[Python] ImportError: cannot import name '...' from partially initialized module '...' (most...

I'm upgrading an application from Django 1.11.25 (Python 2.6) to Django 3.1.3 (Python 3.8.5) and, when I run manage.py makemigrations, I receive this message:

File "/home/eduardo/projdevs/upgrade-intra/corporate/models/section.py", line 9, in <module>
from authentication.models import get_sentinel**

ImportError: cannot import name 'get_sentinel' from partially initialized module 'authentication.models' (most likely due to a circular import) (/home/eduardo/projdevs/upgrade-intra/authentication/models.py)**


My models are:

authentication / models.py

from django.conf import settings
from django.contrib.auth.models import AbstractUser, UserManager
from django.db import models
from django.db.models.signals import post_save
from django.utils import timezone

from corporate.constants import GROUP_SUPPORT
from corporate.models import Phone, Room, Section
from library.exceptions import ErrorMessage
from library.model import update_through_dict
from .constants import INTERNAL_USER, EXTERNAL_USER, SENTINEL_USERNAME, SPECIAL_USER, USER_TYPES_DICT


class UserProfile(models.Model):
user = models.OneToOneField(
'User',
on_delete=models.CASCADE,
unique=True,
db_index=True
)
...
phone = models.ForeignKey('corporate.Phone', on_delete=models.SET_NULL, ...)
room = models.ForeignKey('corporate.Room', on_delete=models.SET_NULL, ...)
section = models.ForeignKey('corporate.Section', on_delete=models.SET_NULL, ...)
objects = models.Manager()
...

class CustomUserManager(UserManager):

def __init__(self, type=None):
super(CustomUserManager, self).__init__()
self.type = type

def get_queryset(self):
qs = super(CustomUserManager, self).get_queryset()
if self.type:
qs = qs.filter(type=self.type).order_by('first_name', 'last_name')
return qs

def get_this_types(self, types):
qs = super(CustomUserManager, self).get_queryset()
qs = qs.filter(type__in=types).order_by('first_name', 'last_name')
return qs

def get_all_excluding(self, types):
qs = super(CustomUserManager, self).get_queryset()
qs = qs.filter(~models.Q(type__in=types)).order_by('first_name', 'last_name')
return qs

class User(AbstractUser):
type = models.PositiveIntegerField('...', default=SPECIAL_USER)
username = models.CharField('...', max_length=256, unique=True)
first_name = models.CharField('...', max_length=40, blank=True)
last_name = models.CharField('...', max_length=80, blank=True)
date_joined = models.DateTimeField('...', default=timezone.now)
previous_login = models.DateTimeField('...', default=timezone.now)

objects = CustomUserManager()
...
def get_profile(self):
if self.type == INTERNAL_USER:
...
return None

def get_or_create_profile(self):
profile = self.get_profile()
if not profile and self.type == INTERNAL_USER:
...
return profile

def update(self, changes):
...

class ExternalUserProxy(User):
objects = CustomUserManager(type=EXTERNAL_USER)

class Meta:
proxy = True
verbose_name = '...'
verbose_name_plural = '...'

class InternalUserProxy(User):
objects = CustomUserManager(type=INTERNAL_USER)

class Meta:
proxy = True
verbose_name = '...'
verbose_name_plural = '...'

def create_profile(sender, instance, created, **kwargs):
if created and instance.type == INTERNAL_USER:
try:
profile = UserProfile()
profile.user = instance
profile.save()
except:
pass

post_save.connect(create_profile, sender=User)

def get_sentinel():
try:
sentinel = User.objects.get(username__exact=SENTINEL_USERNAME)
except User.DoesNotExist:
settings.LOGGER.error("...")
from django.contrib.auth.models import Group
sentinel = User()
sentinel.username = SENTINEL_USERNAME
sentinel.first_name = "..."
sentinel.last_name = "..."
sentinel.set_unusable_password()
sentinel.save()
technical = Group.objects.get(name=GROUP_SUPPORT)
sentinel = User.objects.get(username__exact=SENTINEL_USERNAME)
sentinel.groups.add(technical)
sentinel.save()
return sentinel


corporate / models / __init__.py

...
from .section import Section
...


corporate / models / section.py

from django.conf import settings
from authentication.models import get_sentinel
from .room import Room

class Section(models.Model):
...
boss = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.SET(get_sentinel), ...)
surrogate = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.SET(get_sentinel), ...)
room = models.ForeignKey(Room, on_delete=models.SET_NULL, ...)
is_subordinate_to = models.ForeignKey('self', on_delete=models.SET_NULL, ...)
...


What am I doing wrong?

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