Booleans have two possible values. Are there types that have three possible values? [duplicate]

Answer 1

1. At least one language has this: C#: int? num = null;, now num is actually Nullable<Int32> (the question mark is a "syntactic sugar") see: http://msdn.microsoft.com/en-us/library/1t3y8s4s%28v=vs.80%29.aspx and http://en.wikipedia.org/wiki/Nullable_type
2. Don't know Python, but it depends on your preference: usability (enum or class) vs. efficiency (4 bit fields)

Answer 2

It's generally known as an 'enum', after the C construct of that name.

You can easily create your own class, objects of which must be initialised with values from a set, and you can create the appropriate comparison, equality, and truth functions accordingly.

Answer 3

There is no such built-in types. And enum are also not supported as a type. However, you can use constant as:

class Option:
    NO = 0
    DONT_KNOW = 1
    YES = 2

reply = Option.YES