Python:这是否可以覆盖__eq__和__hash__？

Question

我是Python的新手,我想确保我覆盖__eq__并且__hash__正确,以免以后引起痛苦的错误:

(我正在使用Google App Engine.)

class Course(db.Model):
    dept_code = db.StringProperty()
    number = db.IntegerProperty()
    title = db.StringProperty()
    raw_pre_reqs = db.StringProperty(multiline=True)
    original_description = db.StringProperty()

    def getPreReqs(self):
        return pickle.loads(str(self.raw_pre_reqs))

    def __repr__(self):
        title_msg = self.title if self.title else "Untitled"
        return "%s %s: %s" % (self.dept_code, self.number, title_msg)

    def __attrs(self):
        return (self.dept_code, self.number, self.title, self.raw_pre_reqs, self.original_description)

    def __eq__(self, other):
        return isinstance(other, Course) and self.__attrs() == other.__attrs()

    def __hash__(self):
        return hash(self.__attrs())

一个稍微复杂的类型:

class DependencyArcTail(db.Model):
    ''' A list of courses that is a pre-req for something else '''
    courses = db.ListProperty(db.Key)

    ''' a list of heads that reference this one '''
    forwardLinks = db.ListProperty(db.Key)

    def __repr__(self):
        return "DepArcTail %d: courses='%s' forwardLinks='%s'" % (id(self), getReprOfKeys(self.courses), getIdOfKeys(self.forwardLinks))

    def __eq__(self, other):
        if not isinstance(other, DependencyArcTail):
            return False

        for this_course in self.courses:
            if not (this_course in other.courses):
                return False

        for other_course in other.courses:
            if not (other_course in self.courses):
                return False

        return True

    def __hash__(self):
        return hash((tuple(self.courses), tuple(self.forwardLinks)))

一切都很好看？

更新以反映@ Alex的评论

class DependencyArcTail(db.Model):
    ''' A list of courses that is a pre-req for something else '''
    courses = db.ListProperty(db.Key)

    ''' a list of heads that reference this one '''
    forwardLinks = db.ListProperty(db.Key)

    def __repr__(self):
        return "DepArcTail %d: courses='%s' forwardLinks='%s'" % (id(self), getReprOfKeys(self.courses), getIdOfKeys(self.forwardLinks))

    def __eq__(self, other):
        return isinstance(other, DependencyArcTail) and set(self.courses) == set(other.courses) and set(self.forwardLinks) == set(other.forwardLinks)

    def __hash__(self):
        return hash((tuple(self.courses), tuple(self.forwardLinks)))

Answer 1

第一个很好.第二个问题有两个原因:

1. 可能有重复 .courses
2. 具有相同.courses但不同的两个实体.forwardLinks将比较相等但具有不同的哈希值

我会通过使等式依赖于课程和前向链接来修复第二个,但两者都改变​​为集合(因此没有重复),并且对于散列也是如此.即:

def __eq__(self, other):
    if not isinstance(other, DependencyArcTail):
        return False

    return (set(self.courses) == set(other.courses) and
            set(self.forwardLinks) == set(other.forwardLinks))

def __hash__(self):
    return hash((frozenset(self.courses), frozenset(self.forwardLinks)))

当然,这是假设正向链路是为对象的"实际价值"是至关重要的,否则他们应该从两个被忽略__eq__和__hash__.

编辑:从最多冗余的__hash__调用中移除tuple(并且可能是破坏性的,如@Mark [[tx !!!]]的评论所示); 改变set到frozenset由@Phillips [[TX !!!]在散列,如所建议的评论.