Refactor precondition check into the cache lookup backends

rest_framework/cache_lookups.py

@@ -2,6 +2,7 @@
 Provides a set of pluggable cache policies.
 """
 from django.core.cache import cache
+from rest_framework.exceptions import PreconditionFailed
 
 
 class BaseCacheLookup(object):
@@ -37,6 +38,10 @@ class ETagCacheLookup(BaseCacheLookup):
         cache.set(key, etag)
         return {'ETag': etag}
 
+    def precondition_check(self, obj, request):
+        if self.get_etag(obj) != request.META.get('HTTP_IF_NONE_MATCH'):
+            raise PreconditionFailed
+
     def resource_unchanged(self, request, key):
         etag = cache.get(key)
         header = request.META.get('HTTP_IF_NONE_MATCH')

rest_framework/views.py

@@ -323,10 +323,8 @@ class APIView(View):
             raise exceptions.IfMatchMissing
 
     def cache_precondition_check(self, obj, request):
-        header = request.META.get('HTTP_IF_MATCH')
         for cache_lookup in self.get_cache_lookups():
-            if cache_lookup.get_etag(obj) != header:
+            cache_lookup.precondition_check(obj, request)
-                raise exceptions.PreconditionFailed
 
     # Dispatch methods