http://docs.python.org/library/operator.html#operator.attrgetter
from operator import attrgetter
min_num = min(li,key=attrgetter('number'))
互动会话示例：
>>> li = [SpecialNumber(1), SpecialNumber(3), SpecialNumber(2)]
>>> [i.number for i in li]
[1, 3, 2]
>>> min_num = min(li,key=attrgetter('number'))
>>> print min_num.number
1
602392714
清零编程群