Encode results according to state in one common place, also fixes some backends not preparing exceptions on retry.

celery/backends/amqp.py

@@ -56,11 +56,7 @@ class Backend(BaseBackend):
 
     def store_result(self, task_id, result, status, traceback=None):
         """Send task return value and status."""
-        if status == "DONE":
-            result = self.prepare_result(result)
-        elif status == "FAILURE":
-            result = self.prepare_exception(result)
-
+        result = self.encode_result(result, status)
 
         meta = {"task_id": task_id,
                 "result": result,

celery/backends/base.py

@@ -5,6 +5,8 @@ from celery.serialization import get_pickled_exception
 from celery.serialization import get_pickleable_exception
 from celery.exceptions import TimeoutError
 
+EXCEPTION_STATES = frozenset(["RETRY", "FAILURE"])
+
 
 class BaseBackend(object):
     """The base backend class. All backends should inherit from this."""
@@ -12,6 +14,12 @@ class BaseBackend(object):
     capabilities = []
     TimeoutError = TimeoutError
 
+    def encode_result(self, result, status):
+        if status == "DONE":
+            return self.prepare_value(result)
+        elif status in EXCEPTION_STATES:
+            return self.prepare_exception(result)
+
     def store_result(self, task_id, result, status):
         """Store the result and status of a task."""
         raise NotImplementedError(
@@ -45,10 +53,8 @@ class BaseBackend(object):
         raise NotImplementedError(
                 "get_status is not supported by this backend.")
 
-    def prepare_result(self, result):
-        """Prepare result for storage."""
-        if result is None:
-            return True
+    def prepare_value(self, result):
+        """Prepare value for storage."""
         return result
 
     def get_result(self, task_id):
@@ -126,10 +132,7 @@ class KeyValueStoreBackend(BaseBackend):
 
     def store_result(self, task_id, result, status, traceback=None):
         """Store task result and status."""
-        if status == "DONE":
-            result = self.prepare_result(result)
-        elif status == "FAILURE":
-            result = self.prepare_exception(result)
+        result = self.encode_result(result, status)
         meta = {"status": status, "result": result, "traceback": traceback}
         self.set(self.get_cache_key_for_task(task_id), pickle.dumps(meta))
         return result

celery/backends/database.py

@@ -32,10 +32,7 @@ class Backend(BaseBackend):
 
     def store_result(self, task_id, result, status, traceback=None):
         """Store return value and status of an executed task."""
-        if status == "DONE":
-            result = self.prepare_result(result)
-        elif status in ["FAILURE", "RETRY"]:
-            result = self.prepare_exception(result)
+        result = self.encode_result(result, status)
         TaskMeta.objects.store_result(task_id, result, status,
                                       traceback=traceback)
         return result

celery/backends/mongodb.py

@@ -185,10 +185,7 @@ class Backend(BaseBackend):
         """Store return value and status of an executed task."""
         from pymongo.binary import Binary
 
-        if status == 'DONE':
-            result = self.prepare_result(result)
-        elif status == 'FAILURE':
-            result = self.prepare_exception(result)
+        result = self.encode_result(result, status)
 
         meta = {"_id": task_id,
                 "status": status,