#!/usr/bin/env python

import string
import keyword

alphas = string.letters + '_'
nums = string.digits

print 'Welcome to the Identifier Checker v1.0'
print 'Testees must be at least 2 chars long.'
myInput = raw_input('Identifier to test ? ')
        
if len(myInput) > 1:
    if myInput[0] not in alphas:
        print 'invalid: first symbol must be alphabetic'
    else:
        if keyword.iskeyword(myInput):
            print 'invalid: builtin keyword'
        else:           
            for otherChar in myInput[1:]:
                if otherChar not in alphas + nums:
                        print 'invalid: remaining symbols must be alphanumeric'
                        break
            else:
                print 'Okay as an identifier'

for 循环的else语句是一个可选项，它只在for循环完整的结束，没有遇到break时执行。