Django/venv/lib/python3.7/site-packages/django/contrib/sessions/backends/db.py

import logging

from django.contrib.sessions.backends.base import (
    CreateError, SessionBase, UpdateError,
)
from django.core.exceptions import SuspiciousOperation
from django.db import DatabaseError, IntegrityError, router, transaction
from django.utils import timezone
from django.utils.functional import cached_property


class SessionStore(SessionBase):
    """
    Implement database session store.
    """
    def __init__(self, session_key=None):
        super().__init__(session_key)

    @classmethod
    def get_model_class(cls):
        # Avoids a circular import and allows importing SessionStore when
        # django.contrib.sessions is not in INSTALLED_APPS.
        from django.contrib.sessions.models import Session
        return Session

    @cached_property
    def model(self):
        return self.get_model_class()

    def _get_session_from_db(self):
        try:
            return self.model.objects.get(
                session_key=self.session_key,
                expire_date__gt=timezone.now()
            )
        except (self.model.DoesNotExist, SuspiciousOperation) as e:
            if isinstance(e, SuspiciousOperation):
                logger = logging.getLogger('django.security.%s' % e.__class__.__name__)
                logger.warning(str(e))
            self._session_key = None

    def load(self):
        s = self._get_session_from_db()
        return self.decode(s.session_data) if s else {}

    def exists(self, session_key):
        return self.model.objects.filter(session_key=session_key).exists()

    def create(self):
        while True:
            self._session_key = self._get_new_session_key()
            try:
                # Save immediately to ensure we have a unique entry in the
                # database.
                self.save(must_create=True)
            except CreateError:
                # Key wasn't unique. Try again.
                continue
            self.modified = True
            return

    def create_model_instance(self, data):
        """
        Return a new instance of the session model object, which represents the
        current session state. Intended to be used for saving the session data
        to the database.
        """
        return self.model(
            session_key=self._get_or_create_session_key(),
            session_data=self.encode(data),
            expire_date=self.get_expiry_date(),
        )

    def save(self, must_create=False):
        """
        Save the current session data to the database. If 'must_create' is
        True, raise a database error if the saving operation doesn't create a
        new entry (as opposed to possibly updating an existing entry).
        """
        if self.session_key is None:
            return self.create()
        data = self._get_session(no_load=must_create)
        obj = self.create_model_instance(data)
        using = router.db_for_write(self.model, instance=obj)
        try:
            with transaction.atomic(using=using):
                obj.save(force_insert=must_create, force_update=not must_create, using=using)
        except IntegrityError:
            if must_create:
                raise CreateError
            raise
        except DatabaseError:
            if not must_create:
                raise UpdateError
            raise

    def delete(self, session_key=None):
        if session_key is None:
            if self.session_key is None:
                return
            session_key = self.session_key
        try:
            self.model.objects.get(session_key=session_key).delete()
        except self.model.DoesNotExist:
            pass

    @classmethod
    def clear_expired(cls):
        cls.get_model_class().objects.filter(expire_date__lt=timezone.now()).delete()