如何读取类属性与声明的顺序相同？ [英] How to read class attributes in the same order as declared?

问题描述

我编写一个读取类属性并将它们存储在列表中的元类，但我想要列表（cls.columns）遵守声明顺序（即：mycol2，mycol3，zut，cool，menfin，a我的例子）：

I am writing a metaclass that reads class attributes and store them in a list, but I want the list (cls.columns) to respect the declaration order (that is : mycol2,mycol3,zut,cool,menfin,a in my exemple):

import inspect
import pprint

class Column(object):
    pass
class ListingMeta(type):
    def __new__(meta, classname, bases, classDict):
        cls = type.__new__(meta, classname, bases, classDict)
        cls.columns = inspect.getmembers(cls,lambda o:isinstance(o,Column)) 
        cls.nb_columns = len(cls.columns)
        return cls

class Listing(object):
    __metaclass__ = ListingMeta
    mycol2 = Column()
    mycol3 = Column()
    zut = Column()
    cool = Column()
    menfin = Column()
    a = Column()


pprint.pprint(Listing.columns)

结果：

[('a', <__main__.Column object at 0xb7449d2c>),
 ('cool', <__main__.Column object at 0xb7449aac>),
 ('menfin', <__main__.Column object at 0xb7449a8c>),
 ('mycol2', <__main__.Column object at 0xb73a3b4c>),
 ('mycol3', <__main__.Column object at 0xb744914c>),
 ('zut', <__main__.Column object at 0xb74490cc>)]

这不尊重Listing类的Column（）属性的声明顺序。

This does not respect the declaration order of Column() attributes for Listing class. if I use classDict directly, it does not help neither : How can I proceed ?

推荐答案

在Python 3中，只需使用<在将整个字典传递给类型.__ new __（）之前，请存储它的 .keys（） ，像这样：

In Python 3, just use an OrderedDict and store its .keys() before passing the whole dict to type.__new__(), like this:

import collections 

class OrderedClassMembers(type):
    @classmethod
    def __prepare__(self, name, bases):
        return collections.OrderedDict()

    def __new__(self, name, bases, classdict):
        classdict['__ordered__'] = [key for key in classdict.keys()
                if key not in ('__module__', '__qualname__')]
        return type.__new__(self, name, bases, classdict)

class Something(metaclass=OrderedClassMembers):
    A_CONSTANT = 1

    def first(self):
        ...

    def second(self):
        ...

print(Something.__ordered__)
# ['A_CONSTANT', 'first', 'second']

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