BUG: Fix Series.str.zfill for ArrowDtype string arrays #61485

Author: iabhi4

doc/source/whatsnew/v3.0.0.rst

@@ -725,8 +725,8 @@ Conversion
 
 Strings
 ^^^^^^^
+- Bug in :meth:`Series.str.zfill` raising ``AttributeError`` for ``ArrowDtype(pa.string())``. Now supported via ``_str_zfill`` implementation in ``ArrowExtensionArray`` (:issue:`61485`)
 - Bug in :meth:`Series.value_counts` would not respect ``sort=False`` for series having ``string`` dtype (:issue:`55224`)
--
 
 Interval
 ^^^^^^^^

pandas/core/arrays/arrow/array.py

@@ -2601,6 +2601,12 @@ def _str_wrap(self, width: int, **kwargs) -> Self:
         result = self._apply_elementwise(predicate)
         return type(self)(pa.chunked_array(result))
 
+    def _str_zfill(self, width: int) -> Self:
+        # TODO: Replace with pc.utf8_zfill when supported by arrow
+        predicate = lambda val: val.zfill(width)
+        result = self._apply_elementwise(predicate)
+        return type(self)(pa.chunked_array(result))
+
     @property
     def _dt_days(self) -> Self:
         return type(self)(

pandas/core/strings/accessor.py

@@ -1939,7 +1939,13 @@ def zfill(self, width: int):
             msg = f"width must be of integer type, not {type(width).__name__}"
             raise TypeError(msg)
         f = lambda x: x.zfill(width)
-        result = self._data.array._str_map(f)
+        array = self._data.array
+
+        if hasattr(array, "_str_zfill"):
+            result = array._str_zfill(width)
+        else:
+            result = array._str_map(f)
+
         return self._wrap_result(result)
 
     def slice(self, start=None, stop=None, step=None):

pandas/tests/strings/test_string_array.py

@@ -110,3 +110,19 @@ def test_string_array_extract(nullable_string_dtype):
 
     result = result.astype(object)
     tm.assert_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "values, width, expected",
+    [
+        (["a", "ab", "abc", None], 4, ["000a", "00ab", "0abc", None]),
+        (["1", "-1", "+1", None], 4, ["0001", "-001", "+001", None]),
+        (["1234", "-1234"], 3, ["1234", "-1234"]),
+    ],
+)
+def test_string_array_zfill(nullable_string_dtype, values, width, expected):
+    # GH #61485
+    s = Series(values, dtype=nullable_string_dtype)
+    result = s.str.zfill(width)
+    expected = Series(expected, dtype=nullable_string_dtype)
+    tm.assert_series_equal(result, expected)