request.user.user_permissions.add("can_write")

from django.contrib.auth.decorators import login_required
from django.contrib.auth.models import Permission
from django.http import Http404, HttpResponseRedirect
@login_required
def add_permission(request, permission):
    redirect_to = request.GET.get('next', request.META.get('HTTP_REFERER', '/'))
    if request.user.has_perm(permission):
        return HttpResponseRedirect(redirect_to)
    try:
        package, codename = permission.split('.')
        p = Permission.objects.get(content_type__app=package, codename=codename)
    except Permission.DoesNotExist:
        raise Http404
    request.user.user_permissions.add(p)
    request.user.save()
    return HttpResponseRedirect(redirect_to)

urlpatterns += ('yourapp.views',
    url(r'^permissions/add/(?P<permission>.*)/', 'add_permission', name='add_permission'),
)

http://path/to/yourapp/permissions/add/auth.add_user/
http://path/to/yourapp/permissions/add/auth.change_user/
http://path/to/yourapp/permissions/add/auth.delete_user/

p = Permission.objects.get(content_type__app=package, codename=codename)

content_type__app=package

package, codename = permission.split('.')

p = Permission.objects.get(content_type__app=package, codename=codename)