Python - how can I use generators more succinctly?

Question

(Python 3)

I am using a Python generator to read messages from a queue.

After the consumer reads a queue message, it needs to be able to tell the generator to delete the queue message if it was successfully processed.

In order to .send() to a Python generator, it seems I must first .send(None) to the generator. This is making my code fatter than I think it should be.

Can anyone suggest a way for qconsumer.py to drive the generator with fewer lines of code? I have identified which lines below I am hoping to eliminate.

In short, how can I make the code below more compact, any suggestions for how I can delete lines?

Code below is qconsumer.py:

from qserver import Qserver

myqserver = Qserver()

myproducer = myqserver.producer() # trying to eliminate this line
# first send to a generator must be None
myproducer.send(None) # trying to eliminate this line
for msg in myproducer:
    # do something with message
    print(msg)
    if messageprocessok:
        myproducer.send('delete')

Code below is qserver.py:

# -*- coding: utf-8 -*-
import boto
from boto.sqs.connection import SQSConnection
from boto.sqs.message import Message

QNAME = 'qinbound'
SQSREGION = 'us-west-1'

class Qserver():
    """A simple Q server."""

    def __init__(self, qname=None, sqsregion=None):
        self.qname = qname or QNAME
        self.sqsregion = sqsregion or SQSREGION
        self.sqsconn = boto.sqs.connect_to_region(self.sqsregion)
        self.q_in = self.sqsconn.get_queue(self.qname)

    def producer(self):
        while True:
            qmessage = self.q_in.read(wait_time_seconds=20)
            if qmessage is None:
                continue
            action = (yield qmessage.get_body())
            if action == 'delete':
                # if processing completed ok, clear message from this queue
                self.q_in.delete_message(qmessage)

Answer 1

Your current consumer is throwing away messages because each send call returns one. You should do this instead:

myqserver = Qserver()
myproducer = myqserver.producer() 
messageprocessok = False
while True:
    msg = myproducer.send('delete' if messageprocessok else None)
    # do something with message
    print(msg)

or alternatively:

myqserver = Qserver()
myproducer = myqserver.producer() 
msg = next(myproducer)
while True:
    # do something with message
    print(msg)
    msg = myproducer.send('delete' if messageprocessok else None)

The fact that you need separate calls to Qserver() and myqserver.producer() is simply because you made prouducer a method of a class. Alternatively you could use a stand-alone function, or make a wrapper function that simply returns Qserver().producer(). Here's the stand-alone version:

def producer(qname=None, sqsregion=None):
    qname = qname or QNAME
    sqsregion = sqsregion or SQSREGION
    sqsconn = boto.sqs.connect_to_region(sqsregion)
    q_in = sqsconn.get_queue(qname)
    while True:
        qmessage = q_in.read(wait_time_seconds=20)
        if qmessage is None:
            continue
        action = (yield qmessage.get_body())
        if action == 'delete':
            # if processing completed ok, clear message from this queue
            q_in.delete_message(qmessage)

Answer 2

Having understood what you're trying to do, I think I would avoid mixing send with iteration. Having the myqserver class be an iterator itself seems to make more sense to me:

# -*- coding: utf-8 -*-
import boto
from boto.sqs.connection import SQSConnection
from boto.sqs.message import Message

QNAME = 'qinbound'
SQSREGION = 'us-west-1'

class Qserver():
    """A simple Q server."""
    _current_message = None

    def __init__(self, qname=None, sqsregion=None):
        self.qname = qname or QNAME
        self.sqsregion = sqsregion or SQSREGION
        self.sqsconn = boto.sqs.connect_to_region(self.sqsregion)
        self.q_in = self.sqsconn.get_queue(self.qname)

    def __iter__(self):
        return self

    def __next__(self):
        while True:
            qmessage = self.q_in.read(wait_time_seconds=20)
            if qmessage is not None:
                self._current_message = qmessage
                return qmessage

    next = __next__

    def delete_current(self):
        if self._current_message is not None:
            self.q_in.delete_message(self._current_message)

And usage will be something like:

from qserver import Qserver

myqserver = Qserver()
for msg in myqserver:
    # do something with message
    print(msg)
    if messageprocessok:
        myqserver.delete_current()