Fixed #27318 -- Made cache.set_many() return the list of failed keys.

2 files changed, 12 insertions, 5 deletions

diff --git a/django/core/cache/backends/base.py b/django/core/cache/backends/base.py
index a0ed8275f9..aaf34c042e 100644
--- a/django/core/cache/backends/base.py
+++ b/django/core/cache/backends/base.py
@@ -206,9 +206,13 @@ class BaseCache:
 
         If timeout is given, use that timeout for the key; otherwise use the
         default cache timeout.
+
+        On backends that support it, return a list of keys that failed
+        insertion, or an empty list if all keys were inserted successfully.
         """
         for key, value in data.items():
             self.set(key, value, timeout=timeout, version=version)
+        return []
 
     def delete_many(self, keys, version=None):
         """
diff --git a/django/core/cache/backends/memcached.py b/django/core/cache/backends/memcached.py
index 5af28d7ac3..d49fd148fd 100644
--- a/django/core/cache/backends/memcached.py
+++ b/django/core/cache/backends/memcached.py
@@ -134,11 +134,14 @@ class BaseMemcachedCache(BaseCache):
         return val
 
     def set_many(self, data, timeout=DEFAULT_TIMEOUT, version=None):
-        safe_data = {
-            self.make_key(key, version=version): value
-            for key, value in data.items()
-        }
-        self._cache.set_multi(safe_data, self.get_backend_timeout(timeout))
+        safe_data = {}
+        original_keys = {}
+        for key, value in data.items():
+            safe_key = self.make_key(key, version=version)
+            safe_data[safe_key] = value
+            original_keys[safe_key] = key
+        failed_keys = self._cache.set_multi(safe_data, self.get_backend_timeout(timeout))
+        return [original_keys[k] for k in failed_keys]
 
     def delete_many(self, keys, version=None):
         self._cache.delete_multi(self.make_key(key, version=version) for key in keys)