Get a subscription from a topic

[jgeewax]

In pubsub-usage.rst we have:

client = pubsub.Client()
topic = client.topic('topic-name')
sub = pubsub.Subscription('sub-name', topic)

I think we should consider changing this to

client = pubsub.Client()
topic = client.topic('topic-name')
sub = topic.subscription('sub-name')

[dhermes]

I am 👍

@tseaver WDYT?

[tseaver]

Given that we advertise client.topic() as the "normal" way to instantiate Topic, this would be more consistent.

[dhermes]

@tseaver I'm trying to incorporate this into #910 and Pylint is not happy about cylic-import. I even move the imports from global down to the methods which use them as

# In topic.py
        # pylint: disable=cyclic-import
        from gcloud.pubsub.subscription import Subscription
        # pylint: enable=cyclic-import
# In subscription.py
        # pylint: disable=cyclic-import
        from gcloud.pubsub.topic import Topic
        # pylint: enable=cyclic-import

and it still complains.

Thoughts?

[dhermes]

Ended up solving by factoring out the behavior in the from_api_repr factories into a shared helper library.