Add convenience method to add `Permission`s to a `User`

[Flavio Curella]

When writing tests checking for permissions, it's commonplace to test if a user can access a particular view, check for failure, add the necessary permission, then check again for success:

def mytest(self):
    user = User.objects.create_user(
            username='regularuser',
            password='regularuser',
    )
    response = self.client.get('/mymodel/create/')
    # Usually redirects to some other page, such as `/login/`
    self.assertNotEqual(response.status_code, 200)

    # then we fetch the 'myapp.add_mymodel' permission and add it to `user`
    my_permission = Permission.objects.get_by_natural_key('add_mymodel', 'myapp', 'mymodel')
    user.user_permissions.add(mypermission)

    response = self.client.get('/mymodel/create/')
    # now it works
    self.assertEqual(response.status_code, 200)

I'm wondering if we should have a more convenient way to add permissions. I'm thinking of adding a add_perm method to the User model, that mirrors the already existing ​`has_perm` so that instead of

    my_permission = Permission.objects.get_by_natural_key('add_mymodel', 'myapp', 'mymodel')
    user.user_permissions.add(mypermission)

    response = self.client.get('/mymodel/create/')
    self.assertEqual(response.status_code, 200)

we can have:

    user.add_perm('myapp.add_mymodel')
    response = self.client.get('/mymodel/create/')
    self.assertEqual(response.status_code, 200)

[Tim Graham]

There's the problem described in #25281 that "<app label>.<permission codename>" might not uniquely identify permissions. Anyway, a proposal like this should be made on the DevelopersMailingList to see if there's consensus, however, the existence of ​a helper in Django's test suite to help ease the verbosity here is a good indication that something could be done. I'm bit confused because the ticket's summary talks about adding a manager method to Permission but the description proposes adding a user method. In the latter case, a consideration of custom user models might be needed.

[Flavio Curella]

Thank Tim,

I will work on a proposal for #25281. Once that's resolve, I can move on to this one.