When running locally with gcloud auth providing application default credentials, pubsub is unable to infer a connection, so this will fail:

import pubsub
pubsub.set_default_project('my-project-id')
topic = pubsub.Topic('my-topic')
topic.publish(...)

with EnvironmentError: Connection could not be inferred.

The fix is to set an explicit connection using adc:

credentials = GoogleCredentials.get_application_default()
pubsub.set_default_connection(pubsub.Connection(credentials))

However, storage and datastore do not have this issue. It seems unnecessarily to do this just for pubsub.