python - 我可以在 Python 中迭代一个类吗？

Question

我有一个类在类变量中跟踪其实例，如下所示:

class Foo:
    by_id = {}

    def __init__(self, id):
        self.id = id
        self.by_id[id] = self

我想做的是遍历类的现有实例。我可以这样做:

for foo in Foo.by_id.values():
    foo.do_something()

但这样看起来会更整洁:

for foo in Foo:
    foo.do_something()

这可能吗？我尝试定义一个类方法 __iter__，但没有奏效。

Answer 1

如果你想迭代类，你必须定义一个支持迭代的元类。

x.py:

class it(type):
    def __iter__(self):
        # Wanna iterate over a class? Then ask that class for iterator.
        return self.classiter()

class Foo:
    __metaclass__ = it # We need that meta class...
    by_id = {} # Store the stuff here...

    def __init__(self, id): # new isntance of class
        self.id = id # do we need that?
        self.by_id[id] = self # register istance

    @classmethod
    def classiter(cls): # iterate over class by giving all instances which have been instantiated
        return iter(cls.by_id.values())

if __name__ == '__main__':
    a = Foo(123)
    print list(Foo)
    del a
    print list(Foo)

正如你最后看到的，删除一个实例不会对对象本身产生任何影响，因为它保留在 by_id 字典中。你可以使用 weakrefs 当你

import weakref

然后做

by_id = weakref.WeakValueDictionary()

。这样，只要有一个“强”引用保留它，值就会保留，例如 a 在这种情况下。在del a之后，只有指向对象的弱引用，所以可以gc'ed。

由于WeakValueDictionary()s的警告，我建议使用以下:

[...]
    self.by_id[id] = weakref.ref(self)
[...]
@classmethod
def classiter(cls):
    # return all class instances which are still alive according to their weakref pointing to them
    return (i for i in (i() for i in cls.by_id.values()) if i is not None)

看起来有点复杂，但请确保您获得的是对象而不是 weakref 对象。