使用slot能降低内存使用，同时也不能对实例新增属性。
For classes that primarily serve as simple data structures, you can often greatly reduce the memory footprint of instances by adding the __slots__ attribute to the class defi‐ nition. For example:
class Date:
__slots__ = ['year', 'month', 'day']
def __init__(self, year, month, day):
self.year = year
self.month = month
self.day = day
When you define __slots__, Python uses a much more compact internal representation for instances. Instead of each instance consisting of a dictionary, instances are built around a small fixed-sized array, much like a tuple or list. Attribute names listed in the __slots__ specifier are internally mapped to specific indices within this array. A side effect of using slots is that it is no longer possible to add new attributes to instances— you are restricted to only those attribute names listed in the __slots__ specifier