1# A contains everything
2class A:
3    def __contains__(self, key):
4        return True
5
6a = A()
7print(True in a)
8print(1 in a)
9print(() in a)
10
11# B contains given things
12class B:
13    def __init__(self, items):
14        self.items = items
15    def __contains__(self, key):
16        return key in self.items
17
18b = B([])
19print(1 in b)
20b = B([1, 2])
21print(1 in b)
22print(2 in b)
23print(3 in b)
24