python - 如何按照声明的顺序读取类属性？

Question

我正在编写一个读取类属性并将它们存储在列表中的元类，但我希望列表（cls.columns）尊重声明顺序（即：mycol2, mycol3, zut, cool, menfin,a在我的示例中）：

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>)]

这不尊重类Column()属性的声明顺序Listing。如果我classDict直接使用，它也无济于事。

我该如何进行？

score 40 · Accepted Answer

在当前版本的 Python 中，保留了类排序。有关详细信息，请参阅PEP520。

在该语言的旧版本（3.5 及以下，但不是 2.x）中，您可以提供一个元类，它使用OrderedDict类命名空间。

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']

但是，这种方法对您需要使用自省的现有类没有帮助。

score 16 · Accepted Answer

这是我刚刚开发的解决方法：

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

score 13 · Accepted Answer

For python 3.6, this has become the default behavior. See PEP520: https://www.python.org/dev/peps/pep-0520/

class OrderPreserved:
    a = 1
    b = 2
    def meth(self): pass

print(list(OrderPreserved.__dict__.keys()))
# ['__module__', 'a', 'b', 'meth', '__dict__', '__weakref__', '__doc__']

score 7 · Accepted Answer

1) 由于类定义中的 Python 3.6 属性具有名称在源中出现的相同顺序。此顺序现在保留在新类的__dict__属性中（https://docs.python.org/3.6/whatsnew/3.6.html#whatsnew36-pep520）：

class Column:
    pass

class MyClass:
    mycol2 = Column()
    mycol3 = Column()
    zut = Column()
    cool = Column()
    menfin = Column()
    a = Column()

print(MyClass.__dict__.keys())

你会看到这样的输出（MyClass.__dict__可以像 OrderedDict 一样使用）：

dict_keys(['__module__', 'mycol2', 'mycol3', 'zut', 'cool', 'menfin', 'a', '__dict__', '__weakref__', '__doc__'])

注意 __xxx__python 添加的额外字段，您可能需要忽略它们。

2) 对于以前的Python 3.x版本，您可以使用基于 @Duncan 回答的解决方案，但更简单。我们使用这个事实，该__prepare__方法返回 aOrderDict而不是 simple dict- 因此在调用之前收集的所有属性__new__都将被排序。

from collections import OrderedDict

class OrderedClass(type):
    @classmethod
    def __prepare__(mcs, name, bases): 
         return OrderedDict()

    def __new__(cls, name, bases, classdict):
        result = type.__new__(cls, name, bases, dict(classdict))
        result.__fields__ = list(classdict.keys())
        return result

class Column:
    pass

class MyClass(metaclass=OrderedClass):
    mycol2 = Column()
    mycol3 = Column()
    zut = Column()
    cool = Column()
    menfin = Column()
    a = Column()

现在您可以使用属性__fields__按要求的顺序访问属性：

m = MyClass()
print(m.__fields__)
['__module__', '__qualname__', 'mycol2', 'mycol3', 'zut', 'cool', 'menfin', 'a']

请注意，将有 attrs '__module__'，从班级'__qualname__'出生。type要摆脱它们，您可以按以下方式过滤名称（更改OrderedClass.__new__）：

def __new__(cls, name, bases, classdict):
    result = type.__new__(cls, name, bases, dict(classdict))
    exclude = set(dir(type))
    result.__fields__ = list(f for f in classdict.keys() if f not in exclude)
    return result

它只会给出来自 MyClass 的属性：

['mycol2', 'mycol3', 'zut', 'cool', 'menfin', 'a']

3）这个anwser只适用于python3.x，因为__prepare__python2.7中没有定义

score 6 · Accepted Answer

如果您使用的是 Python 2.x，那么您将需要 Lennart 建议的 hack。如果您使用的是 Python 3.x，请阅读PEP 3115，因为其中包含一个示例，可以满足您的需求。只需修改示例以仅查看您的 Column() 实例：

 # The custom dictionary
 class member_table(dict):
    def __init__(self):
       self.member_names = []

    def __setitem__(self, key, value):
       # if the key is not already defined, add to the
       # list of keys.
       if key not in self:
          self.member_names.append(key)

       # Call superclass
       dict.__setitem__(self, key, value)

 # The metaclass
 class OrderedClass(type):

     # The prepare function
     @classmethod
     def __prepare__(metacls, name, bases): # No keywords in this case
        return member_table()

     # The metaclass invocation
     def __new__(cls, name, bases, classdict):
        # Note that we replace the classdict with a regular
        # dict before passing it to the superclass, so that we
        # don't continue to record member names after the class
        # has been created.
        result = type.__new__(cls, name, bases, dict(classdict))
        result.member_names = classdict.member_names
        return result

 class MyClass(metaclass=OrderedClass):
    # method1 goes in array element 0
    def method1(self):
       pass

    # method2 goes in array element 1
    def method2(self):
       pass

score 3 · Accepted Answer

排除方法的答案：

from collections import OrderedDict
from types import FunctionType


class StaticOrderHelper(type):
    # Requires python3.
    def __prepare__(name, bases, **kwargs):
        return OrderedDict()

    def __new__(mcls, name, bases, namespace, **kwargs):
        namespace['_field_order'] = [
                k
                for k, v in namespace.items()
                if not k.startswith('__') and not k.endswith('__')
                    and not isinstance(v, (FunctionType, classmethod, staticmethod))
        ]
        return type.__new__(mcls, name, bases, namespace, **kwargs)


class Person(metaclass=StaticOrderHelper):
    first_name = 'First Name'
    last_name = 'Last Name'
    phone_number = '000-000'

    @classmethod
    def classmethods_not_included(self):
        pass

    @staticmethod
    def staticmethods_not_included(self):
        pass

    def methods_not_included(self):
        pass


print(Person._field_order)

score -2 · Accepted Answer

-2

我想您应该能够创建一个类，将其替换__dict__为ordered-dict

于 2010-12-16T11:11:33.423 回答

python - 如何按照声明的顺序读取类属性？

7 回答 7

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