Python元类的一些应用
最近刚接触python的元类，网络上有比较详细的介绍，这里是在看Django时候发现一点关于元类的应用，做个笔记。
from django.utils import six class A(type): def __new__(cls, name, parents, attrs): return type.__new__(cls, name, parents, attrs) class C(six.with_metaclass(A)): pass
创建C类的时候，他会先调用__metaclass__属性里面的东西。上面C类中的代码可以是
class C(object): __metaclass__ = A
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', (), {})
如果我们需要做一些小动作，可以在这里下手。
那么现在就在A类继承一个B类，在加上几个新的变量。
from django.utils import six class A(type): def __new__(cls, name, parents, attrs): attrs['name'] = 'sora' attrs['sex'] = 'man' name = 'Hello' parents = (B,) return type.__new__(cls, name, parents, attrs) class B(object): a = 123456 class C(six.with_metaclass(A)): pass c = C() print dir(c) print c.name print c.__class__ print c.a
输出结果:
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'a', 'name', 'sex']
sora
<class '__main__.Hello'>
123456
parents是父类，attrs是一个字典，注意的是，attrs不能写成{'name':'sora','sex':',man'}这样是不对的.name是创建的类名字.
这里就是一些简单的一些应用了，在Django里面，经常要用到class Meta这个东西，Django相关的tastypie里面也有，比如Resource类,看了一下源代码学习了下.
#coding:utf-8
from django.utils import six
class B(object):
opt_1 = 1
opt_2 = 2
opt_3 = 3
def __new__(cls, meta=None):
overrides = {}
if meta:
for override_name in dir(meta):
if not override_name.startswith('_'):
overrides[override_name] = getattr(meta,override_name)
return object.__new__(type(b'B',(cls,),overrides))
class A(type):
def __new__(cls, name, bases, attrs):
new_class = super(A,cls).__new__(cls,name,bases,attrs)
# new_class = type.__new__(cls, name, bases, attrs)
ops = getattr(new_class,'Options',None)
new_class._opt = B(ops)
return new_class
class C(six.with_metaclass(A)):
def show_opt(self):
opt = self._opt
print 'opt_1:',opt.opt_1
print 'opt_2:',opt.opt_2
print 'opt_3:',opt.opt_3
class D(C):
class Options:
opt_1 = 5
d = D()
d.show_opt()
输出结果：
opt_1: 5
opt_2: 2
opt_3: 3
Django里面是class Meta，这里换了下名字。