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）遵守声明顺序（即：mycol2，mycol3，我的例子中有zut，cool，menfin，a：

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Column()类的Listing属性的声明顺序。如果我直接使用classDict，它也无济于事。

我该怎么办？

Answer 1

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及更低版本，但不是2.x）中，您可以提供一个元类，它使用OrderedDict作为类命名空间。

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']

这种方法对现有的课程没有帮助，但是你需要使用内省。

Answer 2

以下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

Answer 3

对于python 3.6，这已成为默认行为。见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__']

Answer 4

如果您使用的是Python 2.x，那么您需要一个像Lennart建议的那样的黑客攻击。如果您使用的是Python 3.x，请阅读PEP 3115，因为其中包含一个可以满足您需求的示例。只需修改示例，只查看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

Answer 5

1）因为类定义中的Python 3.6属性与名称在源中出现的顺序相同。此订单现在保留在新类的__dict__属性（https://docs.python.org/3.6/whatsnew/3.6.html#whatsnew36-pep520）中：

class Column:
    pass

class MyClass(metaclass=OrderedClass):
    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python添加的额外__xxx__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答案的解决方案，但更简单。我们使用这一事实，即__prepare__方法返回OrderDict而非简单dict - 因此，在__new__调用之前收集的所有属性都将被排序。

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__以所需顺序访问属性：

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

请注意，'__module__'课程中会出现名为'__qualname__'，type的人。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的玩家：

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

3）这个anwser仅适用于python3.x，因为python2.7中没有__prepare__定义

Answer 6

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)

Answer 7

我想你应该能够创建一个用__dict__替换ordered-dict的课程

如何按声明的顺序读取类属性？

7 个答案: