When you are changing your table structure and adding a new column to it. Then to make it nullable false you have to assign a default value to it. But you can not do that by only adding default attribue. You have to use server_default for this purpose.
from sqlalchemy import text
#use server_default if table already created to add default value
flag = db.Column(db.Boolean, nullable=False, server_default=text('1'))