BUG: avoid StringArray.__setitem__ to mutate the value being set as side-effect

doc/source/whatsnew/v2.0.0.rst

@@ -1229,6 +1229,7 @@ Strings
 ^^^^^^^
 - Bug in :func:`pandas.api.types.is_string_dtype` that would not return ``True`` for :class:`StringDtype` or :class:`ArrowDtype` with ``pyarrow.string()`` (:issue:`15585`)
 - Bug in converting string dtypes to "datetime64[ns]" or "timedelta64[ns]" incorrectly raising ``TypeError`` (:issue:`36153`)
+- Bug in setting values in a string-dtype column with an array, mutating the array as side effect when it contains missing values (:issue:`51299`)
 -
 
 Interval

pandas/core/arrays/string_.py

@@ -423,7 +423,10 @@ def __setitem__(self, key, value):
             if len(value) and not lib.is_string_array(value, skipna=True):
                 raise TypeError("Must provide strings.")
 
-            value[isna(value)] = libmissing.NA
+            mask = isna(value)
+            if mask.any():
+                value = value.copy()
+                value[isna(value)] = libmissing.NA
 
         super().__setitem__(key, value)
 

pandas/tests/arrays/string_/test_string.py

@@ -72,6 +72,19 @@ def test_setitem_with_scalar_string(dtype):
     tm.assert_extension_array_equal(arr, expected)
 
 
+def test_setitem_with_array_with_missing(dtype):
+    # ensure that when setting with an array of values, we don't mutate the
+    # array `value` in __setitem__(self, key, value)
+    arr = pd.array(["a", "b", "c"], dtype=dtype)
+    value = np.array(["A", None])
+    value_orig = value.copy()
+    arr[[0, 1]] = value
+
+    expected = pd.array(["A", pd.NA, "c"], dtype=dtype)
+    tm.assert_extension_array_equal(arr, expected)
+    tm.assert_numpy_array_equal(value, value_orig)
+
+
 def test_astype_roundtrip(dtype):
     ser = pd.Series(pd.date_range("2000", periods=12))
     ser[0] = None