Backport PR #57489 on branch 2.2.x (REGR: astype introducing decimals when casting from int with na to string) (#57533)

Backport PR #57489: REGR: astype introducing decimals when casting from int with na to string

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

Author: meeseeksmachine

doc/source/whatsnew/v2.2.1.rst

@@ -39,6 +39,7 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrameGroupBy.idxmin`, :meth:`DataFrameGroupBy.idxmax`, :meth:`SeriesGroupBy.idxmin`, :meth:`SeriesGroupBy.idxmax` where values containing the minimum or maximum value for the dtype could produce incorrect results (:issue:`57040`)
 - Fixed regression in :meth:`ExtensionArray.to_numpy` raising for non-numeric masked dtypes (:issue:`56991`)
 - Fixed regression in :meth:`Index.join` raising ``TypeError`` when joining an empty index to a non-empty index containing mixed dtype values (:issue:`57048`)
+- Fixed regression in :meth:`Series.astype` introducing decimals when converting from integer with missing values to string dtype (:issue:`57418`)
 - Fixed regression in :meth:`Series.pct_change` raising a ``ValueError`` for an empty :class:`Series` (:issue:`57056`)
 - Fixed regression in :meth:`Series.to_numpy` when dtype is given as float and the data contains NaNs (:issue:`57121`)
 

pandas/_libs/lib.pyx

@@ -759,7 +759,7 @@ cpdef ndarray[object] ensure_string_array(
             out = arr.astype(str).astype(object)
             out[arr.isna()] = na_value
             return out
-        arr = arr.to_numpy()
+        arr = arr.to_numpy(dtype=object)
     elif not util.is_array(arr):
         arr = np.array(arr, dtype="object")
 

pandas/tests/series/methods/test_astype.py

@@ -673,3 +673,11 @@ def test_astype_timedelta64_with_np_nan(self):
         result = Series([Timedelta(1), np.nan], dtype="timedelta64[ns]")
         expected = Series([Timedelta(1), NaT], dtype="timedelta64[ns]")
         tm.assert_series_equal(result, expected)
+
+    @td.skip_if_no("pyarrow")
+    def test_astype_int_na_string(self):
+        # GH#57418
+        ser = Series([12, NA], dtype="Int64[pyarrow]")
+        result = ser.astype("string[pyarrow]")
+        expected = Series(["12", NA], dtype="string[pyarrow]")
+        tm.assert_series_equal(result, expected)