重复操作放到循环中效率低

import string
alphas = string.letters
nums = string.digits

myinput = raw_input('>')
for x in myinput:
if x in alphas + nums:
balabala.....

import string
#alphas = string.letters
#nums = string.digits
alphanums = string.letters + string.digits

myinput = raw_input('>')
for x in myinput:
if x in alphanums:
balabala.....