BUG: Series inferring new string dtype even if dtype is given for scalar value (#55537)

Author: phofl

pandas/core/construction.py

@@ -540,6 +540,7 @@ def sanitize_array(
     -------
     np.ndarray or ExtensionArray
     """
+    original_dtype = dtype
     if isinstance(data, ma.MaskedArray):
         data = sanitize_masked_array(data)
 
@@ -562,7 +563,11 @@ 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():
+        if (
+            isinstance(data, str)
+            and using_pyarrow_string_dtype()
+            and original_dtype is None
+        ):
             from pandas.core.arrays.string_ import StringDtype
 
             dtype = StringDtype("pyarrow_numpy")

pandas/tests/series/test_constructors.py

@@ -2123,6 +2123,14 @@ def test_series_string_inference_storage_definition(self):
             result = Series(["a", "b"], dtype="string")
         tm.assert_series_equal(result, expected)
 
+    def test_series_constructor_infer_string_scalar(self):
+        # GH#55537
+        with pd.option_context("future.infer_string", True):
+            ser = Series("a", index=[1, 2], dtype="string[python]")
+        expected = Series(["a", "a"], index=[1, 2], dtype="string[python]")
+        tm.assert_series_equal(ser, expected)
+        assert ser.dtype.storage == "python"
+
 
 class TestSeriesConstructorIndexCoercion:
     def test_series_constructor_datetimelike_index_coercion(self):