Django DRY context managers

gistfile1.py

"""
You find yourself doing this all the time?

    >>> car = Car.objects.get(id=1)
    >>> if c.user != request.user:
    >>>     return HttpResponse("This car doesn't belong to you")
    >>> c.brand = request.POST.get('brand_name')
    >>> c.save()

Better to turn it into:

    >>> try:
    >>>     with ensure_instance_owner(car, user) as c:
    >>>         c.brand = request.POST.get('brand_name')
    >>> except NotInstanceOwner:
    >>>     return HttpResponse("This car doesn't belong to you")

"""
class NotInstanceOwner(Exception):
    pass


class EnsureModelInstanceOwner(object):
    def __init__(self, instance, user, owner_field='user'):
        self.instance = instance
        self.owner_field = owner_field
        self.user = user

    def __enter__(self):
        owner = getattr(self.instance, self.owner_field)
        if owner != self.user:
            raise NotInstanceOwner()
        return self.instance

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None:
            self.instance.save()

ensure_instance_owner = EnsureModelInstanceOwner

"""
Now, I  don't like that Car.objects.get, let's make it better:

    >>> try:
    >>>     with get_and_ensure_owner(Car, 1, user) as c:
    >>>         c.brand = request.POST.get('brand_name')
    >>> except Car.DoesNotExist:
    >>>     return HttpResponseNotFound()
    >>> except NotInstanceOwner:
    >>>     return HttpResponse("This car doesn't belong to you")

"""

class GetAndEnsureOwner(object):
    def __init__(self, model_class, pk, user, owner_field='user'):
        self.model_class = model_class
        self.pk = pk
        self.owner_field = owner_field
        self.user = user

    def __enter__(self):
        self.instance = self.model_class.objects.get(pk=self.pk)
        owner = getattr(self.instance, self.owner_field)
        if owner != self.user:
            raise NotInstanceOwnerException()
        return self.instance

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None:
            self.instance.save()

get_and_ensure_owner = GetAndEnsureOwner

# License: DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE

de verdad te parece que es util esto? no es poco DRY tener que darle el el nombre del field que queremos chequear (si no es 'user') ? y si es DRY no te parece poco KISS ? no es sobreingenieria?

Might be over-engineered. Don't know, for me it's useful. See the update (the second context manager). I'm just trying to share and have fun!

Armar context managers para python es una divertido y groso, pero ¿Qué te evita usar los clásicos shortcuts? Esto para mí es más straightforward y djanguero:

from django.shortcuts import get_object_or_404
car = get_object_or_404(Car, pk=1, user=request.user)

+1 a los shortcuts, obvio. El ejemplo completo sería:

from django.shortcuts import get_object_or_404
car = get_object_or_404(Car, pk=1, user=request.user)
if car.user != request.user:
     return HttpResponse("This car doesn't belong to you")
car.brand = request.POST.get('brand_name')
car.save()