python 面向对象十二 元类

1 class ObjectCreator(object):
2     pass

 1 >>> class ObjectCreator(object):
 2 ...     pass
 3 ... 
 4 >>> print(ObjectCreator)    # 可以打印，因为也是个对象
 5 <class '__console__.ObjectCreator'>
 6 >>> print(type(ObjectCreator))
 7 <class 'type'>
 8 >>> def echo(o):
 9 ...     print(o)
10 ... 
11 >>> echo(ObjectCreator)     # 当做参数
12 <class '__console__.ObjectCreator'>
13 >>> hasattr(ObjectCreator, 'new_attribute') 
14 False
15 >>> ObjectCreator.new_attribute = 'foo'    # 增加属性
16 >>> hasattr(ObjectCreator, 'new_attribute')
17 True
18 >>> ObjectCreator.new_attribute
19 'foo'
20 >>> ObjectCreatorMirror = ObjectCreator    # 赋值给变量
21 >>> ObjectCreatorMirror()
22 <__console__.ObjectCreator object at 0x0000000002CB38D0>

1 >>> MyShinyClass = type('MyShinyClass', (), {})
2 >>> MyShinyClass
3 <class '__console__.MyShinyClass'>
4 >>> MyShinyClass()
5 <__console__.MyShinyClass object at 0x0000000002CB3978>

1 >>> MyShiny = type('MyShinyClass', (), {})
2 >>> MyShiny
3 <class '__console__.MyShinyClass'>
4 >>> MyShinyClass
5 <class '__console__.MyShinyClass'>

>>> Foo = type('Foo', (), {'bar': True})
>>> Foo.bar
True
>>> f = Foo()
>>> f.bar
True

1 >>> FooChild=type('FooChild', (Foo,),{})
2 >>> FooChild
3 <class '__console__.FooChild'>
4 >>> FooChild.bar
5 True

>>> def echo_bar(self):
…       print(self.bar)
…
>>> FooChild = type('FooChild', (Foo,), {'echo_bar': echo_bar})
>>> hasattr(Foo, 'echo_bar')
False
>>> hasattr(FooChild, 'echo_bar')
True
>>> my_foo = FooChild()
>>> my_foo.echo_bar()
True

 1 class UpperAttrMetaclass(type):
 2 
 3     def __new__(cls, name, bases, dct):
 4         print(name, bases, dct)
 5         attrs = ((name, value)
 6                  for name, value in dct.items() if not name.startswith('__'))
 7         uppercase_attr = dict((name.upper(), value) for name, value in attrs)
 8         return super(UpperAttrMetaclass,
 9                      cls).__new__(cls, name, bases, uppercase_attr)
10 
11 
12 class Foo(object, metaclass=UpperAttrMetaclass):
13     bar = 'bip'
14 
15 
16 print(hasattr(Foo, 'bar'))
17 # 输出: False
18 print(hasattr(Foo, 'BAR'))
19 # 输出:True
20 
21 f = Foo()
22 print(f.BAR)
23 # 输出:'bip'

 1 sensitive_words_list = ['asshole', 'fuck', 'shit']
 2 
 3 
 4 def detect_sensitive_words(st):
 5     '''检测敏感词汇'''
 6     words_detected = list(filter(
 7         lambda word: word in st.lower(), sensitive_words_list))
 8     if words_detected:
 9         raise NameError('Sensitive words {0} detected in the string "{1}".'
10                         .format(', '.join(map(lambda s: '"%s"' % s,
11                                               words_detected)), st)
12                         )
13 
14 
15 class CleanerMeta(type):
16 
17     def __new__(cls, class_name, bases, attrs):
18         detect_sensitive_words(class_name)  # 检查类名
19 
20         map(detect_sensitive_words, attrs.keys())  # 检查属性名
21 
22         print("Well done! You are a polite coder!")  # 如无异常，输出祝贺消息
23 
24         return super(CleanerMeta, cls).__new__(cls, class_name, bases, attrs)
25         # 重要！这行一定不能漏！！这回调用内建的类构造器来构造类，否则定义好的类将会变成 None
26 
27 
28 class APIBase(object, metaclass=CleanerMeta):
29 
30     pass
31 
32 
33 a = APIBase()
34 print(a.__class__)
35 print(a.__class__.__class__)
36 print(a.__class__.__class__.__class__)

Well done! You are a polite coder!
<class '__main__.APIBase'>
<class '__main__.CleanerMeta'>
<class 'type'>