title: Redis transaction with condition
body: <p>I want to do something like the following:</p>
  <p>Step 1: check Redis server for value x, which indicates free space left on that Redis server
  Step 2: if x &gt; some number, do a transaction on Redis server.</p>
  <p>I am wondering if step 1 and 2 can be atomic as well. This is because multiple clients could be issuing these two steps, and if 1 and 2 are not atomic, then presumably two clients can both execute step 1, satisfied with x, and then both do step 2, which I do not want.</p>
  <p>Is there a good way to do this? I am using the Python Redis client.</p>