使用元类的关键技术：
- type
- __new__()
- __init__()
- metaclass
完整示例如下：
#!/usr/bin/env python
# -*- coding: utf-8 -*-
def make_hook(f):
f.is_hook = 1
return f
class MyType(type):
def __new__(mcls, name, bases, attrs):
if name.startswith('None'):
return None
newattrs = {}
for attrname, attrvalue in attrs.iteritems():
print "name: ", attrname
print "value: ", attrvalue
if getattr(attrvalue, 'is_hook', 0):
newattrs['__%s__' % attrname] = attrvalue
else:
newattrs[attrname] = attrvalue
print newattrs
return super(MyType, mcls).__new__(mcls, name, bases, newattrs)
def __init__(self, name, bases, attrs):
super(MyType, self).__init__(name, bases, attrs)
print "Would register class %s now." % self
def __add__(self, other):
class AutoClass(self, other):
pass
return AutoClass
def unregister(self):
print "Would unregister class %s now." % self
# 通过 __metaclass__ 参数指定类对象的元类
class MyObject:
__metaclass__ = MyType
print type(MyObject)
print repr(MyObject)
"""
name: __module__
value: __main__
name: __metaclass__
value: <class '__main__.MyType'>
{'__module__': '__main__', '__metaclass__': <class '__main__.MyType'>}
Would register class <class '__main__.MyObject'> now.
<class '__main__.MyType'>
<class '__main__.MyObject'>
"""
# 测试在元类中直接返回 None
class NoneSample(MyObject):
pass
print type(NoneSample)
print repr(NoneSample)
"""
<type 'NoneType'>
None
"""
# 测试在元类中添加新的属性
class Example(MyObject):
def __init__(self, value):
self.value = value
@make_hook
def add(self, other):
return self.__class__(self.value + other.value)
"""
name: __module__
value: __main__
name: add
value: <function add at 0x7f525fcb0938>
name: __init__
value: <function __init__ at 0x7f525fcb08c0>
{'__module__': '__main__', '__init__': <function __init__ at 0x7f525fcb08c0>, '__add__': <function add at 0x7f525fcb0938>}
Would register class <class '__main__.Example'> now.
"""
Example.unregister() # Would unregister class <class '__main__.Example'> now.
inst = Example(10)
print inst # <__main__.Example object at 0x7f478cc051d0>
# inst.unregister() # AttributeError: 'Example' object has no attribute 'unregister'
print inst + inst # <__main__.Example object at 0x7fb5abbbb250>
# 测试类对象的 __add__ 方法
class Sibling(MyObject):
pass
"""
name: __module__
value: __main__
{'__module__': '__main__'}
Would register class <class '__main__.Sibling'> now.
"""
ExampleSibling = Example + Sibling
print ExampleSibling # <class '__main__.AutoClass'>
print ExampleSibling.__mro__
"""
(<class '__main__.AutoClass'>,
<class '__main__.Example'>,
<class '__main__.Sibling'>,
<class '__main__.MyObject'>,
<type 'object'>)
"""