python中isdigit

line = "12r45ofjo13jr3 3j"
print line[0:3].isdigit()
返回：false

line = "12345ofjo13jr3 3j"
print line[0:3].isdigit()
返回：true

isdigit是返回line[0:3]是否全为数字，若全为数字则返回true，否则返回false