2017.7.10

1. slots可以控制类不能新添加或者删除属性
   class slots:
   slots = {"x", "y"}
   def init(self, x, y):
   self.x = x
   self.y = y

    class notslots:
        def __init__(self, x, y):
            self.x = x
            self.y = y
   
    s1 = slots(10, 20)
    s1.m = 2    #AttributeError: 'slots' object has no attribute 'm'
    s2 = notslots(10, 20)
    s2.m = 2    #right
   

2. modifier
   修饰器函数，可以对方法有限定的作用，修饰器函数只接受一个参数，即function
   def modifier(function):
   def wraper(*args, *kargs):
   result = function(args, **kargs)
   assert result > 0, "result must bigger than zero"
   return result

        wraper.__name__ = function.__name__
        wraper.__doc__ = function.__doc__
   
        return wraper
   
    @modifier
    def test_func(a, b):
    """
    this is a function count a - b
    when a is smaller than b, raise an exception
   
    for example: test_func(3, 1) => 2
    test_func(3, 4) =>  AssertionError
    """
        return a - b
   

3. modifier2 修饰器函数还以有多个参数
   def modifier2(min_num, max_num):
   def out_wraper(function):
   def wraper(*args, *kargs):
   result = function(args, **kargs)

                if result < min_num:
                    return min_num
                elif result > max_num:
                    return max_num
                else:
                    return result
   
            wraper.__name__ = function.__name__
            wraper.__doc__ = function.__doc__
   
            return wraper
   
      return out_wraper
   
    @modifier2(10, 100)
    def test_func2(a, b):
        return (a ** 3) / b - b
   

4. with
   上下文环境
   with open("D://python/x.txt", "r") as fs:
   for line in fs:
   print(line)

    #可同时多个上下文环境
    with open("D://python/in.txt", "r") as fin, open("D://python/out.txt", "wa") as fout:
        for line fin:
            fout.write(line)
   
    #自定义上下文环境，需实现__enter__(self), __exit__(self, exc_type, exc_val, exc_tb)方法
    #在使用with时，__enter__会被调用，
    #当到达with末尾时，__exit__会被调用，如果没有异常发生，exc_type为None
   

5. 函数性程序设计
   #map, filter, reduce(降低)
   lst = [1,2,3,4,5,6,7,8,9]
   map_lst = list(map(lambda x: x**2, lst))
   filter_lst = list(filter(lambda x: x%2==0, lst))

    #均可以用生成器表达式代替
    map_lst2 = [x**2 for x in lst]
    filter_lst = [x for x in lst if x % 2 == 0]
   
    #reduce_lst
    import functools
    functools.reduce(operator.mul, lst)
    functools.reduce(lambda x,y: x*y, lst)