我想要做一个密码的暴力破解，想要输出所有的四位的数字与小写字母组合，请问用Python该怎么写呢？
def get_pwd(str, num):
if(num == 1):
for x in str:
yield x
else:
for x in str:
for y in get_pwd(str, num-1):
yield x+y
strKey="abcdefghijklmnopqrstuvwxyz0123456789"
for x in get_pwd(strKey,4):
print(x)
可以根据给定字典，穷举规定长度所有字符串