From fe821b9b8b22c6268afeb53bdd1b2e7757d08f95 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Wed, 4 Oct 2023 10:39:20 +0200
Subject: [PATCH] Backport PR #55366: BUG: Inserting ndim=0 array does not
 infer string dtype

---
 doc/source/whatsnew/v2.1.2.rst               |  1 +
 pandas/core/construction.py                  |  5 +++++
 pandas/tests/frame/indexing/test_indexing.py | 13 +++++++++++++
 3 files changed, 19 insertions(+)

diff --git a/doc/source/whatsnew/v2.1.2.rst b/doc/source/whatsnew/v2.1.2.rst
index 60b628c5d658c..0312ec3682f11 100644
--- a/doc/source/whatsnew/v2.1.2.rst
+++ b/doc/source/whatsnew/v2.1.2.rst
@@ -22,6 +22,7 @@ Fixed regressions
 Bug fixes
 ~~~~~~~~~
 - Fixed bug in :meth:`Categorical.equals` if other has arrow backed string dtype (:issue:`55364`)
+- Fixed bug in :meth:`DataFrame.__setitem__` not inferring string dtype for zero-dimensional array with ``infer_string=True`` (:issue:`55366`)
 - Fixed bug in :meth:`DataFrame.idxmin` and :meth:`DataFrame.idxmax` raising for arrow dtypes (:issue:`55368`)
 - Fixed bug in :meth:`Index.insert` raising when inserting ``None`` into :class:`Index` with ``dtype="string[pyarrow_numpy]"`` (:issue:`55365`)
 - Silence ``Period[B]`` warnings introduced by :issue:`53446` during normal plotting activity (:issue:`55138`)
diff --git a/pandas/core/construction.py b/pandas/core/construction.py
index aaac0dc73486f..e661d590ab330 100644
--- a/pandas/core/construction.py
+++ b/pandas/core/construction.py
@@ -562,7 +562,12 @@ def sanitize_array(
     if not is_list_like(data):
         if index is None:
             raise ValueError("index must be specified when data is not list-like")
+        if isinstance(data, str) and using_pyarrow_string_dtype():
+            from pandas.core.arrays.string_ import StringDtype
+
+            dtype = StringDtype("pyarrow_numpy")
         data = construct_1d_arraylike_from_scalar(data, len(index), dtype)
+
         return data
 
     elif isinstance(data, ABCExtensionArray):
diff --git a/pandas/tests/frame/indexing/test_indexing.py b/pandas/tests/frame/indexing/test_indexing.py
index 370cbf0f33174..6d4eedb49ff83 100644
--- a/pandas/tests/frame/indexing/test_indexing.py
+++ b/pandas/tests/frame/indexing/test_indexing.py
@@ -1905,6 +1905,19 @@ def test_adding_new_conditional_column() -> None:
     tm.assert_frame_equal(df, expected)
 
 
+def test_add_new_column_infer_string():
+    # GH#55366
+    pytest.importorskip("pyarrow")
+    df = DataFrame({"x": [1]})
+    with pd.option_context("future.infer_string", True):
+        df.loc[df["x"] == 1, "y"] = "1"
+    expected = DataFrame(
+        {"x": [1], "y": Series(["1"], dtype="string[pyarrow_numpy]")},
+        columns=Index(["x", "y"], dtype="string[pyarrow_numpy]"),
+    )
+    tm.assert_frame_equal(df, expected)
+
+
 class TestSetitemValidation:
     # This is adapted from pandas/tests/arrays/masked/test_indexing.py
     # but checks for warnings instead of errors.