     if (
         can_convert_to_column(others)
@@ -4756,9 +4760,23 @@ def _get_cols_list(others):
         If others is a list-like object (in our case lists & tuples)
         just another Series/Index, great go ahead with concatenation.
         """
-        cols_list = [column.as_column(frame, dtype="str") for frame in others]
+        cols_list = [
+            column.as_column(frame.reindex(parent_index), dtype="str")
+            if (
+                isinstance(frame, cudf.Series)
+                and not frame.index.equals(parent_index)
+            )
+            else column.as_column(frame, dtype="str")
+            for frame in others
+        ]
+
         return cols_list
     elif others is not None:
+        if isinstance(others, cudf.Series) and not others.index.equals(
+            parent_index
+        ):
+            others = others.reindex(parent_index)
+
         return [column.as_column(others, dtype="str")]
     else:
         raise TypeError(