python 隨機生成固定長度的字串

from random import Random

#隨機生成4到20位的用戶名
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

#隨機生成11個字符的手機號碼
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()