permissions: Allow permissions to be composed (#5753)

* permissions: Allow permissions to be composed

Implement a system to compose permissions with and / or.
This is performed by returning an `OperationHolder` instance that keeps the
permission classes and type of composition (and / or).
When called it will return a AND/OR instance that will then delegate the
permission check to the operands.

* permissions: Add documentation about composed permissions

* Fix documentation typo in permissions

docs/api-guide/permissions.md

@@ -102,6 +102,27 @@ Or, if you're using the `@api_view` decorator with function based views.
 
 __Note:__ when you set new permission classes through class attribute or decorators you're telling the view to ignore the default list set over the __settings.py__ file.
 
+Provided they inherit from `rest_framework.permissions.BasePermission`, permissions can be composed using standard Python bitwise operators. For example, `IsAuthenticatedOrReadOnly` could be written:
+
+    from rest_framework.permissions import BasePermission, IsAuthenticated
+    from rest_framework.response import Response
+    from rest_framework.views import APIView
+
+    class ReadOnly(BasePermission):
+        def has_permission(self, request, view):
+            return request.method in SAFE_METHODS
+
+    class ExampleView(APIView):
+        permission_classes = (IsAuthenticated|ReadOnly)
+
+        def get(self, request, format=None):
+            content = {
+                'status': 'request was permitted'
+            }
+            return Response(content)
+
+__Note:__ it only supports & -and- and | -or-.
+
 ---
 
 # API Reference

rest_framework/permissions.py

@@ -4,12 +4,76 @@ Provides a set of pluggable permission policies.
 from __future__ import unicode_literals
 
 from django.http import Http404
+from django.utils import six
 
 from rest_framework import exceptions
 
 SAFE_METHODS = ('GET', 'HEAD', 'OPTIONS')
 
 
+class OperandHolder:
+    def __init__(self, operator_class, op1_class, op2_class):
+        self.operator_class = operator_class
+        self.op1_class = op1_class
+        self.op2_class = op2_class
+
+    def __call__(self, *args, **kwargs):
+        op1 = self.op1_class(*args, **kwargs)
+        op2 = self.op2_class(*args, **kwargs)
+        return self.operator_class(op1, op2)
+
+
+class AND:
+    def __init__(self, op1, op2):
+        self.op1 = op1
+        self.op2 = op2
+
+    def has_permission(self, request, view):
+        return (
+            self.op1.has_permission(request, view) &
+            self.op2.has_permission(request, view)
+        )
+
+    def has_object_permission(self, request, view, obj):
+        return (
+            self.op1.has_object_permission(request, view, obj) &
+            self.op2.has_object_permission(request, view, obj)
+        )
+
+
+class OR:
+    def __init__(self, op1, op2):
+        self.op1 = op1
+        self.op2 = op2
+
+    def has_permission(self, request, view):
+        return (
+            self.op1.has_permission(request, view) |
+            self.op2.has_permission(request, view)
+        )
+
+    def has_object_permission(self, request, view, obj):
+        return (
+            self.op1.has_object_permission(request, view, obj) |
+            self.op2.has_object_permission(request, view, obj)
+        )
+
+
+class BasePermissionMetaclass(type):
+    def __and__(cls, other):
+        return OperandHolder(AND, cls, other)
+
+    def __or__(cls, other):
+        return OperandHolder(OR, cls, other)
+
+    def __rand__(cls, other):
+        return OperandHolder(AND, other, cls)
+
+    def __ror__(cls, other):
+        return OperandHolder(OR, other, cls)
+
+
+@six.add_metaclass(BasePermissionMetaclass)
 class BasePermission(object):
     """
     A base class from which all permission classes should inherit.

tests/test_permissions.py

@@ -540,3 +540,45 @@ class CustomPermissionsTests(TestCase):
             detail = response.data.get('detail')
             self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
             self.assertEqual(detail, self.custom_message)
+
+
+class FakeUser:
+    def __init__(self, auth=False):
+        self.is_authenticated = auth
+
+
+class PermissionsCompositionTests(TestCase):
+    def test_and_false(self):
+        request = factory.get('/1', format='json')
+        request.user = FakeUser(auth=False)
+        composed_perm = permissions.IsAuthenticated & permissions.AllowAny
+        assert composed_perm().has_permission(request, None) is False
+
+    def test_and_true(self):
+        request = factory.get('/1', format='json')
+        request.user = FakeUser(auth=True)
+        composed_perm = permissions.IsAuthenticated & permissions.AllowAny
+        assert composed_perm().has_permission(request, None) is True
+
+    def test_or_false(self):
+        request = factory.get('/1', format='json')
+        request.user = FakeUser(auth=False)
+        composed_perm = permissions.IsAuthenticated | permissions.AllowAny
+        assert composed_perm().has_permission(request, None) is True
+
+    def test_or_true(self):
+        request = factory.get('/1', format='json')
+        request.user = FakeUser(auth=True)
+        composed_perm = permissions.IsAuthenticated | permissions.AllowAny
+        assert composed_perm().has_permission(request, None) is True
+
+    def test_several_levels(self):
+        request = factory.get('/1', format='json')
+        request.user = FakeUser(auth=True)
+        composed_perm = (
+            permissions.IsAuthenticated &
+            permissions.IsAuthenticated &
+            permissions.IsAuthenticated &
+            permissions.IsAuthenticated
+        )
+        assert composed_perm().has_permission(request, None) is True