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.

Answer 2

Here is the workaround I juste developped :

import inspect  class Column(object):     creation_counter = 0     def __init__(self):         self.creation_order = Column.creation_counter         Column.creation_counter+=1  class ListingMeta(type):     def __new__(meta, classname, bases, classDict):         cls = type.__new__(meta, classname, bases, classDict)         cls.columns = sorted(inspect.getmembers(cls,lambda o:isinstance(o,Column)),key=lambda i:i[1].creation_order)          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()   for colname,col in Listing.columns:     print colname,'=>',col.creation_order