Backport PR #37034 on branch 1.1.x (REGR: Fix casting of None to str during astype) (#37076)

Co-authored-by: Daniel Saxton <[email protected]>

Author: meeseeksmachine

doc/source/whatsnew/v1.1.4.rst

@@ -18,6 +18,7 @@ Fixed regressions
 - Fixed regression where attempting to mutate a :class:`DateOffset` object would no longer raise an ``AttributeError`` (:issue:`36940`)
 - Fixed regression where :meth:`DataFrame.agg` would fail with :exc:`TypeError` when passed positional arguments to be passed on to the aggregation function (:issue:`36948`).
 - Fixed regression in :class:`RollingGroupby` with ``sort=False`` not being respected (:issue:`36889`)
+- Fixed regression in :meth:`Series.astype` converting ``None`` to ``"nan"`` when casting to string (:issue:`36904`)
 - Fixed regression in :class:`RollingGroupby` causing a segmentation fault with Index of dtype object (:issue:`36727`)
 
 .. ---------------------------------------------------------------------------

pandas/core/dtypes/cast.py

@@ -922,7 +922,9 @@ def astype_nansafe(arr, dtype, copy: bool = True, skipna: bool = False):
         dtype = pandas_dtype(dtype)
 
     if issubclass(dtype.type, str):
-        return lib.ensure_string_array(arr.ravel(), skipna=skipna).reshape(arr.shape)
+        return lib.ensure_string_array(
+            arr.ravel(), skipna=skipna, convert_na_value=False
+        ).reshape(arr.shape)
 
     elif is_datetime64_dtype(arr):
         if is_object_dtype(dtype):

pandas/tests/series/methods/test_astype.py

@@ -1,7 +1,7 @@
 import numpy as np
 import pytest
 
-from pandas import Interval, Series, Timestamp, date_range
+from pandas import NA, Interval, Series, Timestamp, date_range
 import pandas._testing as tm
 
 
@@ -55,3 +55,13 @@ def test_astype_from_float_to_str(self, dtype):
         result = s.astype(str)
         expected = Series(["0.1"])
         tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "value, string_value", [(None, "None"), (np.nan, "nan"), (NA, "<NA>")],
+    )
+    def test_astype_to_str_preserves_na(self, value, string_value):
+        # https://github.com/pandas-dev/pandas/issues/36904
+        s = Series(["a", "b", value], dtype=object)
+        result = s.astype(str)
+        expected = Series(["a", "b", string_value], dtype=object)
+        tm.assert_series_equal(result, expected)