The StringBool class is accessible via the formencode.validators module.
Values like 'true' and 'false' are considered True and False, respectively; anything in true_values is true, anything in false_values is false, case-insensitive). The first item of those lists is considered the preferred form.
>>> s = StringBoolean()
>>> s.to_python('yes'), s.to_python('no')
(True, False)
>>> s.to_python(1), s.to_python('N')
(True, False)
>>> s.to_python('ye')
Traceback (most recent call last):
...
Invalid: Value should be 'true' or 'false'
Messages
See the source for more information.