From 1c6c3e3a930c0db92294445c25f213efb6ec1fe7 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Sat, 9 Dec 2023 23:55:10 +0100
Subject: [PATCH] Fixup

---
 pandas/core/strings/accessor.py  | 11 +++++------
 pandas/tests/strings/test_cat.py | 21 ++++++++++++++++-----
 2 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/pandas/core/strings/accessor.py b/pandas/core/strings/accessor.py
index 75866c6f6013a..631030d637fae 100644
--- a/pandas/core/strings/accessor.py
+++ b/pandas/core/strings/accessor.py
@@ -689,19 +689,18 @@ def cat(
             result = cat_safe(all_cols, sep)
 
         out: Index | Series
+        if isinstance(self._orig.dtype, CategoricalDtype):
+            # We need to infer the new categories.
+            dtype = self._orig.dtype.categories.dtype  # type: ignore[assignment]
+        else:
+            dtype = self._orig.dtype
         if isinstance(self._orig, ABCIndex):
             # add dtype for case that result is all-NA
-            dtype = None
             if isna(result).all():
                 dtype = object
 
             out = Index(result, dtype=dtype, name=self._orig.name)
         else:  # Series
-            if isinstance(self._orig.dtype, CategoricalDtype):
-                # We need to infer the new categories.
-                dtype = self._orig.dtype.categories.dtype  # type: ignore[assignment]
-            else:
-                dtype = self._orig.dtype
             res_ser = Series(
                 result, dtype=dtype, index=data.index, name=self._orig.name, copy=False
             )
diff --git a/pandas/tests/strings/test_cat.py b/pandas/tests/strings/test_cat.py
index 284932491a65e..c1e7ad6e02779 100644
--- a/pandas/tests/strings/test_cat.py
+++ b/pandas/tests/strings/test_cat.py
@@ -98,14 +98,18 @@ def test_str_cat_categorical(
 
     with option_context("future.infer_string", infer_string):
         s = Index(["a", "a", "b", "a"], dtype=dtype_caller)
-        s = s if box == Index else Series(s, index=s)
+        s = s if box == Index else Series(s, index=s, dtype=s.dtype)
         t = Index(["b", "a", "b", "c"], dtype=dtype_target)
 
-        expected = Index(["ab", "aa", "bb", "ac"])
+        expected = Index(
+            ["ab", "aa", "bb", "ac"], dtype=object if dtype_caller == "object" else None
+        )
         expected = (
             expected
             if box == Index
-            else Series(expected, index=Index(s, dtype=dtype_caller))
+            else Series(
+                expected, index=Index(s, dtype=dtype_caller), dtype=expected.dtype
+            )
         )
 
         # Series/Index with unaligned Index -> t.values
@@ -123,12 +127,19 @@ def test_str_cat_categorical(
 
         # Series/Index with Series having different Index
         t = Series(t.values, index=t.values)
-        expected = Index(["aa", "aa", "bb", "bb", "aa"])
+        expected = Index(
+            ["aa", "aa", "bb", "bb", "aa"],
+            dtype=object if dtype_caller == "object" else None,
+        )
         dtype = object if dtype_caller == "object" else s.dtype.categories.dtype
         expected = (
             expected
             if box == Index
-            else Series(expected, index=Index(expected.str[:1], dtype=dtype))
+            else Series(
+                expected,
+                index=Index(expected.str[:1], dtype=dtype),
+                dtype=expected.dtype,
+            )
         )
 
         result = s.str.cat(t, sep=sep)