TST: Add additional test for future warning when call Series.str.cat(Series.str)

doc/source/whatsnew/v1.5.0.rst

@@ -866,6 +866,7 @@ Strings
 ^^^^^^^
 - Bug in :meth:`str.startswith` and :meth:`str.endswith` when using other series as parameter _pat_. Now raises ``TypeError`` (:issue:`3485`)
 - Bug in :meth:`Series.str.zfill` when strings contain leading signs, padding '0' before the sign character rather than after as ``str.zfill`` from standard library (:issue:`20868`)
+- Bug in :meth:`Series.str.cat` only concatenating the longest string when ``others`` is a ``Series.str`` object (:issue:`28277`)
 -
 
 Interval

pandas/core/strings/accessor.py

@@ -398,6 +398,8 @@ def _get_series_list(self, others):
         elif isinstance(others, np.ndarray) and others.ndim == 2:
             others = DataFrame(others, index=idx)
             return [others[x] for x in others]
+        elif isinstance(others, type(self)):
+            return [others._data]
         elif is_list_like(others, allow_sets=False):
             others = list(others)  # ensure iterators do not get read twice etc
 

pandas/tests/strings/test_cat.py

@@ -376,3 +376,11 @@ def test_cat_different_classes(klass):
     result = s.str.cat(klass(["x", "y", "z"]))
     expected = Series(["ax", "by", "cz"])
     tm.assert_series_equal(result, expected)
+
+
+def test_cat_on_series_dot_str():
+    ps = Series(["AbC", "de", "FGHI", "j", "kLLLm"])
+
+    res = ps.str.cat(others=ps.str)
+    expected = Series(["AbCAbC", "dede", "FGHIFGHI", "jj", "kLLLmkLLLm"])
+    tm.assert_series_equal(res, expected)