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我一直试图了解Django（以及Wagtail＆＃39; Stream-field）是如何工作的。这样做我了解了元类，并相信能够掌握原理。也就是说，究竟SIX如何执行with_metaclass函数对我来说仍然有点模糊。以下是特定问题的代码：
models.py
class BlogPage(Page):
blogElement = StreamField([
('heading', blocks.CharBlock(classname="full title")),
('paragraph', blocks.TextBlock()),
('picture', ImageChooserBlock()),
], default=[])
wagtailcore＆gt; 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＆gt;块＆gt; stream_block.py
class StreamBlock(six.with_metaclass(DeclarativeSubBlocksMetaclass, BaseStreamBlock)):
pass
six.py
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虚拟元类，用实际的元类代替它自己。 （2）这是如何工作的？ （3）我们如何通过with_metaclass函数对元类生成进行排序？ （4）doBaseStreamBlock在哪里？
困扰我的部分是我们定义
[1] class metaclass(meta):
但只能通过以下方式调用：
[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。此类的实例包含DeclarativeSubBlockMetaclass作为类型，＆＃39; temporary_class＆＃39; 作为没有基础或属性的名称。
在[1]中，我们定义了元类类，它似乎正在进行实际的元类工作。这里我们开发一个类生成器，它根据基数和名称生成DeclarativeSubBlockMetaclass类型的类（以meta形式传入）。
但是，由于对[1]的唯一调用来自[2]，我们所做的只是实例化＆lt; temp_class＆＃39;类型 DeclarativeSubBlockMetaclass 没有任何基础或属性。
我们如何用描述（1）中描述的实际元类替换这个临时虚拟元类？
我试图咨询这六个人的文档，但是找不到能解决我困惑的事情。
任何建议都将受到高度赞赏。
非常感谢你 ž
仅适用于上下文：
我在上面的six.with_metaclass调用中包含了两个类的代码：
DeclarativeSubBlocksMetaclass
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
BaseStreamBlock
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()
答案 0 :(得分：2)
好的 - 我想我明白了。问题的关键在于
return meta(name, bases, d)
：
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', (), {})
以下是我认为它在sudo代码中的作用：
(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>>, {})
（7）中解释的步骤是解释的内容。本质上，SIX所做的是通过规定的步骤来创建一个虚拟元类，它称之为temporary_class。由于DeclarativeSubBlocksMetaClass也是元类，因此它使用BaseStreamBlock基础生成一个新类。
我希望这是有道理的。
Z