tests/basics: Add test for containment of a subclass of a native type.

tests/basics/subclass_native_containment.py

@@ -0,0 +1,22 @@
+# test containment operator on subclass of a native type
+
+class mylist(list):
+    pass
+
+class mydict(dict):
+    pass
+
+class mybytes(bytes):
+    pass
+
+l = mylist([1, 2, 3])
+print(0 in l)
+print(1 in l)
+
+d = mydict({1:1, 2:2})
+print(0 in l)
+print(1 in l)
+
+b = mybytes(b'1234')
+print(0 in b)
+print(b'1' in b)