pipeline.hget() returns StrictPipeline, NOT value of key

[pothiers]

I'm unable to use hget on a pipeline to get an actual key value:

red = redis.StrictRedis()
pl = red.pipeline()

ercnt = pl.hget(ecnt,rid)
logging.debug('Got error on action. ercnt="{}"'.format(ercnt))
# => DEBUG Got error on action.      
ercnt="StrictPipeline<ConnectionPool<Connection<host=localhost,port=6379,db=0>>>

If I replace above get with:
errcnt=red.hget(ecnt,rid)
I get a value. But I worry that its an unprotected one.

[andymccurdy]

That's how pipelines work. The HGET doesn't get run until you run pl.execute(), which returns a list of the return values for each command.

[pothiers]

Ok, thanks for the clarification! That makes sense. Does that mean there is no way to conditionaly run commands within a pipeline based upon values in the DB?

I have a TRY/EXCEPT in the pipeline and would like to alter redis values as part of exception cleanup. I want such cleanup to be part of the same transaction.

My simplified flow is essentially:

id = red.brpop(activeq)
pl - red.pipeline()
pl.watch(...)
pl.multi()

try:
    # Do some action
except:
   cnt = hget('error_count',id)
   if cnt > maxerrors:
      pl.lpush(inactiveq, id)
   else:
      pl.lpush(activeq, id)

pl.save()
pl.execute()

[andymccurdy]

You're on the right track with using WATCH. If you create a pipeline and execute the WATCH command, the pipeline is in a special mode where the commands you run will immediately execute and return until you run pl.multi(). So you can do something like this:

pl.watch('foo', 'bar')
# get some values from redis you need in the transaction
foo = pl.get('foo')
bar = pl.get('bar')

# switches to normal pipeline mode where commands get buffered
pl.multi()

# do some writes with the foo and bar values
pl.sadd(foo)
pl.sadd(bar)

# execute the pipeline
pl.execute()

Note that pl.execute() can raise a WatchError exception. Check out the pipeline section of the redis-py README: https://github.com/andymccurdy/redis-py/blob/master/README.rst#pipelines

I believe the second half of that section is exactly what you're looking for, along with some convenience optimizations.

[pothiers]

Perfect! Thanks! I'll read and adjust.

[puncha]

Great! I was really confused that why the conditional checks can be used in Redis pipeline. Thanks for your explain on special mode.