python 3.2 插件工厂 : instantiation from class/metaclass

Question

我是从这里的信息中提取的: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了对象；但是当我尝试使用与注册表关联的类对象时，我收到错误消息，提示我传递的参数数量不正确。 (似乎是在调用元类 new 而不是我的构造函数...？？)

我不清楚它失败的原因，因为我认为我应该能够使用“可调用”语法从类对象创建一个实例。

似乎我正在将元类而不是类本身放入注册表中？但是我没有看到在 new 调用中访问类本身的简单方法。

这是我的代码示例，它无法实例化变量“d”:

registry = [] # list of subclassesclass 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 modulesclass SpamPlugin(Plugin):    def __init__(self, stuff):        self.stuff = stuffclass BaconPlugin(Plugin):    def __init__(self, stuff):        self.stuff = stuffc = 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，就像任何常规类方法一样。您可能想要注册从 super(PluginMetaclass, cls).__new__(..) 获取的新创建的类对象。

这个修改后的版本适用于 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

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'>)]

编辑:@drone115b 还通过在 PluginMetaclass 中使用 __init__ 而不是 __new__ 解决了这个问题。在大多数情况下，这可能是更好的方法。