如何按在Python类中声明的顺序获取属性? [英] How to get attributes in the order they are declared in a Python class?

问题描述

如 PEP435 中所述，可以定义enum方式:

As described in PEP435, an enum can be defined this way:

class Color(Enum):
    red = 1
    green = 2
    blue = 3

Color的结果enum members可以按照定义顺序进行迭代:Color.red, Color.green, Color.blue.

And the resulting enum members of Color can be iterated in definition order: Color.red, Color.green, Color.blue.

这让我想起了Django中的Form，其中可以按照在Form子类中声明的顺序来渲染字段.他们通过维护字段计数器来实现此目的，每次实例化一个新字段时，计数器值都会增加.

This reminds me of Form in Django, in which fields can be rendered in the order they are declared in a Form subclass. They implemented this by maintaining a field counter, every time a new field is instantiated the counter value get incremented.

但是在Color的定义中，我们没有类似FormField的东西，我们如何实现呢?

But in the definition of Color, we don't have something like a FormField, how can we implement this?

推荐答案

在Python 3中，您可以自定义使用元类声明类的名称空间.例如，您可以使用OrderedDict:

In Python 3, you can customize the namespace that a class is declared in with the metaclass. For example, you can use an OrderedDict:

from collections import OrderedDict

class EnumMeta(type):

    def __new__(mcls, cls, bases, d):
        print(d)
        return type.__new__(mcls, cls, bases, d)

    @classmethod
    def __prepare__(mcls, cls, bases):
        return OrderedDict()


class Color(metaclass=EnumMeta):
    red = 1
    green = 2
    blue = 3

此打印

OrderedDict([('__module__', '__main__'), ('red', 1), ('green', 2), ('blue', 3)])

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