Where do I place functions for setting foreign key values in Django?

Question

I have two functions that I would like to add to my Django project, get_netloc which would run on the the Document.source_url and return a value to input into the set_source function. The set_sorce also take a list of tuples, from the SOURCE_CHOICES in the Source model. This set_source would act like a @property setter in a class, which I have a hunch is what I need to do. I am just not sure where these functions belong and how to implement them correctly. I am even wondering if they need to be in the forms, though the documents can also come from S3 via a lambda function.

Here is the code I have:

from django.db import models
from urllib.parse import urlparse

def get_netloc(url):
    try:
        return urlparse(url).netloc
    except:
        return 'other'

def set_source(netloc, list_tuples):
    for i in list_tuples:
        return netloc if netloc in i else 'other'

class Source(models.Model):
    OTHER = 'other'
    STACK = 'www.stackoverflow.com'
    TWITTER = 'www.twitter.com'
    REDDIT = 'www.reddit.com'
    SOURCE_CHOICES = [
        (OTHER, 'Other'),
        (STACK, 'StackOverflow'),
        (TWITTER, 'Twitter'),
        (REDDIT, 'Reddit'),
    ]
    name = models.CharField('Source Name', max_length=18, choices=SOURCE_CHOICES,
        default=OTHER)

    def __str__(self):
        return self.name

class Document(models.Model):
    name = models.CharField('Name', max_length=200)
    full_text = models.TextField('Text', blank=True, default='')
    source_url = models.URLField(blank=True)
    source = models.ForeignKey(Source, null=True, on_delete=models.PROTECT)