Python重载学习手记

今天学习了一下Python的操作符重载，总结了几点比较神奇的东东：

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关于iter：
Technically, iteration contexts work by calling the iter built-in function to try to
find an _ _iter_ _ method, which is expected to return an iterator object. If it’s

provided,Python then repeatedly calls this iterator object’s next method to produce
items until a StopIteration exception is raised. If no such _ _iter_ _ method is found,
Python falls back on the _ _getitem_ _ scheme, and repeatedly indexes by offsets as
before, until an IndexError exception is raised.
所以为了使用iter，我们必须重载__iter__，然后再定义一个next方法，例子如下：
class Squares:
 def _ _init_ _(self, start, stop): # Save state when created
  self.value = start - 1
  self.stop = stop
 def _ _iter_ _(self): # Get iterator object on iter( )
  return self
 def next(self): # Return a square on each iteration
  if self.value == self.stop:
   raise StopIteration
  self.value += 1
  return self.value ** 2

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利用__setattr__的时候，自己赋值不可以使用self.name = value，因为这个语句也是用了__setattr__

，这样重复使用，出错。要使用self.__dict__['name'] = value

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利用__getattr__建立“私有”成员变量：
利用重载的__setattr__在每次取之前判断一下私有成员名字当中有没有，来实现私有，代码如下（取自

Learning Python）
class PrivateExc(Exception): pass

class Privacy:
    def __setattr__(self,attrname, value):
        if attrname in self.privates:
            raise PrivateExc(attrname, self)
        else:
            self.__dict__[attrname] = value
class Test1(Privacy):
    privates = ['age']

class Test2(Privacy):
    privates = ['name', 'pay']
    def __init__(self):
        self.__dict__['name'] = 'Tom'

x = Test1()
y = Test2()

x.name = 'Bob'
y.name = 'Sue' #This will raise the exception!!!

y.age = 30
x.age = 40 #And so will this!

 

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先找__str__再找__repr__，因此多用__repr__
在交互界面中输入x，调用的是repr(x)，而输入print x，调用的是str(x)

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__getitem__用来下标运算

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