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).