BUG: Fixed PandasArray.__setitem__ with str

Closes pandas-dev#28118

Author: TomAugspurger

doc/source/whatsnew/v1.0.0.rst

@@ -198,7 +198,7 @@ Build Changes
 ExtensionArray
 ^^^^^^^^^^^^^^
 
--
+- Bug in :class:`arrays.PandasArray` when setting a scalar string (:issue:`28118`)
 -
 
 

pandas/core/arrays/numpy_.py

@@ -8,6 +8,7 @@
 from pandas.util._decorators import Appender
 from pandas.util._validators import validate_fillna_kwargs
 
+from pandas.core.dtypes.common import is_object_dtype
 from pandas.core.dtypes.dtypes import ExtensionDtype
 from pandas.core.dtypes.generic import ABCIndexClass, ABCSeries
 from pandas.core.dtypes.inference import is_array_like, is_list_like
@@ -236,7 +237,13 @@ def __setitem__(self, key, value):
             value = np.asarray(value)
 
         values = self._ndarray
-        t = np.result_type(value, values)
+        if isinstance(value, str):
+            if is_object_dtype(self.dtype._dtype):
+                t = np.dtype(object)
+            else:
+                t = self.dtype._dtype
+        else:
+            t = np.result_type(value, values)
         if t != self._ndarray.dtype:
             values = values.astype(t, casting="safe")
             values[key] = value

pandas/tests/arrays/test_numpy.py

@@ -211,3 +211,11 @@ def test_basic_binop():
     result = x + x
     expected = PandasArray(np.array([2, 4, 6]))
     tm.assert_extension_array_equal(result, expected)
+
+
+@pytest.mark.parametrize("dtype", [None, object])
+def test_setitem_object_typecode(dtype):
+    arr = PandasArray(np.array(["a", "b", "c"], dtype=dtype))
+    arr[0] = "t"
+    expected = PandasArray(np.array(["t", "b", "c"], dtype=dtype))
+    tm.assert_extension_array_equal(arr, expected)