在已有数据的表中新增字段时，会弹出下面的步骤让你来操作
Tracking file by folder pattern: migrations
It is impossible to add a non-nullable field 'Publisher' to book without specifying a default. This is because the database needs something to populate existing rows.
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows with a null value for this column) #这是现在就输入一个默认值
2) Quit and manually define a default value in models.py. #这是退出后自己输入
Select an option:
解决方法1： 在输入框中选择 1 会弹出下面的提示
Please enter the default value as valid Python.
The datetime and django.utils.timezone modules are available, so it is possible to provide e.g. timezone.now as a value.
Type 'exit' to exit this prompt
然后输入默认的值："1" 这里的值要记得加引号
解决方法2：在models.py文件中 将新增的字段添加默认值
例子：addr =models.CharField(max_length,default="默认值")