Deleting an attribute has to be guarded, too.

src/RestrictedPython/transformer.py

@@ -835,8 +835,9 @@ def visit_Attribute(self, node):
         """Checks and mutates attribute access/assignment.
 
         'a.b' becomes '_getattr_(a, "b")'
-
         'a.b = c' becomes '_write_(a).b = c'
+        'del a.b' becomes 'del _write_(a).b'
+
         The _write_ function should return a security proxy.
         """
         if node.attr.startswith('_') and node.attr != '_':
@@ -861,7 +862,7 @@ def visit_Attribute(self, node):
             copy_locations(new_node, node)
             return new_node
 
-        elif isinstance(node.ctx, ast.Store):
+        elif isinstance(node.ctx, (ast.Store, ast.Del)):
             node = self.node_contents_visit(node)
             new_value = ast.Call(
                 func=ast.Name('_write_', ast.Load()),

tests/transformer/test_transformer.py

@@ -310,6 +310,23 @@ def test_transformer__RestrictingNodeTransformer__visit_Attribute__5(
     assert glb['a'].b == 'it works'
 
 
+@pytest.mark.parametrize(*e_exec)
+def test_transformer__RestrictingNodeTransformer__visit_Attribute__5_5(
+        e_exec, mocker):
+    """It transforms deleting of an attribute to `_write_`."""
+    glb = {
+        '_write_': mocker.stub(),
+        'a': mocker.stub(),
+    }
+    glb['a'].b = 'it exists'
+    glb['_write_'].return_value = glb['a']
+
+    e_exec("del a.b", glb)
+
+    glb['_write_'].assert_called_once_with(glb['a'])
+    assert not hasattr(glb['a'], 'b')
+
+
 DISALLOW_TRACEBACK_ACCESS = """
 try:
     raise Exception()