我应该使用元类，课程装饰器还是覆盖__new__方法？

Question

这是我的问题。我希望以下课程具有一堆属性属性。我可以把它们全部写出来 foo 和 bar, ，或基于我见过的其他一些示例，看起来我可以使用类装饰器，元素或覆盖 __new__ 自动设置属性的方法。我只是不确定“正确”的方法是什么。

class Test(object):
    def calculate_attr(self, attr):
        # do calculaty stuff
        return attr

    @property
    def foo(self):
        return self.calculate_attr('foo')

    @property
    def bar(self):
        return self.calculate_attr('bar')

Answer 1

魔术是不好的。它使您的代码更难理解和维护。您几乎不需要元类或 __new__.

看来您的用例可以用非常简单的代码实现（只有一小段魔术）：

class Test(object):
    def calculate_attr(self, attr):
        return something

    def __getattr__(self, name):
        return self.calculate_attr(name)

Answer 2

A metaclass's __new__ does not become the __new__ for the class you make—it's used to make the class itself. The actual class object is returned by the metaclass. A new instance of a class is returned by __new__.

Consider the following (insane) code:

def MyMetaClass(name, bases, dict):
    print "name", name
    print "bases", bases
    print "dict", dict
    return 7

class C('hello', 'world'):
    __metaclass__ = MyMetaClass

    foo = "bar"

    def baz(self, qux):
        pass

print "C", C

(I used a function instead of a class as the metaclass. Any callable can be used as a metaclass, but many people choose to right theirs as classes that inherit from type with new overrided. The differences between that an a function are subtle.)

It outputs

name C
bases ('hello', 'world')
dict {'baz': <function baz at 0x4034c844>, '__module__': '__main__', 'foo': 'bar', '__metaclass__': <function MyMetaClass at 0x40345c34>}
C 7

Does that help you better make sense of what metaclasses are?

You will very seldom need to define a metaclass of your own.

Answer 3

当创建新类 - 非实例 - 时，使用元类。这样，您可以例如注册类（Django做到这一点，并使用它例如在数据库中创建表）。自从 class 是您可以将其视为课堂装饰的指令。