i'm using this method as button, but i get error NameError: global name 'project_id' is not defined

here is my method

@api.multi

def add_project_followers(self):

for follower in project_id.message_follower_ids:

if follower not in task_id.message_follower_ids:

task_id.message_follower_ids = [(6, 0, project_id.message_follower_ids.ids)]

It was an example! Please read the documentation how to write functions in Odoo / Python. In your case you try to apply a variable which doesn't exist. It should be sth like:

@api.multi

def add_project_followers(self):

for task in self:

for follower in task.project_id.message_follower_ids:

if follower not in task.message_follower_ids:

task.message_follower_ids = [(4, follower.id)]