如何在保存之前更改Django表单字段值?

if request.method == 'POST':
    userf = UsersModelForm(request.POST)
    username = userf.data['username']
    password = userf.data['password']
    passwordrepeat = userf.data['passwordrepeat']
    email = userf.data['email']
if request.method == 'POST':
    userf = UsersM