python 随机生成固定长度的字串

from random import Random
#随机生成4到20位的用户名
def random_username():
    username = ''
    chars = 'AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz0123456789-_'
    length = len(chars) - 1
    random = Random()
    for i in range(4,20):
        username +=chars[random.randint(0, length)]
    print username
    return username
#随机生成11个字符的手机号码
def random_phonenum(numlength=11):
    num =''
    chars='1234567890'
    length=len(chars)-1
    rd=Random()
    for j in range(numlength):
        num +=chars[rd.randint(0,length)]
    print num
    return num
if __name__ == '__main__':
    random_username()
    random_phonenum()