在Python中，枚举和我们在对象中定义的类变量时一样的，每一个类变量就是一个枚举项，访问枚举项的方式为：类名加上类变量。

虽然这样是可以解决问题的，但是并不严谨，也不怎么安全，比如：

1、枚举类中，不应该存在key相同的枚举项（类变量）

2、不允许在类外直接修改枚举项的值

下面来看看如何实现枚举类型：

import operator
class EnumValue(object):
    def __init__(self, parent_name, name, value):
        self._parent_name = parent_name
        self._name = name
        self._value = value
    def _parents_equal(self, other):
        return (
            hasattr(other, '_parent_name')
            and self._parent_name == other._parent_name)
    def _check_parents_equal(self, other):
        if not self._parents_equal(other):
            raise TypeError(
                'This operation is valid only for enum values of the same type')
    def __eq__(self, other):
        return self._parents_equal(other) and self._value == other._value
    def __ne__(self, other):
        return not self.__eq__(other)
    def __lt__(self, other):
        self._check_parents_equal(other)
        return self._value < other._value
    def __le__(self, other):
        self._check_parents_equal(other)
        return self._value <= other._value
    def __gt__(self, other):
        self._check_parents_equal(other)
        return self._value > other._value
    def __ge__(self, other):
        self._check_parents_equal(other)
        return self._value >= other._value
    def __hash__(self):
        return hash(self._parent_name + str(self._value))
    def __repr__(self):
        return '{}({!r}, {!r}, {!r})'.format(
            self.__class__.__name__, self._parent_name, self._name, self._value)
    def __int__(self):
        return int(self._value)
    def __str__(self):
        return str(self._name)
class EnumMetaclass(type):
    def __new__(cls, name, bases, dct):
        uppercased = dict((k.upper(), v) for k, v in dct.items())
        new_dct = dict(
            name=name,
            _enums_by_str=dict(
                (k, EnumValue(name, k, v)) for k, v in uppercased.items()),
            _enums_by_int=dict(
                (v, EnumValue(name, k, v)) for k, v in uppercased.items()),
        )
        return super(EnumMetaclass, cls).__new__(cls, name, bases, new_dct)
    def __getattr__(cls, name):
        try:
            return cls.__getitem__(name)
        except KeyError:
            raise AttributeError
    def __getitem__(cls, name):
        try:
            name = name.upper()
        except AttributeError:
            pass
        try:
            return cls._enums_by_str[name]
        except KeyError:
            return cls._enums_by_int[name]
    def __repr__(cls):
        return '{}({!r}, {})'.format(
            cls.__class__.__name__,
            cls.name,
            ', '.join('{}={}'.format(v._name, v._value)
                      for v in sorted(cls._enums_by_str.values())))
    def values(cls):
        return sorted(cls._enums_by_str.values())
    def _values_comparison(cls, item, comparison_operator):
        """
        Return a list of values such that comparison_operator(value, item) is
        True.
        """
        return sorted(
            [v for v in cls._enums_by_str.values()
             if comparison_operator(v, item)])
    def values_lt(cls, item):
        return cls._values_comparison(item, operator.lt)
    def values_le(cls, item):
        return cls._values_comparison(item, operator.le)
    def values_gt(cls, item):
        return cls._values_comparison(item, operator.gt)
    def values_ge(cls, item):
        return cls._values_comparison(item, operator.ge)
    def values_ne(cls, item):
        return cls._values_comparison(item, operator.ne)
def enum_factory(name, **kwargs):
    return EnumMetaclass(name, (), kwargs)

Python中枚举的测试:

import unittest
class EnumTestCase(unittest.TestCase):
    def test_repr(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            repr(ProfileAction),
            "EnumMetaclass('ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)")
    def test_value_repr(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            repr(ProfileAction.VIEW), "EnumValue('ProfileAction', 'VIEW', 1)")
    def test_attribute_error(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        with self.assertRaises(AttributeError):
            ProfileAction.ASDFASDF
    def test_cast_to_str(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(str(ProfileAction.VIEW), 'VIEW')
    def test_cast_to_int(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(int(ProfileAction.VIEW), 1)
    def test_access_by_str(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(ProfileAction['VIEW'], ProfileAction.VIEW)
    def test_access_by_int(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(ProfileAction[1], ProfileAction.VIEW)
    def test_equality(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(ProfileAction.VIEW, ProfileAction.VIEW)
        self.assertEqual(ProfileAction['VIEW'], ProfileAction.VIEW)
        self.assertEqual(ProfileAction[1], ProfileAction.VIEW)
    def test_inequality(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertNotEqual(ProfileAction.VIEW, ProfileAction.EDIT_OWN)
        self.assertNotEqual(ProfileAction['VIEW'], ProfileAction.EDIT_OWN)
        self.assertNotEqual(ProfileAction[1], ProfileAction.EDIT_OWN)
        DashboardAction = enum_factory(
            'DashboardAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertNotEqual(ProfileAction.VIEW, DashboardAction.VIEW)
    def test_invalid_comparison(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        DashboardAction = enum_factory(
            'DashboardAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        with self.assertRaises(TypeError) as cm:
            ProfileAction.VIEW < DashboardAction.EDIT_OWN
        self.assertEqual(
            str(cm.exception),
            'This operation is valid only for enum values of the same type')
    def test_values(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            ProfileAction.values(), [
                EnumValue('ProfileAction', 'VIEW', 1),
                EnumValue('ProfileAction', 'EDIT_OWN', 2),
                EnumValue('ProfileAction', 'EDIT_PUBLIC', 3),
                EnumValue('ProfileAction', 'EDIT_FULL', 4),
            ])
    def test_values_lt(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            ProfileAction.values_lt(ProfileAction.EDIT_PUBLIC), [
                EnumValue('ProfileAction', 'VIEW', 1),
                EnumValue('ProfileAction', 'EDIT_OWN', 2),
            ])
    def test_values_le(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            ProfileAction.values_le(ProfileAction.EDIT_PUBLIC), [
                EnumValue('ProfileAction', 'VIEW', 1),
                EnumValue('ProfileAction', 'EDIT_OWN', 2),
                EnumValue('ProfileAction', 'EDIT_PUBLIC', 3),
            ])
    def test_values_gt(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            ProfileAction.values_gt(ProfileAction.EDIT_PUBLIC), [
                EnumValue('ProfileAction', 'EDIT_FULL', 4),
            ])
    def test_values_ge(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            ProfileAction.values_ge(ProfileAction.EDIT_PUBLIC), [
                EnumValue('ProfileAction', 'EDIT_PUBLIC', 3),
                EnumValue('ProfileAction', 'EDIT_FULL', 4),
            ])
    def test_values_ne(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        self.assertEqual(
            ProfileAction.values_ne(ProfileAction.EDIT_PUBLIC), [
                EnumValue('ProfileAction', 'VIEW', 1),
                EnumValue('ProfileAction', 'EDIT_OWN', 2),
                EnumValue('ProfileAction', 'EDIT_FULL', 4),
            ])
    def test_intersection_with_same_type(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        set_a = set([ProfileAction.VIEW, ProfileAction.EDIT_OWN])
        set_b = set([ProfileAction.VIEW, ProfileAction.EDIT_PUBLIC])
        self.assertEqual(set_a & set_b, set([ProfileAction.VIEW]))
    def test_intersection_with_different_types(self):
        ProfileAction = enum_factory(
            'ProfileAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        DashboardAction = enum_factory(
            'DashboardAction', VIEW=1, EDIT_OWN=2, EDIT_PUBLIC=3, EDIT_FULL=4)
        set_a = set([ProfileAction.VIEW, ProfileAction.EDIT_OWN])
        set_b = set([DashboardAction.VIEW, DashboardAction.EDIT_PUBLIC])
        self.assertEqual(set_a & set_b, set([]))