Updates

* Failing test for setting a sequence into a scalar
* Fixed implementation

Author: TomAugspurger

pandas/core/arrays/datetimelike.py

@@ -492,6 +492,10 @@ def __setitem__(
 
         if is_list_like(value):
             is_slice = isinstance(key, slice)
+
+            if lib.is_scalar(key):
+                raise ValueError("setting an array element with a sequence.")
+
             if (not is_slice
                     and len(key) != len(value)
                     and not com.is_bool_indexer(key)):

pandas/tests/extension/base/setitem.py

@@ -182,3 +182,8 @@ def test_setitem_slice_array(self, data):
         arr = data[:5].copy()
         arr[:5] = data[-5:]
         self.assert_extension_array_equal(arr, data[-5:])
+
+    def test_setitem_scalar_key_sequence_raise(self, data):
+        arr = data[:5].copy()
+        with pytest.raises(ValueError):
+            arr[0] = arr[[0, 1]]

pandas/tests/extension/decimal/array.py

@@ -108,6 +108,8 @@ def astype(self, dtype, copy=True):
 
     def __setitem__(self, key, value):
         if pd.api.types.is_list_like(value):
+            if pd.api.types.is_scalar(key):
+                raise ValueError("setting an array element with a sequence.")
             value = [decimal.Decimal(v) for v in value]
         else:
             value = decimal.Decimal(value)