Python3设计一个函数产生指定长度的验证码，验证码由大小写字母和数字构成

实例代码如下：

import random
def generate_code(code_len):
    all_chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
    last_pos = len(all_chars) - 1
    code = ''
    for _ in range(code_len):
    	index = random.randint(0, last_pos)
    	code += all_chars[index]
    return code

if __name__=='__main__':
	generate_code(5)