在已有數據的表中新增字段時，會彈出下面的步驟讓你來操作
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) #這是現在就輸入一個默認值
2) Quit and manually define a default value in models.py. #這是退出后自己輸入
Select an option:
解決方法1： 在輸入框中選擇 1 會彈出下面的提示
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
然后輸入默認的值："1" 這里的值要記得加引號
解決方法2：在models.py文件中 將新增的字段添加默認值
例子：addr =models.CharField(max_length,default="默認值")