title: Using nx option when doing zadd in redis-py (redis ver 2.8.4)
body: <p>I'm using redis 2.8.4 in a python project of mine. In a particular sorted set, I want to <code>zadd</code> with the <code>nx</code> option. The <code>nx</code> option isn't documented in <a href="https://redis-py.readthedocs.io/en/latest/" rel="nofollow noreferrer">redis-py</a> docs, and according <a href="https://redis.io/commands/zadd" rel="nofollow noreferrer">to the docs</a>, it only works with redis 3.0.2 or greater.</p>
  
  <p>So what's an alternative pattern I can follow to simulate the <code>nx</code> option when doing <code>zadd</code>?</p>