python获取指定长度的字符串

from random import Random
def random_str(randomlength=31):
    str = ''
    chars = 'abcdefghijklmnopqrstuvwxyz0123456789'
    length = len(chars) - 1
    random = Random()
    for i in range(randomlength):
        str+=chars[random.randint(0, length)]
    return str

x = random_str()
print(x)