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).
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