Question

I can use a boolean as a list index:

isTrue = True
print ('No','Yes')[isTrue]  -> 'Yes'

but if I define a class,

class YeaOrNay(object):
    def __init__(self, val):
        self.val = bool(val)

    def __nonzero__(self):
        return self.val

    def __int__(self):
        return int(self.val)

    def __str__(self):
        return ('Nay','Yea')[self.val]

and try to use it the same way:

isTrue = YeaOrNay(True)
print ('No','Yes')[isTrue]

I get an error

TypeError: tuple indices must be integers, not YeaOrNay

I can work around it,

print ('No','Yes')[int(isTrue)]  ->  'Yes'

but I would really like to know if there is some magic class method I can add to YeaOrNay to make it ‘just work’.

Answer 1

In python, bool is a subclass of int. That’s why you can use a bool as an index of the tuple. However, YeaOrNea is clearly neither and thus the error. If you insist on using this object as an index to a tuple, subclass int (or bool) to trick it to make it work.

class YeaOrNea(int):
    ...

Otherwise do it the right way.

print('Yes' if isTrue else 'No')