i have '__contains__' ,why error

Question

class a(object):
    def a(self):
        return True
    __contains__=a

b=a()
print 2 in b#why error

thanks

Answer 1

__contains__ is meant to take an argument. a doesn't accept an argument.

The following is your example with a working __contains__:

>>> class a(object):
...     def a(self, item):
...         return True
...     __contains__=a
...
>>> b=a()
>>> print 2 in b
True

Answer 2

The signature of __contains__ is:

object.__contains__(self, item)

as per documentation. You need to extend your "a" method:

def a(self, item)

class a(object):
    def a(self, item):
        return True
    __contains__=a