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.....

每次循环都会把alphas和nums相加，效率低，正确做法应该提前把二者相加的值付给一个变量，对这个变量进行迭代。正确做法如下。

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.....