什么是甲状腺?它们用于什么?
当前回答
什么是Metaclasses?你用它们用于什么?
>>> Class(...)
instance
>>> Metaclass(...)
Class
>>> type('Foo', (object,), {}) # requires a name, bases, and a namespace
<class '__main__.Foo'>
每当你创建一个类时,你都会使用一个类型:
class Foo(object):
'demo'
>>> Foo
<class '__main__.Foo'>
>>> isinstance(Foo, type), isinstance(Foo, object)
(True, True)
name = 'Foo'
bases = (object,)
namespace = {'__doc__': 'demo'}
Foo = type(name, bases, namespace)
>>> Foo.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
'__module__': '__main__', '__weakref__': <attribute '__weakref__'
of 'Foo' objects>, '__doc__': 'demo'})
(在 __dict__: __module__ 类的内容上有一个侧笔记,因为类必须知道它们在哪里定义,而 __dict__ 和 __weakref__ 是因为我们不定义 __slots__ - 如果我们定义 __slots__ 我们会在例子中节省一些空间,因为我们可以通过排除它们来排除 __dict__ 和 __weakref__。
>>> Baz = type('Bar', (object,), {'__doc__': 'demo', '__slots__': ()})
>>> Baz.__dict__
mappingproxy({'__doc__': 'demo', '__slots__': (), '__module__': '__main__'})
我们可以像任何其他类定义一样扩展类型:
>>> Foo
<class '__main__.Foo'>
class Type(type):
def __repr__(cls):
"""
>>> Baz
Type('Baz', (Foo, Bar,), {'__module__': '__main__', '__doc__': None})
>>> eval(repr(Baz))
Type('Baz', (Foo, Bar,), {'__module__': '__main__', '__doc__': None})
"""
metaname = type(cls).__name__
name = cls.__name__
parents = ', '.join(b.__name__ for b in cls.__bases__)
if parents:
parents += ','
namespace = ', '.join(': '.join(
(repr(k), repr(v) if not isinstance(v, type) else v.__name__))
for k, v in cls.__dict__.items())
return '{0}(\'{1}\', ({2}), {{{3}}})'.format(metaname, name, parents, namespace)
def __eq__(cls, other):
"""
>>> Baz == eval(repr(Baz))
True
"""
return (cls.__name__, cls.__bases__, cls.__dict__) == (
other.__name__, other.__bases__, other.__dict__)
>>> class Bar(object): pass
>>> Baz = Type('Baz', (Foo, Bar,), {'__module__': '__main__', '__doc__': None})
>>> Baz
Type('Baz', (Foo, Bar,), {'__module__': '__main__', '__doc__': None})
但是,与 eval(repr(Class))的进一步检查是不可能的(因为函数将是相当不可能从他们的默认 __repr__ 的 eval 。
from collections import OrderedDict
class OrderedType(Type):
@classmethod
def __prepare__(metacls, name, bases, **kwargs):
return OrderedDict()
def __new__(cls, name, bases, namespace, **kwargs):
result = Type.__new__(cls, name, bases, dict(namespace))
result.members = tuple(namespace)
return result
class OrderedMethodsObject(object, metaclass=OrderedType):
def method1(self): pass
def method2(self): pass
def method3(self): pass
def method4(self): pass
>>> OrderedMethodsObject.members
('__module__', '__qualname__', 'method1', 'method2', 'method3', 'method4')
>>> inspect.getmro(OrderedType)
(<class '__main__.OrderedType'>, <class '__main__.Type'>, <class 'type'>, <class 'object'>)
而且它大约有正确的回报(除非我们能找到代表我们的功能的方式,否则我们就不能再评估):
>>> OrderedMethodsObject
OrderedType('OrderedMethodsObject', (object,), {'method1': <function OrderedMethodsObject.method1 at 0x0000000002DB01E0>, 'members': ('__module__', '__qualname__', 'method1', 'method2', 'method3', 'method4'), 'method3': <function OrderedMet
hodsObject.method3 at 0x0000000002DB02F0>, 'method2': <function OrderedMethodsObject.method2 at 0x0000000002DB0268>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'OrderedMethodsObject' objects>, '__doc__': None, '__d
ict__': <attribute '__dict__' of 'OrderedMethodsObject' objects>, 'method4': <function OrderedMethodsObject.method4 at 0x0000000002DB0378>})
其他回答
上面的答案是正确的。
但读者可能来到这里寻找关于类似名称的内部课程的答案,他们在受欢迎的图书馆,如Django和WTForms。
相反,这些是班级的命令之内的名称空间,它们是用内部班级为可读性而建造的。
在这个特殊的例子领域,抽象是显而易见地与作者模型的领域分开。
from django.db import models
class Author(models.Model):
name = models.CharField(max_length=50)
email = models.EmailField()
class Meta:
abstract = True
另一个例子是WTForms的文档:
from wtforms.form import Form
from wtforms.csrf.session import SessionCSRF
from wtforms.fields import StringField
class MyBaseForm(Form):
class Meta:
csrf = True
csrf_class = SessionCSRF
name = StringField("name")
这个合成不会在Python编程语言中得到特别的处理. Meta 不是这里的一个关键词,也不会引发 meta 类行为. 相反,第三方图书馆代码在 Django 和 WTForms 等包中,在某些类的构建者和其他地方读到这个属性。
这些声明的存在改变了具有这些声明的类别的行为. 例如,WTForms 阅读 self.Meta.csrf 以确定表格是否需要一个 csrf 字段。
请注意,在Python 3.6中,引入了一个新的Dunder方法 __init_subclass__(cls, **kwargs),以取代许多常见的使用案例为MetaClass。
下面是另一个例子,它可以用于什么:
您可以使用甲状腺来改变其例子(类)的功能。
class MetaMemberControl(type):
__slots__ = ()
@classmethod
def __prepare__(mcs, f_cls_name, f_cls_parents, # f_cls means: future class
meta_args=None, meta_options=None): # meta_args and meta_options is not necessarily needed, just so you know.
f_cls_attr = dict()
if not "do something or if you want to define your cool stuff of dict...":
return dict(make_your_special_dict=None)
else:
return f_cls_attr
def __new__(mcs, f_cls_name, f_cls_parents, f_cls_attr,
meta_args=None, meta_options=None):
original_getattr = f_cls_attr.get('__getattribute__')
original_setattr = f_cls_attr.get('__setattr__')
def init_getattr(self, item):
if not item.startswith('_'): # you can set break points at here
alias_name = '_' + item
if alias_name in f_cls_attr['__slots__']:
item = alias_name
if original_getattr is not None:
return original_getattr(self, item)
else:
return super(eval(f_cls_name), self).__getattribute__(item)
def init_setattr(self, key, value):
if not key.startswith('_') and ('_' + key) in f_cls_attr['__slots__']:
raise AttributeError(f"you can't modify private members:_{key}")
if original_setattr is not None:
original_setattr(self, key, value)
else:
super(eval(f_cls_name), self).__setattr__(key, value)
f_cls_attr['__getattribute__'] = init_getattr
f_cls_attr['__setattr__'] = init_setattr
cls = super().__new__(mcs, f_cls_name, f_cls_parents, f_cls_attr)
return cls
class Human(metaclass=MetaMemberControl):
__slots__ = ('_age', '_name')
def __init__(self, name, age):
self._name = name
self._age = age
def __getattribute__(self, item):
"""
is just for IDE recognize.
"""
return super().__getattribute__(item)
""" with MetaMemberControl then you don't have to write as following
@property
def name(self):
return self._name
@property
def age(self):
return self._age
"""
def test_demo():
human = Human('Carson', 27)
# human.age = 18 # you can't modify private members:_age <-- this is defined by yourself.
# human.k = 18 # 'Human' object has no attribute 'k' <-- system error.
age1 = human._age # It's OK, although the IDE will show some warnings. (Access to a protected member _age of a class)
age2 = human.age # It's OK! see below:
"""
if you do not define `__getattribute__` at the class of Human,
the IDE will show you: Unresolved attribute reference 'age' for class 'Human'
but it's ok on running since the MetaMemberControl will help you.
"""
if __name__ == '__main__':
test_demo()
金星是强大的,有很多事情(如猴子魔法)你可以用它,但要小心,这可能只是你知道的。
Metaclasses 是做“类”的工作的秘密酱油,新风格对象的默认 metaclass 被称为“类型”。
class type(object)
| type(object) -> the object's type
| type(name, bases, dict) -> a new type
Metaclasses 取 3 args. 'name', 'bases' 和 'dict'
查找这个例子类定义中的名称、基础和字符号来源于哪里。
class ThisIsTheName(Bases, Are, Here):
All_the_code_here
def doesIs(create, a):
dict
def test_metaclass(name, bases, dict):
print 'The Class Name is', name
print 'The Class Bases are', bases
print 'The dict has', len(dict), 'elems, the keys are', dict.keys()
return "yellow"
class TestName(object, None, int, 1):
__metaclass__ = test_metaclass
foo = 1
def baz(self, arr):
pass
print 'TestName = ', repr(TestName)
# output =>
The Class Name is TestName
The Class Bases are (<type 'object'>, None, <type 'int'>, 1)
The dict has 4 elems, the keys are ['baz', '__module__', 'foo', '__metaclass__']
TestName = 'yellow'
现在,一个实际上意味着什么的例子,这将自动使列表中的变量“属性”设置在课堂上,并设置为无。
def init_attributes(name, bases, dict):
if 'attributes' in dict:
for attr in dict['attributes']:
dict[attr] = None
return type(name, bases, dict)
class Initialised(object):
__metaclass__ = init_attributes
attributes = ['foo', 'bar', 'baz']
print 'foo =>', Initialised.foo
# output=>
foo => None
请注意,启动者获得的魔法行为是通过拥有金属类的 init_属性而没有转移到启动者的子类。
这里是一个更具体的例子,显示如何可以创建一个在创建一个类时执行一个行动的甲型类的“类型”。
class MetaSingleton(type):
instance = None
def __call__(cls, *args, **kw):
if cls.instance is None:
cls.instance = super(MetaSingleton, cls).__call__(*args, **kw)
return cls.instance
class Foo(object):
__metaclass__ = MetaSingleton
a = Foo()
b = Foo()
assert a is b
>>> class ObjectCreator(object):
... pass
>>> my_object = ObjectCreator()
>>> print(my_object)
<__main__.ObjectCreator object at 0x8974f2c>
>>> class ObjectCreator(object):
... pass
>>> print(JustAnotherVariable)
<class '__main__.ObjectCreator'>
>>> print(JustAnotherVariable())
<__main__.ObjectCreator object at 0x8997b4c>
>>> def choose_class(name):
... if name == 'foo':
... class Foo(object):
... pass
... return Foo # return the class, not an instance
... else:
... class Bar(object):
... pass
... return Bar
...
>>> MyClass = choose_class('foo')
>>> print(MyClass) # the function returns a class, not an instance
<class '__main__.Foo'>
>>> print(MyClass()) # you can create an object from this class
<__main__.Foo object at 0x89c6d4c>
>>> print(type(1))
<type 'int'>
>>> print(type("1"))
<type 'str'>
>>> print(type(ObjectCreator))
<type 'type'>
>>> print(type(ObjectCreator()))
<class '__main__.ObjectCreator'>
type(name, bases, attrs)
>>> class MyShinyClass(object):
... pass
>>> MyShinyClass = type('MyShinyClass', (), {}) # returns a class object
>>> print(MyShinyClass)
<class '__main__.MyShinyClass'>
>>> print(MyShinyClass()) # create an instance with the class
<__main__.MyShinyClass object at 0x8997cec>
>>> class Foo(object):
... bar = True
>>> Foo = type('Foo', (), {'bar':True})
>>> print(Foo)
<class '__main__.Foo'>
>>> print(Foo.bar)
True
>>> f = Foo()
>>> print(f)
<__main__.Foo object at 0x8a9b84c>
>>> print(f.bar)
True
>>> class FooChild(Foo):
... pass
>>> FooChild = type('FooChild', (Foo,), {})
>>> print(FooChild)
<class '__main__.FooChild'>
>>> print(FooChild.bar) # bar is inherited from Foo
True
>>> def echo_bar(self):
... print(self.bar)
...
>>> FooChild = type('FooChild', (Foo,), {'echo_bar': echo_bar})
>>> hasattr(Foo, 'echo_bar')
False
>>> hasattr(FooChild, 'echo_bar')
True
>>> my_foo = FooChild()
>>> my_foo.echo_bar()
True
>>> def echo_bar_more(self):
... print('yet another method')
...
>>> FooChild.echo_bar_more = echo_bar_more
>>> hasattr(FooChild, 'echo_bar_more')
True
MyClass = MetaClass()
my_object = MyClass()
MyClass = type('MyClass', (), {})
>>> age = 35
>>> age.__class__
<type 'int'>
>>> name = 'bob'
>>> name.__class__
<type 'str'>
>>> def foo(): pass
>>> foo.__class__
<type 'function'>
>>> class Bar(object): pass
>>> b = Bar()
>>> b.__class__
<class '__main__.Bar'>
>>> age.__class__.__class__
<type 'type'>
>>> name.__class__.__class__
<type 'type'>
>>> foo.__class__.__class__
<type 'type'>
>>> b.__class__.__class__
<type 'type'>
class Foo(object):
__metaclass__ = something...
[...]
class Foo(Bar):
pass
设置 meta 类的合成已在 Python 3 中更改:
class Foo(object, metaclass=something):
...
class Foo(object, metaclass=something, kwarg1=value1, kwarg2=value2):
...
# the metaclass will automatically get passed the same argument
# that you usually pass to `type`
def upper_attr(future_class_name, future_class_parents, future_class_attrs):
"""
Return a class object, with the list of its attribute turned
into uppercase.
"""
# pick up any attribute that doesn't start with '__' and uppercase it
uppercase_attrs = {
attr if attr.startswith("__") else attr.upper(): v
for attr, v in future_class_attrs.items()
}
# let `type` do the class creation
return type(future_class_name, future_class_parents, uppercase_attrs)
__metaclass__ = upper_attr # this will affect all classes in the module
class Foo(): # global __metaclass__ won't work with "object" though
# but we can define __metaclass__ here instead to affect only this class
# and this will work with "object" children
bar = 'bip'
>>> hasattr(Foo, 'bar')
False
>>> hasattr(Foo, 'BAR')
True
>>> Foo.BAR
'bip'
# remember that `type` is actually a class like `str` and `int`
# so you can inherit from it
class UpperAttrMetaclass(type):
# __new__ is the method called before __init__
# it's the method that creates the object and returns it
# while __init__ just initializes the object passed as parameter
# you rarely use __new__, except when you want to control how the object
# is created.
# here the created object is the class, and we want to customize it
# so we override __new__
# you can do some stuff in __init__ too if you wish
# some advanced use involves overriding __call__ as well, but we won't
# see this
def __new__(upperattr_metaclass, future_class_name,
future_class_parents, future_class_attrs):
uppercase_attrs = {
attr if attr.startswith("__") else attr.upper(): v
for attr, v in future_class_attrs.items()
}
return type(future_class_name, future_class_parents, uppercase_attrs)
class UpperAttrMetaclass(type):
def __new__(cls, clsname, bases, attrs):
uppercase_attrs = {
attr if attr.startswith("__") else attr.upper(): v
for attr, v in attrs.items()
}
return type(clsname, bases, uppercase_attrs)
class UpperAttrMetaclass(type):
def __new__(cls, clsname, bases, attrs):
uppercase_attrs = {
attr if attr.startswith("__") else attr.upper(): v
for attr, v in attrs.items()
}
return type.__new__(cls, clsname, bases, uppercase_attrs)
class UpperAttrMetaclass(type):
def __new__(cls, clsname, bases, attrs):
uppercase_attrs = {
attr if attr.startswith("__") else attr.upper(): v
for attr, v in attrs.items()
}
# Python 2 requires passing arguments to super:
return super(UpperAttrMetaclass, cls).__new__(
cls, clsname, bases, uppercase_attrs)
# Python 3 can use no-arg super() which infers them:
return super().__new__(cls, clsname, bases, uppercase_attrs)
class Foo(object, metaclass=MyMetaclass, kwarg1=value1):
...
class MyMetaclass(type):
def __new__(cls, clsname, bases, dct, kwargs1=default):
...
使用金属玻璃代码的复杂性背后的原因不是由于金属玻璃,而是因为你通常使用金属玻璃来制作依赖于入观、操纵遗产、如 __dict__ 等的旋转物品。
有几个理由这样做:
為什麼要使用MetaClass?
现在,大问题:为什么你会使用一些模糊的错误漏洞功能?
如果你想知道你是否需要它们,你不会(真正需要它们的人肯定知道他们需要它们,不需要解释为什么)。
Python Guru 蒂姆·彼得斯
class Person(models.Model):
name = models.CharField(max_length=30)
age = models.IntegerField()
person = Person(name='bob', age='35')
print(person.age)
最后一句话
首先,你知道,类是可以创造例子的物体。
>>> class Foo(object): pass
>>> id(Foo)
142630324
99%的时间你需要课堂变化,你更好地使用这些。
但98%的时间,你根本不需要课堂变化。
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