举例讲解Python中metaclass元类的创建与使用

from warnings import warn
#元类需要继承type类
class ReqStrSugRepr(type):
  def __init__(cls, name, bases, attrd):
  #构造函数需要传递的参数为类名，基类，类属性字典
    super(ReqStrSugRepr, cls).__init__(name, bases, attrd)
    # 判断__str__字符串是否在类的属性字典里
    if '__str__' not in attrd:
      raise TypeError('Class requires overriding of __str__()')

    if '__repr__' not in attrd:
      warn('Class suggests overriding of __repr__()\n', stacklevel=3)

class Foo(object):
  #给类指定元类 
  __metaclass__ = ReqStrSugRepr

  def foo(self):
    pass
#这一段代码不用创建类来测试，直接运行一下就会报错，可见元类的功力。

# <type 'int'>
# <type 'str'>
type(1)     
type('mink')

X = type('X', (object,), dict(a=1))
# 等于
class X(object):
  a = 1

def say(self):
  print 'hello'

X = type('X', (object,), dict(say=say))
x = X()

# pirnt hello
x.say()

class PrivateMetaclass(type):
  def __new__(cls, name, parents, attrs):
    attrs = dict(('__%s' % k, v) for k, v in attrs.itmes())
    return super(PrivateMetaclass, cls).__new__(cls, name, parents, attrs)

class A(object):
  __metaclass__ = PrivateMetaclass
  a = 1
  b = 2

a = A()
# raise AttributeError
print a.a, a.b 

# print 1, 2
print a.__a, a.__b