2

在这个问题中给出的示例枚举代码中，转载如下，为什么TOKEN包含元类的实现__contains__和__repr__来自元类的实现EnumerationType？

from ctypes import *

class EnumerationType(type(c_uint)):  
    def __new__(metacls, name, bases, dict):  
        if not "_members_" in dict:  
            _members_ = {}  
            for key,value in dict.items():  
                if not key.startswith("_"):  
                    _members_[key] = value  
            dict["_members_"] = _members_  
        cls = type(c_uint).__new__(metacls, name, bases, dict)  
        for key,value in cls._members_.items():  
            globals()[key] = value  
        return cls  

    def __contains__(self, value):
        return value in self._members_.values()

    def __repr__(self):
        return "<Enumeration %s>" % self.__name__

class Enumeration(c_uint):
    __metaclass__ = EnumerationType
    _members_ = {}
    def __init__(self, value):
        for k,v in self._members_.items():
            if v == value:
                self.name = k
                break
        else:
            raise ValueError("No enumeration member with value %r" % value)
        c_uint.__init__(self, value)


    @classmethod
    def from_param(cls, param):
        if isinstance(param, Enumeration):
            if param.__class__ != cls:
                raise ValueError("Cannot mix enumeration members")
            else:
                return param
        else:
            return cls(param)

    def __repr__(self):
        return "<member %s=%d of %r>" % (self.name, self.value, self.__class__)

class TOKEN(Enumeration):
    _members_ = {'T_UNDEF':0, 'T_NAME':1, 'T_NUMBER':2, 'T_STRING':3, 'T_OPERATOR':4, 'T_VARIABLE':5, 'T_FUNCTION':6}

我希望以下代码会引发__contains__未实现的异常，但是，我收到了True False.

print 2 in TOKEN, 7 in TOKEN
4

1 回答 1

2

Enumeration和都是TOKEN的实例EnumerationType：

>>> isinstance(Enumeration, EnumerationType)
True
>>> isinstance(TOKEN, EnumerationType)
True

并且在类中查找新样式类实例的特殊方法，例如repr(TOKEN)等价于type(TOKEN).__repr__(TOKEN)，即EnumerationType.__repr__(TOKEN)。

于 2009-12-01T11:42:16.487 回答