如何创建一个元类？
这个问题在这里已经有了答案：
在这个元类中有两个关键的方法：
__prepare__和 __new__ __prepare__允许您在创建类时提供用作名称空间的自定义映射（如OrderedDict ）。 您必须返回您选择的任何名称空间的实例。 如果你没有实现__prepare__则使用正常的dict 。
__new__负责实际创建/修改最终课程。
一个简单的，不需要额外的元类将如下所示：
class Meta(type):
def __prepare__(metaclass, cls, bases):
return dict()
def __new__(metacls, cls, bases, clsdict):
return super().__new__(metacls, cls, bases, clsdict)
一个简单的例子：
假设你需要一些简单的验证代码来运行你的属性 - 就像它必须是一个int或者一个str 。 没有元类，你的类将看起来像这样：
class Person:
weight = ValidateType('weight', int)
age = ValidateType('age', int)
name = ValidateType('name', str)
正如你所看到的，你必须重复两次属性的名称。 这使得拼写错误和恼人的错误成为可能。
一个简单的元类可以解决这个问题：
class Person(metaclass=Validator):
weight = ValidateType(int)
age = ValidateType(int)
name = ValidateType(str)
这是元类的样子（不使用__prepare__因为它不是必需的）：
class Validator(type):
def __new__(metacls, cls, bases, clsdict):
# search clsdict looking for ValidateType descriptors
for name, attr in clsdict.items():
if isinstance(attr, ValidateType):
attr.name = name
attr.attr = '_' + name
# create final class and return it
return super().__new__(metacls, cls, bases, clsdict)
样品运行：
p = Person()
p.weight = 9
print(p.weight)
p.weight = '9'
生产：
9
Traceback (most recent call last):
File "simple_meta.py", line 36, in <module>
p.weight = '9'
File "simple_meta.py", line 24, in __set__
(self.name, self.type, value))
TypeError: weight must be of type(s) <class 'int'> (got '9')
笔记
这个例子很简单，它也可以用一个类装饰器完成，但是大概一个实际的元类会做得更多。
在Python 2.x中，所述__prepare__方法不存在，并且类通过包括类变量speficies其元类__metaclass__ = ... ，如下所示：
class Person(object):
__metaclass__ = ValidateType
'ValidateType'类供参考：
class ValidateType:
def __init__(self, type):
self.name = None # will be set by metaclass
self.attr = None # will be set by metaclass
self.type = type
def __get__(self, inst, cls):
if inst is None:
return self
else:
return inst.__dict__[self.attr]
def __set__(self, inst, value):
if not isinstance(value, self.type):
raise TypeError('%s must be of type(s) %s (got %r)' %
(self.name, self.type, value))
else:
inst.__dict__[self.attr] = value
我刚刚写了一个元类的完整评论的例子。 它在Python 2.7中。 我在这里分享它，希望它可以帮助您了解更多关于__new__ ， __init__ ， __call__ ， __dict__方法和有界/无界在Python的概念，以及使用元类的。
我觉得元类的问题在于，它有太多的地方可以做同样的事情 ，或者类似但有一些细微差别 。 所以我的评论和测试案例主要强调在什么地方写什么 ，在某些地方的什么地方以及某个对象可以访问哪些地方 。
该示例尝试构建类工厂，同时维护格式良好的类定义。
from pprint import pprint
from types import DictType
class FactoryMeta(type):
""" Factory Metaclass """
# @ Anything "static" (bounded to the classes rather than the instances)
# goes in here. Or use "@classmethod" decorator to bound it to meta.
# @ Note that these members won't be visible to instances, you have to
# manually add them to the instances in metaclass' __call__ if you wish
# to access them through a instance directly (see below).
extra = "default extra"
count = 0
def clsVar(cls):
print "Class member 'var': " + str(cls.var)
@classmethod
def metaVar(meta):
print "Metaclass member 'var': " + str(meta.var)
def __new__(meta, name, bases, dict):
# @ Metaclass' __new__ serves as a bi-functional slot capable for
# initiating the classes as well as alternating the meta.
# @ Suggestion is putting majority of the class initialization code
# in __init__, as you can directly reference to cls there; saving
# here for anything you want to dynamically added to the meta (such
# as shared variables or lazily GC'd temps).
# @ Any changes here to dict will be visible to the new class and their
# future instances, but won't affect the metaclass. While changes
# directly through meta will be visible to all (unless you override
# it later).
dict['new_elem'] = "effective"
meta.var = "Change made to %s by metaclass' __new__" % str(meta)
meta.count += 1
print "================================================================"
print " Metaclass's __new__ (creates class objects)"
print "----------------------------------------------------------------"
print "Bounded to object: " + str(meta)
print "Bounded object's __dict__: "
pprint(DictType(meta.__dict__), depth = 1)
print "----------------------------------------------------------------"
print "Parameter 'name': " + str(name)
print "Parameter 'bases': " + str(bases)
print "Parameter 'dict': "
pprint(dict, depth = 1)
print "n"
return super(FactoryMeta, meta).__new__(meta, name, bases, dict)
def __init__(cls, name, bases, dict):
# @ Metaclass' __init__ is the standard slot for class initialization.
# Classes' common variables should mainly goes in here.
# @ Any changes here to dict won't actually affect anything. While
# changes directly through cls will be visible to the created class
# and its future instances. Metaclass remains untouched.
dict['init_elem'] = "defective"
cls.var = "Change made to %s by metaclass' __init__" % str(cls)
print "================================================================"
print " Metaclass's __init__ (initiates class objects)"
print "----------------------------------------------------------------"
print "Bounded to object: " + str(cls)
print "Bounded object's __dict__: "
pprint(DictType(cls.__dict__), depth = 1)
print "----------------------------------------------------------------"
print "Parameter 'name': " + str(name)
print "Parameter 'bases': " + str(bases)
print "Parameter 'dict': "
pprint(dict, depth = 1)
print "n"
return super(FactoryMeta, cls).__init__(name, bases, dict)
def __call__(cls, *args):
# @ Metaclass' __call__ gets called when a class name is used as a
# callable function to create an instance. It is called before the
# class' __new__.
# @ Instance's initialization code can be put in here, although it
# is bounded to "cls" rather than instance's "self". This provides
# a slot similar to the class' __new__, where cls' members can be
# altered and get copied to the instances.
# @ Any changes here through cls will be visible to the class and its
# instances. Metaclass remains unchanged.
cls.var = "Change made to %s by metaclass' __call__" % str(cls)
# @ "Static" methods defined in the meta which cannot be seen through
# instances by default can be manually assigned with an access point
# here. This is a way to create shared methods between different
# instances of the same metaclass.
cls.metaVar = FactoryMeta.metaVar
print "================================================================"
print " Metaclass's __call__ (initiates instance objects)"
print "----------------------------------------------------------------"
print "Bounded to object: " + str(cls)
print "Bounded object's __dict__: "
pprint(DictType(cls.__dict__), depth = 1)
print "n"
return super(FactoryMeta, cls).__call__(*args)
class Factory(object):
""" Factory Class """
# @ Anything declared here goes into the "dict" argument in the metaclass'
# __new__ and __init__ methods. This provides a chance to pre-set the
# member variables desired by the two methods, before they get run.
# @ This also overrides the default values declared in the meta.
__metaclass__ = FactoryMeta
extra = "overridng extra"
def selfVar(self):
print "Instance member 'var': " + str(self.var)
@classmethod
def classFactory(cls, name, bases, dict):
# @ With a factory method embedded, the Factory class can act like a
# "class incubator" for generating other new classes.
# @ The dict parameter here will later be passed to the metaclass'
# __new__ and __init__, so it is the right place for setting up
# member variables desired by these two methods.
dict['class_id'] = cls.__metaclass__.count # An ID starts from 0.
# @ Note that this dict is for the *factory product classes*. Using
# metaclass as callable is another way of writing class definition,
# with the flexibility of employing dynamically generated members
# in this dict.
# @ Class' member methods can be added dynamically by using the exec
# keyword on dict.
exec(cls.extra, dict)
exec(dict['another_func'], dict)
return cls.__metaclass__(name + ("_%02d" % dict['class_id']), bases, dict)
def __new__(cls, function):
# @ Class' __new__ "creates" the instances.
# @ This won't affect the metaclass. But it does alter the class' member
# as it is bounded to cls.
cls.extra = function
print "================================================================"
print " Class' __new__ ("creates" instance objects)"
print "----------------------------------------------------------------"
print "Bounded to object: " + str(cls)
print "Bounded object's __dict__: "
pprint(DictType(cls.__dict__), depth = 1)
print "----------------------------------------------------------------"
print "Parameter 'function': n" + str(function)
print "n"
return super(Factory, cls).__new__(cls)
def __init__(self, function, *args, **kwargs):
# @ Class' __init__ initializes the instances.
# @ Changes through self here (normally) won't affect the class or the
# metaclass; they are only visible locally to the instances.
# @ However, here you have another chance to make "static" things
# visible to the instances, "locally".
self.classFactory = self.__class__.classFactory
print "================================================================"
print " Class' __init__ (initiates instance objects)"
print "----------------------------------------------------------------"
print "Bounded to object: " + str(self)
print "Bounded object's __dict__: "
pprint(DictType(self.__dict__), depth = 1)
print "----------------------------------------------------------------"
print "Parameter 'function': n" + str(function)
print "n"
return super(Factory, self).__init__(*args, **kwargs)
# @ The metaclass' __new__ and __init__ will be run at this point, where the
# (manual) class definition hitting its end.
# @ Note that if you have already defined everything well in a metaclass, the
# class definition can go dummy with simply a class name and a "pass".
# @ Moreover, if you use class factories extensively, your only use of a
# manually defined class would be to define the incubator class.
输出看起来像这样（适合更好的演示）：
================================================================
Metaclass's __new__ (creates class objects)
----------------------------------------------------------------
Bounded to object: <class '__main__.FactoryMeta'>
Bounded object's __dict__:
{ ...,
'clsVar': <function clsVar at 0x00000000029BC828>,
'count': 1,
'extra': 'default extra',
'metaVar': <classmethod object at 0x00000000029B4B28>,
'var': "Change made to <class '__main__.FactoryMeta'> by metaclass' __new__"}
----------------------------------------------------------------
Parameter 'name': Factory
Parameter 'bases': (<type 'object'>,)
Parameter 'dict':
{ ...,
'classFactory': <classmethod object at 0x00000000029B4DC8>,
'extra': 'overridng extra',
'new_elem': 'effective',
'selfVar': <function selfVar at 0x00000000029BC6D8>}
================================================================
Metaclass's __init__ (initiates class objects)
----------------------------------------------------------------
Bounded to object: <class '__main__.Factory'>
Bounded object's __dict__:
{ ...,
'classFactory': <classmethod object at 0x00000000029B4DC8>,
'extra': 'overridng extra',
'new_elem': 'effective',
'selfVar': <function selfVar at 0x00000000029BC6D8>,
'var': "Change made to <class '__main__.Factory'> by metaclass' __init__"}
----------------------------------------------------------------
Parameter 'name': Factory
Parameter 'bases': (<type 'object'>,)
Parameter 'dict':
{ ...,
'classFactory': <classmethod object at 0x00000000029B4DC8>,
'extra': 'overridng extra',
'init_elem': 'defective',
'new_elem': 'effective',
'selfVar': <function selfVar at 0x00000000029BC6D8>}
调用序列是元类' __new__然后是__init__ 。 __call__不会被调用。
如果我们创建一个实例，
func1 = (
"def printElems(self):n"
" print "Member new_elem: " + self.new_elemn"
" print "Member init_elem: " + self.init_elemn"
)
factory = Factory(func1)
输出是：
================================================================
Metaclass's __call__ (initiates instance objects)
----------------------------------------------------------------
Bounded to object: <class '__main__.Factory'>
Bounded object's __dict__:
{ ...,
'classFactory': <classmethod object at 0x00000000029B4DC8>,
'extra': 'overridng extra',
'metaVar': <bound method type.metaVar of <class '__main__.FactoryMeta'>>,
'new_elem': 'effective',
'selfVar': <function selfVar at 0x00000000029BC6D8>,
'var': "Change made to <class '__main__.Factory'> by metaclass' __call__"}
================================================================
Class' __new__ ("creates" instance objects)
----------------------------------------------------------------
Bounded to object: <class '__main__.Factory'>
Bounded object's __dict__:
{ ...,
'classFactory': <classmethod object at 0x00000000029B4DC8>,
'extra': 'def printElems(self):n print "Member new_elem: " + self.new_elemn print "Member init_elem: " + self.init_elemn',
'metaVar': <bound method type.metaVar of <class '__main__.FactoryMeta'>>,
'new_elem': 'effective',
'selfVar': <function selfVar at 0x00000000029BC6D8>,
'var': "Change made to <class '__main__.Factory'> by metaclass' __call__"}
----------------------------------------------------------------
Parameter 'function':
def printElems(self):
print "Member new_elem: " + self.new_elem
print "Member init_elem: " + self.init_elem
================================================================
Class' __init__ (initiates instance objects)
----------------------------------------------------------------
Bounded to object: <__main__.Factory object at 0x00000000029BB7B8>
Bounded object's __dict__:
{'classFactory': <bound method FactoryMeta.classFactory of <class '__main__.Factory'>>}
----------------------------------------------------------------
Parameter 'function':
def printElems(self):
print "Member new_elem: " + self.new_elem
print "Member init_elem: " + self.init_elem
元类' __call__被调用，然后是类__new__和__init__ 。
比较每个对象的打印成员，您可以发现它们何时何地被添加或更改，就像我在代码中所评论的一样。
我也运行以下测试用例：
factory.clsVar() # Will raise exception
Factory.clsVar()
factory.metaVar()
factory.selfVar()
func2 = (
"@classmethodn"
"def printClassID(cls):n"
" print "Class ID: %02d" % cls.class_idn"
)
ProductClass1 = factory.classFactory("ProductClass", (object, ), { 'another_func': func2 })
product = ProductClass1()
product.printClassID()
product.printElems() # Will raise exception
ProductClass2 = Factory.classFactory("ProductClass", (Factory, ), { 'another_func': "pass" })
ProductClass2.printClassID() # Will raise exception
ProductClass3 = ProductClass2.classFactory("ProductClass", (object, ), { 'another_func': func2 })
你可以自己运行，看看它是如何工作的。
请注意，我故意将动态生成的类的名称与它们分配的变量名称不同。 这是为了显示哪些名称实际上是有效的。
另外需要注意的是，我将“static”放在引号中，我将其引用为C ++中的概念，而不是Python装饰器。 传统上我是一名C ++程序员，所以我仍然喜欢思考。
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