import random


def generate_code(code_len=4):
    """
    生成指定长度验证码
    :param code_len: 验证码的长度(默认4个字符)
    :return: 由大小写英文字母以及数字构成的随机验证码
    """
    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


random_code = generate_code()
print(random_code)