I have a 3-valued Logical class in my dbf module.
It implements False/True/Unknown as singleton values Falsth/Truth/Unknown. It implements all the comparison operators, and also allows comparison with the Python singletons False/True/None (None taken to mean unknown). Any comparison with an Unknown value results in Unknown, and an attempt to implicitly use an Unknown value in an if statement (or other bool context) will raise a TypeError, although Truth and Falsth can be used in boolean contexts, and Unknown can be compared against directly.
Because it is not possible to override the and, or, and not behavior the type overrides the bitwise operators &, |, and ~.
It also implements __index__ with Unknown having the value of 2.
Example:
from dbf import Logical, Truth, Falsth, Unknown
middle_name = raw_input("What is the middle name? ['?' if unknown] ").strip()
if middle_name == '?':
middle_name = ''
middle_exists = Unknown
elif middle_name:
middle_exists = Truth
else:
middle_exists = Falsth
.
.
.
if middle_exists is Unknown:
print "The middle name is unknown."
elif middle_exists:
print "The middle name is %s." % middle_name
else:
print "This person does not have a middle name."
