Overriding of perms_map in subclasses of DjangoModelPermissions

[hoefling]

Following use case: I have a bunch of views that use vanilla DjangoModelPermissions:

class V1(ModelViewSet):
    permission_classes = (DjangoModelPermissions, )
    ...

I want to add another view that should also respect view permissions. My naive approach was to:

1. Subclass DjangoModelPermissions
2. Override perms_map
3. Use the custom permissions class in the view

Stub:

class PermsWithView(permissions.DjangoModelPermissions):
    def __init__(self):
        self.perms_map['GET'] = ['%(app_label)s.view_%(model_name)s']

class V2(ModelViewSet):
    permission_classes = (PermsWithView, )
    ...

The problem here is that since perms_map is a class attibute, the view permissions are applied to all views using DjangoModelPermissions:

In [1]: from rest_framework import permissions

In [2]: from my_stuff import PermsWithView

In [3]: permissions.DjangoModelPermissions.perms_map
Out[3]:
{'GET': [],
 'OPTIONS': [],
 'HEAD': [],
 'POST': ['%(app_label)s.add_%(model_name)s'],
 'PUT': ['%(app_label)s.change_%(model_name)s'],
 'PATCH': ['%(app_label)s.change_%(model_name)s'],
 'DELETE': ['%(app_label)s.delete_%(model_name)s']}

In [4]: my_perm = PermsWithView()

In [5]: permissions.DjangoModelPermissions.perms_map
Out[5]:
{'GET': ['%(app_label)s.view_%(model_name)s'],
 'OPTIONS': [],
 'HEAD': [],
 'POST': ['%(app_label)s.add_%(model_name)s'],
 'PUT': ['%(app_label)s.change_%(model_name)s'],
 'PATCH': ['%(app_label)s.change_%(model_name)s'],
 'DELETE': ['%(app_label)s.delete_%(model_name)s']}

Is this an expected behaviour? Of course, it isn't hard to work around the limitation, e.g. by using

class PermsWithView(permissions.DjangoModelPermissions):
    def __init__(self):
        self._perms_map = super().perms_map.copy()
        self._perms_map['GET'] = ['%(app_label)s.view_%(model_name)s']

    @property
    def perms_map(self):
        return self._perms_map

but I'm curious whether the default behaviour is as designed.

[hoefling]

Hmm, yeah actually just creating a copy is already sufficient:

class PermsWithView(permissions.DjangoModelPermissions):
    def __init__(self):
        self.perms_map = super().perms_map.copy()
        self.perms_map['GET'] = ['%(app_label)s.view_%(model_name)s']

This question looks like just noise to me now, as this is just how Python behaves and not really a DRF issue.