Backport PR pandas-dev#55655 on branch 2.1.x (BUG: infer_string not inferring string dtype when NA is first value) (pandas-dev#55666)

Backport PR pandas-dev#55655: BUG: infer_string not inferring string dtype when NA is first value

Co-authored-by: Patrick Hoefler <[email protected]>

Author: meeseeksmachine

doc/source/whatsnew/v2.1.2.rst

@@ -35,6 +35,7 @@ Bug fixes
 - Fixed bug in :meth:`Series.str.extractall` for :class:`ArrowDtype` dtype being converted to object (:issue:`53846`)
 - Fixed bug where PDEP-6 warning about setting an item of an incompatible dtype was being shown when creating a new conditional column (:issue:`55025`)
 - Silence ``Period[B]`` warnings introduced by :issue:`53446` during normal plotting activity (:issue:`55138`)
+- Fixed bug in :class:`Series` constructor not inferring string dtype when ``NA`` is the first value and ``infer_string`` is set (:issue:` 55655`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_212.other:

pandas/_libs/lib.pyx

@@ -2642,6 +2642,9 @@ def maybe_convert_objects(ndarray[object] objects,
             else:
                 seen.object_ = True
                 break
+        elif val is C_NA:
+            seen.object_ = True
+            continue
         else:
             seen.object_ = True
             break

pandas/tests/series/test_constructors.py

@@ -2131,6 +2131,14 @@ def test_series_constructor_infer_string_scalar(self):
         tm.assert_series_equal(ser, expected)
         assert ser.dtype.storage == "python"
 
+    def test_series_string_inference_na_first(self):
+        # GH#55655
+        pytest.importorskip("pyarrow")
+        expected = Series([pd.NA, "b"], dtype="string[pyarrow_numpy]")
+        with pd.option_context("future.infer_string", True):
+            result = Series([pd.NA, "b"])
+        tm.assert_series_equal(result, expected)
+
 
 class TestSeriesConstructorIndexCoercion:
     def test_series_constructor_datetimelike_index_coercion(self):