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https://odoo-new-api-guide-line.readthedocs.io/en/latest/decorator.html 

Method and decorator

New decorators are just mapper around the new API. The decorator are mandatory as webclient and HTTP controller are not compliant with new API.

api namespace decorators will detect signature using variable name and decide to match old signature or not.

Recognized variable names are:

cr, cursor, uid, user, user_id, id, ids, context

@api.returns

This decorator guaranties unity of returned value. It will return a RecordSet of specified model based on original returned value:

@api.returns('res.partner')
def afun(self):
    ...
    return x  # a RecordSet

And if an old API function calls a new API function it will automatically convert it into a list of ids

All decorators inherits from this decorator to upgrade or downgrade the returned value.

@api.one

This decorator loops automatically on Records of RecordSet for you. Self is redefined as current record:

@api.one
def afun(self):
    self.name = 'toto'

Note

Caution: the returned value is put in a list. This is not always supported by the web client, e.g. on button action methods. In that case, you should use @api.multi to decorate your method, and probably call self.ensure_one() in the method definition.

@api.multi

Self will be the current RecordSet without iteration. It is the default behavior:

@api.multi
def afun(self):
    len(self)

@api.model

This decorator will convert old API calls to decorated function to new API signature. It allows to be polite when migrating code.

@api.model
def afun(self):
    pass

@api.constrains

This decorator will ensure that decorated function will be called on create, write, unlink operation. If a constraint is met the function should raise a openerp.exceptions.Warning with appropriate message.

@api.depends

This decorator will trigger the call to the decorated function if any of the fields specified in the decorator is altered by ORM or changed in the form:

@api.depends('name', 'an_other_field')
def afun(self):
    pass

Note

when you redefine depends you have to redefine all @api.depends, so it loses some of his interest.

View management

One of the great improvement of the new API is that the depends are automatically inserted into the form for you in a simple way. You do not have to worry about modifying views anymore.

@api.onchange

This decorator will trigger the call to the decorated function if any of the fields specified in the decorator is changed in the form:

@api.onchange('fieldx')
def do_stuff(self):
   if self.fieldx == x:
      self.fieldy = 'toto'

In previous sample self corresponds to the record currently edited on the form. When in on_change context all work is done in the cache. So you can alter RecordSet inside your function without being worried about altering database. That’s the main difference with @api.depends

At function return, differences between the cache and the RecordSet will be returned to the form.

View management

One of the great improvement of the new API is that the onchange are automatically inserted into the form for you in a simple way. You do not have to worry about modifying views anymore.

Warning and Domain

To change domain or send a warning just return the usual dictionary. Be careful not to use @api.one in that case as it will mangle the dictionary (put it in a list, which is not supported by the web client).

@api.noguess

This decorator prevent new API decorators to alter the output of a method

Hi,

The @api.model decorator in Odoo is used when you need to do something with the model itself and don’t need to modify or check some exact model’s record or records. If the method doesn’t need to work with specific records, then it should be decorated with @api.model.

Refer to the blog:

https://www.cybrosys.com/blog/an-overview-of-method-decorators-in-odoo-17

Hope it helps

Hello when using @api.model in your method, odoo will understand that in this method you will not use anything (fields) in 'self'

Ex:

Using @api.model when

@api.model

def _get_blabla(self):

​return {}

Not using @api.model when

def _get_blabla(self):

​return self.name