what is __contains__ do, which one can call __contains__ function.

Question

the next is my code:

class a(object):
    d='ddd'
    def __contains__(self):
        if self.d:return true
b=a()
print b.contains('d')#error
print contains(b,'d')#error

thanks

Answer 1

to get your code to do something (although nothing useful):

class a(object):
    d='ddd'
    def __contains__(self, m):
        if self.d: return True
b=a()

>>> 'd' in b
True

The docs.

Answer 2

if self.d:return true

self.d is the string 'ddd'. Non-empty strings are always truthy: when you use if on 'ddd' it will always act as if you'd said if True:.

I think what you probably meant is:

def __contains__(self, item):
    return item in self.d

in is the operator that calls the __contains__ method behind the scenes.

Answer 3

Like all special methods (with "magic names" that begin and end in __), __contains__ is not meant to be called directly (except in very specific cases, such as up=calls to the superclass): rather, such methods are called as part of the operation of built-ins and operators. In the case of __contains__, the operator in question is in -- the "containment check" operator.

With your class a as you present it (except for fixing your typo, and using True instead of true!-), and b as its instance, print 'x' in b will print True -- and so will any other containment check on b, since b always returns True (because self.d, a non-empty string, is true).