tests/basics: Add test for builtin "delattr".

tests/basics/builtin_delattr.py

@@ -0,0 +1,18 @@
+# test builtin delattr
+
+class A: pass
+a = A()
+a.x = 1
+print(a.x)
+
+delattr(a, 'x')
+
+try:
+    a.x
+except AttributeError:
+    print('AttributeError')
+
+try:
+    delattr(a, 'x')
+except AttributeError:
+    print('AttributeError')