How to read class attributes in the same order as declared?

Question

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 example):

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)

Result:

[('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>)]

This does not respect the declaration order of Column() attributes for Listing class. If I use classDict directly, it does not help either.

How can I proceed?

Answer 1

In the current version of Python, the class ordering is preserved. See PEP520 for details.

In older versions of the language (3.5 and below, but not 2.x), you can provide a metaclass which uses an OrderedDict for the class namespace.

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']

This approach doesn't help you with existing classes, however, where you'll need to use introspection.