Hi Stackoverflow社区
我一直在试图了解Django（和Wagtail的Stream field）是如何在引擎盖下工作的。这样做，我学会了元类，并相信有一个原则上的处理。也就是说，对于我来说，SIX如何执行with_元类函数仍然有点模糊。下面是代码，后面跟着一个特定的问题：
模型.py
class BlogPage(Page):
blogElement = StreamField([
('heading', blocks.CharBlock(classname="full title")),
('paragraph', blocks.TextBlock()),
('picture', ImageChooserBlock()),
], default=[])
wagtailcore>fields.py
class StreamField(models.Field):
def __init__(self, block_types, **kwargs):
if isinstance(block_types, Block):
self.stream_block = block_types
elif isinstance(block_types, type):
self.stream_block = block_types()
else:
self.stream_block = StreamBlock(block_types)
super(StreamField, self).__init__(**kwargs)
wagtailcore>blocks>stream_block.py（瓦盖尔核心>街区>stream_block.py）
class StreamBlock(six.with_metaclass(DeclarativeSubBlocksMetaclass, BaseStreamBlock)):
pass
6.py年
def with_metaclass(meta, *bases):
"""Create a base class with a metaclass."""
# This requires a bit of explanation: the basic idea is to make a dummy
# metaclass for one level of class instantiation that replaces itself with
# the actual metaclass.
class metaclass(meta):
def __new__(cls, name, this_bases, d):
return meta(name, bases, d)
return type.__new__(metaclass, 'temporary_class', (), {})
问题
（1）描述建议我们生成一个临时的伪元类，它用实际的元类替换自己。
（2）这是如何工作的？
（3）如何通过with_元类函数对元类生成进行排序？
（4）doesBaseStreamBlock在哪里？
让我困惑的是我们定义了
[1] class metaclass(meta):
但只能通过：
[2] return type.__new__(metaclass, 'temporary_class', (), {})
在[2]中，我们实例化了在[1]中定义的类元类。这个类的实例包含DeclarativeSubBlockMetaclass作为类型，而'temporary_class'作为名称，没有基或属性。
在[1]中，我们定义了一个元类类，它似乎在执行实际的元类工作。在这里，我们开发了一个类生成器，它基于基和名称生成DeclarativeSubBlockMetaclass（作为meta传入）类型的类。
但是，由于对[1]的唯一调用来自[2]，我们似乎所做的只是实例化DeclarativeSubBlockMetaclass类型的“临时类”，而不需要任何基或属性。
如何用描述（1）中描述的实际元类替换此临时伪元类？
我试着去查阅六人组的文件，但找不到任何能解决我困惑的东西。
任何建议都将不胜感激。
非常感谢你
Z轴
只是为了上下文：
我包含了在上面的6.with_元类调用中使用的两个类的代码：
声明SubBlocksMetaClass
class DeclarativeSubBlocksMetaclass(BaseBlock):
"""
Metaclass that collects sub-blocks declared on the base classes.
(cheerfully stolen from https://github.com/django/django/blob/master/django/forms/forms.py)
"""
def __new__(mcs, name, bases, attrs):
# Collect sub-blocks declared on the current class.
# These are available on the class as `declared_blocks`
current_blocks = []
for key, value in list(attrs.items()):
if isinstance(value, Block):
current_blocks.append((key, value))
value.set_name(key)
attrs.pop(key)
current_blocks.sort(key=lambda x: x[1].creation_counter)
attrs['declared_blocks'] = collections.OrderedDict(current_blocks)
new_class = (super(DeclarativeSubBlocksMetaclass, mcs).__new__(mcs, name, bases, attrs))
# Walk through the MRO, collecting all inherited sub-blocks, to make
# the combined `base_blocks`.
base_blocks = collections.OrderedDict()
for base in reversed(new_class.__mro__):
# Collect sub-blocks from base class.
if hasattr(base, 'declared_blocks'):
base_blocks.update(base.declared_blocks)
# Field shadowing.
for attr, value in base.__dict__.items():
if value is None and attr in base_blocks:
base_blocks.pop(attr)
new_class.base_blocks = base_blocks
return new_class
基流块
class BaseStreamBlock(Block):
def __init__(self, local_blocks=None, **kwargs):
self._constructor_kwargs = kwargs
super(BaseStreamBlock, self).__init__(**kwargs)
# create a local (shallow) copy of base_blocks so that it can be supplemented by local_blocks
self.child_blocks = self.base_blocks.copy()
if local_blocks:
for name, block in local_blocks:
block.set_name(name)
self.child_blocks[name] = block
self.dependencies = self.child_blocks.values()
最佳答案
好吧-我想我已经想好了。问题的关键在于
return meta(name, bases, d)
with_元类函数：
def with_metaclass(meta, *bases):
"""Create a base class with a metaclass."""
# This requires a bit of explanation: the basic idea is to make a dummy
# metaclass for one level of class instantiation that replaces itself with
# the actual metaclass.
class metaclass(meta):
def __new__(cls, name, this_bases, d):
return meta(name, bases, d)
return type.__new__(metaclass, 'temporary_class', (), {})
下面是我认为它在sudo代码中的工作原理：
(1) with_metaclass takes <<DeclarativeSubBlocksMetaclass>> as meta; and <<BaseStreamBlock>> as bases
(2) class metaclass(meta) --> the class metaclass is then created extending <<DeclarativeSubBlockMetaclass>> as the class type
(3) def __new__(cls, name, this_bases, d): Only rarely will you have to worry about __new__. Usually, you'll just define __init__ and let the default __new__ pass the constructor arguments to it. __new__ takes care of creating the object and assigning memory space to it. This __new__ method is a class method that gets called when you create an instance of the class and it gets called before __init__. Its main job is to allocate the memory that the object that you are creating uses. It can also be used to set up any aspect of the instance of the class that is immutable Because classes are kind of immutable (they cannot be changed), overloading __new_ is the best place to overload how they are created.
(4) return meta(name, bases, d) --> the class definition ends with returning a <<DeclarativeSubBlockMetaclass>> with the arguments (name, base = BaseStreamBlock, d)
NOTE: We only define the class in 1 - 3; we are not instantiating it this comes below
(5) return type.__new__(metaclass, 'temporary_class', (), {}) --> Here we are using the classic metaclass syntax. This syntax usually looks like this: return type.__new__(cls, name, bases, attrs). We are using this syntax to instantiate the metaclass we defined in (3) and (4). One might think that it is confusing that temporary_class', (), {} are passed on as the 'name', 'bases', and 'attrs' arguments. BUT...
(6) ... when the instantiation arrives at return meta(name,bases,d) we notice that meta doesn't take 'this_bases' as an argument but 'bases'. It derives this value from the arguments which were passed to (1) with_metaclasses. As such bases in this instance == <<BaseStreamBlock>>
(7) Therefore, when we instantiate type.__new__(metaclass, 'temporary_class', (), {}) we essentially execute <<DeclarativeSubBlocksMetaClass>>('temporary_class', <<BaseStreamBlock>>, {})
在（7）中解释的步骤就是解释所说的。实际上，SIX所做的是通过指定的步骤来创建一个虚拟元类，它称之为临时类。由于DeclarativeSubBlocksMetaClass也是一个元类，因此它使用BaseStreamBlock基类生成一个新类。
我希望这有道理。
Z轴