In python, how do I make a class a generator?

Question

I'm working with the varnish API in varnish 4. The way it produces log entries is by Dispatch'ing against it, and passing in a callback. Simple version:

class vlog(object):
    def __init__(self):
        self.body = []
    def callback(self, vap, cbd, priv):
        self.body.append(cbd)
    def main(self):
        self.vlog = varnishapi.VarnishLog(sopath="libvarnishapi.so")
        while True:
            ret = self.vlog.Dispatch(self.callback)
            if ret == 0:
                time.sleep(0.1)

I need to do several more things in self.callback to aggregate the data properly, but whatever.

My REAL question is this: How can I turn the above class into a generator? My ideal usage would be something like this:

    vlog_inst = vlog()
    for log_aggregate in vlog_inst:
        pass

Simply putting a 'yield' statement in the callback function never triggers iteration. (surprising to me, adding the yield statement also causes all of my print statements to produce no output as well... I'm obviously missing something.)

Define `__iter__` and `next`(`__next__` in Python 3). There has to be a dup for this. — Ashwini Chaudhary, Aug 05 '15 at 14:28
Since this is a datasource leveraging a callback, I can't 'restart' iteration with a next() function... the callback has nothing I can give a 'return' data too... and next would force me to 'reconnect' to the log-stream, which would mean I loose data. Also, since the datasource is effectively infinite, generators are a better approach. — Jason, Aug 05 '15 at 14:51

score 2 · Answer 1 · answered Aug 05 '15 at 14:30

You want your class to be iterable. A generator is a way to implement an iterable, but that's not required to make a class iterable.

You could implement an __iter__ method; if you make it a generator function you don't need anything else:

def __iter__(self):
    while True:
        yield self.vlog.Dispatch(self.callback)

In python, how do I make a class a generator?

1 Answers1