Python：覆盖eq和hash的这种方式是否可行？

Question

Python：覆盖eq和hash的这种方式是否可行？

24

我是Python的新手，我想确保正确重写了__eq__和__hash__，以免以后出现痛苦的错误：

（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())

稍微复杂一些的类型：

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)))

一切看起来都好吗？

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)))

- Nick Heiner

1个回答

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- Alex Martelli · Accepted Answer

第一个没问题。第二个有两个问题：

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

我会通过使相等性取决于课程和转发链接来解决第二个问题，但是对于集合的更改（因此没有重复项），哈希也是相同的。即：

```python class Entity: def __init__(self, courses, forwardLinks): self.courses = set(courses) self.forwardLinks = set(forwardLinks)

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

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

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，这是基于前向链接对于对象的“真实价值”至关重要的假设，否则它们应该从__eq__和__hash__中省略。

编辑：从__hash__中删除对tuple的调用，这在最好的情况下是多余的（并可能会造成损害，正如@Mark的评论所建议的那样[[tx！]]）；在哈希过程中将set更改为frozenset，正如@Phillips的评论所建议的那样[[tx！]]。

Python：覆盖__eq__和__hash__的这种方式是否可行？

Python：覆盖eq和hash的这种方式是否可行？