在 Python 或任何其他语言中,我们对每个变量或对象都有一个类型. 在 Python 中,我们可以使用 Type() 函数来获得任何类型(变量、对象等)。

通过在课堂定义中的 meta 类关键词,我们可以自定义课堂创建过程。

class meta(type):
    pass
class baseclass(metaclass=meta): # This is Mestaclass
    pass
class derivedclass(baseclass):
    pass
print(type(meta))
print(type(baseclass))
print(type(derivedclass))

在定义新类时,如果没有定义的甲型,则使用默认类型甲型;如果一个甲型不是类型(例)的对象(例),则在这种情况下,它直接用作甲型。

当班级声明执行时,Python 首先将班级声明的身体作为一个正常的代码块执行。 结果的名称空间(dict)保留了班级的属性. 金属阶级通过观察班级的基层(金属阶级继承),在 __金属阶级__属性的班级(如果有)或 __金属阶级__全球变量来确定。

def make_hook(f):
    """Decorator to turn 'foo' method into '__foo__'"""
    f.is_hook = 1
    return f

class MyType(type):
    def __new__(mcls, name, bases, attrs):

        if name.startswith('None'):
            return None

        # Go over attributes and see if they should be renamed.
        newattrs = {}
        for attrname, attrvalue in attrs.iteritems():
            if getattr(attrvalue, 'is_hook', 0):
                newattrs['__%s__' % attrname] = attrvalue
            else:
                newattrs[attrname] = attrvalue

        return super(MyType, mcls).__new__(mcls, name, bases, newattrs)

    def __init__(self, name, bases, attrs):
        super(MyType, self).__init__(name, bases, attrs)

        # classregistry.register(self, self.interfaces)
        print "Would register class %s now." % self

    def __add__(self, other):
        class AutoClass(self, other):
            pass
        return AutoClass
        # Alternatively, to autogenerate the classname as well as the class:
        # return type(self.__name__ + other.__name__, (self, other), {})

    def unregister(self):
        # classregistry.unregister(self)
        print "Would unregister class %s now." % self

class MyObject:
    __metaclass__ = MyType


class NoneSample(MyObject):
    pass

# Will print "NoneType None"
print type(NoneSample), repr(NoneSample)

class Example(MyObject):
    def __init__(self, value):
        self.value = value
    @make_hook
    def add(self, other):
        return self.__class__(self.value + other.value)

# Will unregister the class
Example.unregister()

inst = Example(10)
# Will fail with an AttributeError
#inst.unregister()

print inst + inst
class Sibling(MyObject):
    pass

ExampleSibling = Example + Sibling
# ExampleSibling is now a subclass of both Example and Sibling (with no
# content of its own) although it will believe it's called 'AutoClass'
print ExampleSibling
print ExampleSibling.__mro__

除了发布的答案,我可以说,一个甲状腺可以定义一个类的行为,所以,你可以明确设置你的甲状腺,每当Python获得一个关键词类,然后它开始搜索甲状腺,如果它没有找到 - 默认甲状腺类型用于创建一个类的对象,使用 __metaclass__属性,你可以设置你的甲状腺类:

class MyClass:
   __metaclass__ = type
   # write here other method
   # write here one more method

print(MyClass.__metaclass__)

它将产生这样的产量:

class 'type'

当然,你可以创建自己的金属类来定义使用你的类创建的任何类的行为。

要做到这一点,您的默认金属类型类必须继承,因为这是主要金属类:

class MyMetaClass(type):
   __metaclass__ = type
   # you can write here any behaviour you want

class MyTestClass:
   __metaclass__ = MyMetaClass

Obj = MyTestClass()
print(Obj.__metaclass__)
print(MyMetaClass.__metaclass__)

产量将是:

class '__main__.MyMetaClass'
class 'type'

此分類上一篇: tl;dr version

类型(obj)函数会给你一个对象的类型。

一个阶级的类型( )是它的甲型阶级。

使用甲状腺:

class Foo(object):
    __metaclass__ = MyMetaClass

一个类的类是一个类的类 - 一个类的身体是转移到一个类的论点,它被用来构建一个类。

在这里,你可以阅读如何使用金属玻璃来自定义课堂建筑。

类型实际上是一类 - 创建另一个类的类型. 大多数类型是类型的子类型. 类型接收新类作为其第一个论点,并提供到类对象的访问,如下所述的细节:

>>> class MetaClass(type):
...     def __init__(cls, name, bases, attrs):
...         print ('class name: %s' %name )
...         print ('Defining class %s' %cls)
...         print('Bases %s: ' %bases)
...         print('Attributes')
...         for (name, value) in attrs.items():
...             print ('%s :%r' %(name, value))
... 

>>> class NewClass(object, metaclass=MetaClass):
...    get_choch='dairy'
... 
class name: NewClass
Bases <class 'object'>: 
Defining class <class 'NewClass'>
get_choch :'dairy'
__module__ :'builtins'
__qualname__ :'NewClass'

注:

请注意,课堂在任何时候都没有被暂停;创建课堂的简单行为引发了金属课堂的执行。

甲特克拉斯(甲特克拉斯)是一类,讲述了(某些)其他类应该是如何形成的。

这是一个案例,我看到甲状腺作为解决我的问题:我有一个真正复杂的问题,可能可以是不同的解决,但我选择用甲状腺解决它。 由于复杂性,这是我写的几个模块之一,在模块上的评论超过了编写的代码的数量。

#!/usr/bin/env python

# Copyright (C) 2013-2014 Craig Phillips.  All rights reserved.

# This requires some explaining.  The point of this metaclass excercise is to
# create a static abstract class that is in one way or another, dormant until
# queried.  I experimented with creating a singlton on import, but that did
# not quite behave how I wanted it to.  See now here, we are creating a class
# called GsyncOptions, that on import, will do nothing except state that its
# class creator is GsyncOptionsType.  This means, docopt doesn't parse any
# of the help document, nor does it start processing command line options.
# So importing this module becomes really efficient.  The complicated bit
# comes from requiring the GsyncOptions class to be static.  By that, I mean
# any property on it, may or may not exist, since they are not statically
# defined; so I can't simply just define the class with a whole bunch of
# properties that are @property @staticmethods.
#
# So here's how it works:
#
# Executing 'from libgsync.options import GsyncOptions' does nothing more
# than load up this module, define the Type and the Class and import them
# into the callers namespace.  Simple.
#
# Invoking 'GsyncOptions.debug' for the first time, or any other property
# causes the __metaclass__ __getattr__ method to be called, since the class
# is not instantiated as a class instance yet.  The __getattr__ method on
# the type then initialises the class (GsyncOptions) via the __initialiseClass
# method.  This is the first and only time the class will actually have its
# dictionary statically populated.  The docopt module is invoked to parse the
# usage document and generate command line options from it.  These are then
# paired with their defaults and what's in sys.argv.  After all that, we
# setup some dynamic properties that could not be defined by their name in
# the usage, before everything is then transplanted onto the actual class
# object (or static class GsyncOptions).
#
# Another piece of magic, is to allow command line options to be set in
# in their native form and be translated into argparse style properties.
#
# Finally, the GsyncListOptions class is actually where the options are
# stored.  This only acts as a mechanism for storing options as lists, to
# allow aggregation of duplicate options or options that can be specified
# multiple times.  The __getattr__ call hides this by default, returning the
# last item in a property's list.  However, if the entire list is required,
# calling the 'list()' method on the GsyncOptions class, returns a reference
# to the GsyncListOptions class, which contains all of the same properties
# but as lists and without the duplication of having them as both lists and
# static singlton values.
#
# So this actually means that GsyncOptions is actually a static proxy class...
#
# ...And all this is neatly hidden within a closure for safe keeping.
def GetGsyncOptionsType():
    class GsyncListOptions(object):
        __initialised = False

    class GsyncOptionsType(type):
        def __initialiseClass(cls):
            if GsyncListOptions._GsyncListOptions__initialised: return

            from docopt import docopt
            from libgsync.options import doc
            from libgsync import __version__

            options = docopt(
                doc.__doc__ % __version__,
                version = __version__,
                options_first = True
            )

            paths = options.pop('<path>', None)
            setattr(cls, "destination_path", paths.pop() if paths else None)
            setattr(cls, "source_paths", paths)
            setattr(cls, "options", options)

            for k, v in options.iteritems():
                setattr(cls, k, v)

            GsyncListOptions._GsyncListOptions__initialised = True

        def list(cls):
            return GsyncListOptions

        def __getattr__(cls, name):
            cls.__initialiseClass()
            return getattr(GsyncListOptions, name)[-1]

        def __setattr__(cls, name, value):
            # Substitut option names: --an-option-name for an_option_name
            import re
            name = re.sub(r'^__', "", re.sub(r'-', "_", name))
            listvalue = []

            # Ensure value is converted to a list type for GsyncListOptions
            if isinstance(value, list):
                if value:
                    listvalue = [] + value
                else:
                    listvalue = [ None ]
            else:
                listvalue = [ value ]

            type.__setattr__(GsyncListOptions, name, listvalue)

    # Cleanup this module to prevent tinkering.
    import sys
    module = sys.modules[__name__]
    del module.__dict__['GetGsyncOptionsType']

    return GsyncOptionsType

# Our singlton abstract proxy class.
class GsyncOptions(object):
    __metaclass__ = GetGsyncOptionsType()