ZADD correctly returns None in certain edge cases when incr=True

When incr=True and xx=True and an element is specified that doesn't exist
the Redis server returns None. redis-py now does this as well.

Fixes #1084

3 files changed, 11 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index a48933c..952cdda 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,4 +1,7 @@
 * 3.0.2 (in development)
+    * ZADD now returns None when xx=True and incr=True and an element
+      is specified that doesn't exist in the sorted set. This matches
+      what the server returns in this case. #1084
     * Added client_kill_filter that accepts various filters to identify
       and kill clients. Thanks Theofanis Despoudis. #1098
     * Fixed a race condition that occurred when unsubscribing and
diff --git a/redis/client.py b/redis/client.py
index 4f08fbd..c0d738c 100755
--- a/redis/client.py
+++ b/redis/client.py
@@ -307,6 +307,8 @@ def bool_ok(response):
 
 
 def parse_zadd(response, **options):
+    if response is None:
+        return None
     if options.get('as_score'):
         return float(response)
     return int(response)
diff --git a/tests/test_commands.py b/tests/test_commands.py
index 144b94c..71c0f18 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -1073,6 +1073,12 @@ class TestRedisCommands(object):
         assert r.zadd('a', {'a1': 1}) == 1
         assert r.zadd('a', {'a1': 4.5}, incr=True) == 5.5
 
+    def test_zadd_incr_with_xx(self, r):
+        # this asks zadd to incr 'a1' only if it exists, but it clearly
+        # doesn't. Redis returns a null value in this case and so should
+        # redis-py
+        assert r.zadd('a', {'a1': 1}, xx=True, incr=True) is None
+
     def test_zcard(self, r):
         r.zadd('a', {'a1': 1, 'a2': 2, 'a3': 3})
         assert r.zcard('a') == 3