How to reference class instance outside method - Python

Question

I'm writing Django app, I want to create different directory for every different Trip.

This code doesn't work (using method):

class TripPhoto(models.Model):
    def __str__(self):
        return self.trip_id.title
    trip_id = models.ForeignKey(Trip, on_delete=models.CASCADE)
    def get_trip_title(self):
        return self.trip_id.title
    photo = models.ImageField(upload_to="reservations/" + get_trip_title() + "/") # NEED FIX

photo = models.ImageField(upload_to="reservations/" + get_trip_title() + "/") # NEED FIX TypeError: get_trip_title() missing 1 required positional argument: 'self'

This code doesn't work too (using attribute):

class TripPhoto(models.Model):
    def __str__(self):
        return self.trip_id.title
    trip_id = models.ForeignKey(Trip, on_delete=models.CASCADE)
    photo = models.ImageField(upload_to="reservations/" + str(trip_id.title) + "/") # NEED FIX

photo = models.ImageField(upload_to="reservations/" + str(trip_id.title) + "/") # NEED FIX AttributeError: 'ForeignKey' object has no attribute 'title'

I know that I can use self.trip_id.title, because I used it in another part of the code.

Is there any way to use self (instance reference?) outside of class method?

the problem is that the upload_to argument is passed (and computed) on class initiation. at that time, there are no instances of TripPhoto at all, and therefore, no self, no trip_id and no trip title. cosider changing your design for not requiring that (or, don't use the image field to store the image) — Mr. Nun.
– Mr. Nun., Commented Mar 7, 2018 at 13:28
@Mr.Nun. Actually neverwalkaloners answer is pretty good. — Qback
– Qback, Commented Mar 7, 2018 at 13:46
It is. apparently, Imagefield is able to use a 'callable' that gets instance and filename which is independent on the TripPhoto instance. however. regarding you question, it is still not possible to refer to the instance 'self' the way you want to. — Mr. Nun.
– Mr. Nun., Commented Mar 7, 2018 at 13:51

neverwalkaloner · Accepted Answer · 2018-03-07 11:38:09Z

3

Declare function outside of the model, instance will be passed to it with first argument:

def get_trip_title(instance, filename):
    # file will be uploaded to MEDIA_ROOT/reservations/title/<filename>
    return 'reservations/{0}/{1}'.format(instance.trip_id.title, filename)

class TripPhoto(models.Model):
    photo = models.FileField(upload_to=get_trip_title)

answered Mar 7, 2018 at 11:38

neverwalkaloner

47.6k7 gold badges101 silver badges103 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Qback Over a year ago

Your answer is not exactly what I was looking for, but it works perfectly for me, so upvote :). But I guess that there is probably no way to call something like self outside method..

Collectives™ on Stack Overflow

How to reference class instance outside method - Python

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related