正数补前面补0

n = ""

s = n.zfill(5)

assert s == ""

zfill()也可以给负数补0

n = "-123"

s = n.zfill(5)

assert s == "-0123"

对于纯数字，我们也可以通过格式化的方式来补0

n = 123

s = "%05d" % n

assert s == ""