Question

我正在搜索这里的信息：Metaclass not being called in subclasses

我的问题是我无法使用此类注册表创建对象的实例。如果我使用“常规”构造方法，那么它似乎正确地实例化对象;但是当我尝试使用与注册表关联的类对象时，我得到一个错误，我传递的参数数量不正确。（似乎是调用了元类 new 而不是我的构造函数...... ??）

我不清楚为什么它会失败，因为我认为我应该能够通过使用“callable”语法从类对象创建一个实例。

似乎我把metaclass放在注册表而不是类本身？但我没有看到在新电话中访问课程本身的简单方法。

这是我的代码示例，它无法实例化变量'd'：

registry = [] # list of subclasses

class PluginMetaclass(type):
    def __new__(cls, name, bases, attrs):
        print(cls)
        print(name)
        registry.append((name, cls))
        return super(PluginMetaclass, cls).__new__(cls, name, bases, attrs)

class Plugin(metaclass=PluginMetaclass):
    def __init__(self, stuff):
        self.stuff = stuff

# in your plugin modules
class SpamPlugin(Plugin):
    def __init__(self, stuff):
        self.stuff = stuff

class BaconPlugin(Plugin):
    def __init__(self, stuff):
        self.stuff = stuff

c = SpamPlugin(0)
b = BaconPlugin(0)
mycls = registry[1][1]
d = mycls(0)

感谢您的帮助。

Answer 1

我认为您遇到的问题是传递给元类构造函数的cls参数实际上是对元类的引用，而不是正在创建的类。由于__new__是PluginMetaclass的类方法，因此它与该类相关联，就像任何常规的类方法一样。您可能希望从super(PluginMetaclass, cls).__new__(..)注册新创建的类对象。

这个修改过的版本对我有用3.2：

class PluginMetaclass(type):
    def __new__(cls, name, bases, attrs):
        print("Called metaclass: %r" % cls)
        print("Creating class with name: %r" % name)
        newclass = super(PluginMetaclass, cls).__new__(cls, name, bases, attrs)
        print("Registering class: %r" % newclass)
        registry.append((name, newclass))
        return newclass

并且print()调用显示了幕后发生的事情：

>>> registry = []
>>>
>>> class Plugin(metaclass=PluginMetaclass):
...     def __init__(self, stuff):
...         self.stuff = stuff
...
Called metaclass: <class '__main__.PluginMetaclass'>
Creating class with name: 'Plugin'
Registering class: <class '__main__.Plugin'>
>>> class SpamPlugin(Plugin):
...     def __init__(self, stuff):
...         self.stuff = stuff
...
Called metaclass: <class '__main__.PluginMetaclass'>
Creating class with name: 'SpamPlugin'
Registering class: <class '__main__.SpamPlugin'>
>>> class BaconPlugin(Plugin):
...     def __init__(self, stuff):
...         self.stuff = stuff
...
Called metaclass: <class '__main__.PluginMetaclass'>
Creating class with name: 'BaconPlugin'
Registering class: <class '__main__.BaconPlugin'>
>>> c = SpamPlugin(0)
>>> b = BaconPlugin(0)
>>> mycls = registry[1][1]
>>> d = mycls(0)
>>> d
<__main__.SpamPlugin object at 0x010478D0>
>>> registry
[('Plugin', <class '__main__.Plugin'>), 
('SpamPlugin', <class '__main__.SpamPlugin'>), 
('BaconPlugin', <class '__main__.BaconPlugin'>)]

修改：@ drone115b也在__init__中使用__new__代替PluginMetaclass解决了此问题。在大多数情况下，这可能是更好的方法。

python 3.2插件工厂：从类/元类实例化

1 个答案: