布尔值有两个可能的值.是否存在具有三个可能值的类型?

In Python I'd do that with a wrapper object that holds one of those three values; I'd use True, False, and None. Since the implicit truthiness value of a Boolean-like object with three possible values is problematic, we'll solve that by disallowing that entirely (raising an exception in __nonzero__(), or in Python 3, __bool__()), thus requiring that comparisons always be done explicitly, using in, ==, or !=. We'll implement equality as identity so that only the specific singleton values True, False, and None are matched.

class Tristate(object):

    def __init__(self, value=None):
       if any(value is v for v in (True, False, None)):
          self.value = value
       else:
           raise ValueError("Tristate value must be True, False, or None")

    def __eq__(self, other):
       return (self.value is other.value if isinstance(other, Tristate)
               else self.value is other)

    def __ne__(self, other):
       return not self == other

    def __nonzero__(self):   # Python 3: __bool__()
       raise TypeError("Tristate object may not be used as a Boolean")

    def __str__(self):
        return str(self.value)

    def __repr__(self):
        return "Tristate(%s)" % self.value

用法:

t = Tristate(True)
t == True           # True
t != False          # True
t in (True, False)  # True
bool(t)             # Exception!
if t: print "woo"   # Exception!

使用 Tristate 对象时，您必须明确指定要匹配的值，即 foo == True 或 bar != None.您也可以执行 foo in (False, None) 来匹配多个值(当然 in 两个值与 != 相同单个值).如果您希望能够对这些对象执行其他逻辑操作，您可以将它们实现为方法，或者可能通过覆盖某些运算符(但遗憾的是，逻辑 not、and 和 or 不可覆盖，尽管有 建议添加).