Python类装饰器实现方法详解

instances = {} # 全局变量，管理实例
def getInstance(aClass, *args):
  if aClass not in instances:
    instances[aClass] = aClass(*args)
  return instances[aClass]   #每一个类只能存在一个实例
def singleton(aClass):
  def onCall(*args):
    return getInstance(aClass,*args)
  return onCall
为了使用它，装饰用来强化单体模型的类：
@singleton    # Person = singleton(Person)
class Person:
  def __init__(self,name,hours,rate):
    self.name = name
    self.hours = hours
    self.rate = rate
  def pay(self):
    return self.hours * self.rate
@singleton    # Spam = singleton(Spam)
class Spam:
  def __init__(self,val):
    self.attr = val
bob = Person('Bob',40,10)
print(bob.name,bob.pay())
sue = Person('Sue',50,20)
print(sue.name,sue.pay())
X = Spam(42)
Y = Spam(99)
print(X.attr,Y.attr)

def singleton(aClass):
 instance = None
 def onCall(*args):
 nonlocal instance
 if instance == None:
  instance = aClass(*args)
 return instance
 return onCall

class singleton:
 def __init__(self,aClass):
 self.aClass = aClass
 self.instance = None
 def __call__(self,*args):
 if self.instance == None:
  self.instance = self.aClass(*args)
 return self.instance

class Wrapper:
 def __init__(self,obj):
 self.wrapped = obj
 def __getattr__(self,attrname):
 print('Trace:',attrname)
 return getattr(self.wrapped,attrname)
>>> x = Wrapper([1,2,3])
>>> x.append(4)
Trace: append
>>> x.wrapped
[1, 2, 3, 4]
>>>
>>> x = Wrapper({'a':1,'b':2})
>>> list(x.keys())
Trace: keys
['b', 'a']

def Tracer(aClass):
  class Wrapper:
    def __init__(self,*args,**kargs):
      self.fetches = 0
      self.wrapped = aClass(*args,**kargs)
    def __getattr__(self,attrname):
      print('Trace:'+attrname)
      self.fetches += 1
      return getattr(self.wrapped,attrname)
  return Wrapper
@Tracer
class Spam:
  def display(self):
    print('Spam!'*8)
@Tracer
class Person:
  def __init__(self,name,hours,rate):
    self.name = name
    self.hours = hours
    self.rate = rate
  def pay(self):
    return self.hours * self.rate
food = Spam()
food.display()
print([food.fetches])
bob = Person('Bob',40,50)
print(bob.name)
print(bob.pay())
print('')
sue = Person('Sue',rate=100,hours = 60)
print(sue.name)
print(sue.pay())
print(bob.name)
print(bob.pay())
print([bob.fetches,sue.fetches])

traceMe = False
def trace(*args):
  if traceMe:
    print('['+ ' '.join(map(str,args))+ ']')
def Private(*privates):
  def onDecorator(aClass):
    class onInstance:
      def __init__(self,*args,**kargs):
        self.wrapped = aClass(*args,**kargs)
      def __getattr__(self,attr):
        trace('get:',attr)
        if attr in privates:
          raise TypeError('private attribute fetch:'+attr)
        else:
          return getattr(self.wrapped,attr)
      def __setattr__(self,attr,value):
        trace('set:',attr,value)
        if attr == 'wrapped': # 这里捕捉对wrapped的赋值
          self.__dict__[attr] = value
        elif attr in privates:
          raise TypeError('private attribute change:'+attr)
        else: # 这里捕捉对wrapped.attr的赋值
          setattr(self.wrapped,attr,value)
    return onInstance
  return onDecorator
if __name__ == '__main__':
  traceMe = True
  @Private('data','size')
  class Doubler:
    def __init__(self,label,start):
      self.label = label
      self.data = start
    def size(self):
      return len(self.data)
    def double(self):
      for i in range(self.size()):
        self.data[i] = self.data[i] * 2
    def display(self):
      print('%s => %s'%(self.label,self.data))
  X = Doubler('X is',[1,2,3])
  Y = Doubler('Y is',[-10,-20,-30])
  print(X.label)
  X.display()
  X.double()
  X.display()
  print(Y.label)
  Y.display()
  Y.double()
  Y.label = 'Spam'
  Y.display()
  # 这些访问都会引发异常
  """
  print(X.size())
  print(X.data)
  X.data = [1,1,1]
  X.size = lambda S:0
  print(Y.data)
  print(Y.size())