理解%r和%s的区别
%r会重现所表达的对象，%s会将所有转成字符串
eg1:
print(\'i am %s years old\' % 22)
print(\'i am %r years old\' % 22)
>>>>>>>>>>>>>>>>>>>>>>>>
i am 22 years old
i am 22 years old
eg2:
t = \'i am %d years old\' % 22
print(\'i said %s\' % t)
print(\'i said %r\' % t)
>>>>>>>>>>>>>>>>>>>>>>>>
i said i am 22 years old
i said \'i am 22 years old\'
eg3:
import datetime
d = datetime.date.today()
print(\'%s\' % d)
print(\'%r\' % d)
>>>>>>>>>>>>>>>>>>>>>>>>
2017-6-26
datetime.date(2017, 6, 26)