BUG: Fixed PandasArray.__setitem__ with str (pandas-dev#28119)

* BUG: Fixed PandasArray.__setitem__ with str

Closes pandas-dev#28118

Author: TomAugspurger

doc/source/whatsnew/v1.0.0.rst

@@ -240,7 +240,7 @@ Sparse
 ExtensionArray
 ^^^^^^^^^^^^^^
 
--
+- Bug in :class:`arrays.PandasArray` when setting a scalar string (:issue:`28118`, :issue:`28150`).
 -
 
 

pandas/core/arrays/numpy_.py

@@ -235,15 +235,8 @@ def __setitem__(self, key, value):
         if not lib.is_scalar(value):
             value = np.asarray(value)
 
-        values = self._ndarray
-        t = np.result_type(value, values)
-        if t != self._ndarray.dtype:
-            values = values.astype(t, casting="safe")
-            values[key] = value
-            self._dtype = PandasDtype(t)
-            self._ndarray = values
-        else:
-            self._ndarray[key] = value
+        value = np.asarray(value, dtype=self._ndarray.dtype)
+        self._ndarray[key] = value
 
     def __len__(self) -> int:
         return len(self._ndarray)

pandas/tests/arrays/test_numpy.py

@@ -211,3 +211,18 @@ 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)
+
+
+def test_setitem_no_coercion():
+    # https://github.com/pandas-dev/pandas/issues/28150
+    arr = PandasArray(np.array([1, 2, 3]))
+    with pytest.raises(ValueError, match="int"):
+        arr[0] = "a"