Python mixin 扩展类属性

Question

试图弄清楚如何为 Django 管理命令编写一些 mixins 来包装BaseCommand.option_list 不会丢失当前类或任何继承类/mixin 的值。目标是避免在我的命令中执行 BaseCommand.option_list + MyCommonOptionMixin.option_list + MyOtherCommonOptionMixin.option_list + (local command options)。

例子:

class BaseCommmand(object):
    option_list = (
        # Default options here.
    )

    # Rest of BaseCommand code. 

我用一些常用选项定义了一个 mixin:

class MyCommonOptionMixin(object):
    option_list = (
        # Some common option/options I wish to have available in many commands
    )

    def __getattribute__(self, name):
        values = super(MyCommonOptionMixin, self).__getattribute__(name)
        if name == 'option_list':
            for option in self.option_list:
                if option not in values:
                    values += option,
        return values

也许我还有一个，只是为了涵盖我有多个的情况。 mixins 都覆盖了 __getattribute__

class MyOtherCommonOptionMixin(object):
    option_list = (
        # Maybe I have another mixin I want to define separately
    )

    # Tried this, does not work with more than one mixin. 
    def __getattribute__(self, name):
        values = super(MyOtherCommonOptionMixin, self).__getattribute__(name)
        if name == 'option_list':
            for option in self.option_list:
                if option not in values:
                    values += option,
        return values

    # Works if the mixin defines the option_list under a different name, e.g. "_mymixin_options"
    # Then access at self._mymixin_options instead of self.option_list


class MyCommand(MyCommonOptionMixin, MyOtherCommonOptionMixin, BaseCommand):
    option_list = BaseCommand.option_list + (
        # Local defined options.
    )

如果 mixin 对 option_list 属性使用相同的名称，我就会遇到冲突。有没有比在 mixin 中唯一命名 option_list 并覆盖 __getattribute__ 更简洁的方法来实现这个目标？

Answer 1

documentation中的建议是显式连接各种选项列表。也就是说，如果您想走这条路，我认为自定义元类是正确的方法。像这样的东西:

class CommandMetaclass(type):
    def __new__(mcl, name, bases, dct):
        # start with the option_list in the class we're creating
        # use set() to avoid duplicates
        option_list = set(dct.get('option_list', tuple()))

        # add the options from each base class
        for base in bases:
            option_list.update(base.__dict__.get('option_list', tuple()))

        # replace the original option_list with our combined version
        dct['option_list'] = tuple(option_list)

        return type.__new__(mcl, name, bases, dct)

class MyCommand(MyCommonOptionMixin, MyOtherCommonOptionMixin, BaseCommand):
    __metaclass__ = CommandMetaclass

    option_list = ( 
        # Local defined options.
    )