Hi,

For user-defined class, it's up to the class to do the right implementation in my opinion. It's true the description is wrong though. 

x in y means that x exist inside of y (so that the execution of y.__contain__(x) is executed successfully and (I guess) doesn't return None,False or 0).

I'll modify the doc to be :
For user-defined classes which define the __contains__() method, x in y is false if y.__contains__(x) is returning either None,False or 0.  Otherwise, x in y return true.

Regards,
Eric Lafontaine