Reactive charms fail with "ImportError: cannot import name 'defaults'" on trusty/mitaka test scenarios due to charms_openstack.charm import

[Pedro Guimarães]

I am getting errors on trusty-mitaka functional testing for Designate charm.
Both Designate's and Designate-Bind's hook install fails with following traceback:

2018-08-23 12:18:18 ERROR juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms/reactive/__init__.py", line 70, in main
    bus.discover()
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms/reactive/bus.py", line 416, in discover
    _register_handlers_from_file(search_path, filepath)
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms/reactive/bus.py", line 446, in _register_handlers_from_file
    _load_module(root, filepath)
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms/reactive/bus.py", line 434, in _load_module
    return importlib.import_module(package + module)
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/importlib/__init__.py", line 109, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 2254, in _gcd_import
  File "<frozen importlib._bootstrap>", line 2237, in _find_and_load
  File "<frozen importlib._bootstrap>", line 2226, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1200, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1129, in _exec
  File "<frozen importlib._bootstrap>", line 1471, in exec_module
  File "<frozen importlib._bootstrap>", line 321, in _call_with_frames_removed
  File "/var/lib/juju/agents/unit-designate-bind-14/charm/reactive/layer_openstack.py", line 3, in <module>
    import charms_openstack.charm as charm
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms_openstack/charm/__init__.py", line 19, in <module>
    from charms_openstack.charm.defaults import use_defaults
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms_openstack/charm/defaults.py", line 5, in <module>
    from charms_openstack.charm.classes import OpenStackCharm
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms_openstack/charm/classes.py", line 19, in <module>
    from charms_openstack.charm.core import (
  File "/var/lib/juju/agents/unit-designate-bind-14/.venv/lib/python3.4/site-packages/charms_openstack/charm/core.py", line 24, in <module>
    from charms_openstack.charm import defaults as os_defaults
ImportError: cannot import name 'defaults'

Trusty is running Python version: 3.4.3

Both Designate and Designate-Bind are charm.reactive.
Other charms in same functional test pass hook "install": https://pastebin.canonical.com/p/2rzYVz4StY/

[Pedro Guimarães (pguimaraes)]

Could simulate this bug by running tests/gate-basic-trusty-mitaka on designate charm.

Module "defaults" is only called once on core.py: https://github.com/openstack/charms.openstack/blob/master/charms_openstack/charm/core.py#L908

Moving the import inside function "upgrade_if_available" solves this issue and all tests finish with OK

Btw, gate-basic-bionic-queens pass for Designate charm with no issues:
https://review.openstack.org/#/c/593607/

Also tried with xenial version and hook install worked correctly.
Xenial runs Python 3.5.2. Therefore, I guess it is a python 3.4 -> 3.5 issue.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/595880

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/595880
Committed: https://git.openstack.org/cgit/openstack/charms.openstack/commit/?id=194a2d65a9562056614ad16e9aeb7a20597a26f8
Submitter: Zuul
Branch: master

commit 194a2d65a9562056614ad16e9aeb7a20597a26f8
Author: Pedro <email address hidden>
Date: Thu Aug 23 22:04:55 2018 +0200

    Import statement removed on core.py

    from charms_openstack.charm import defaults was removed
    to allow Trusty's python 3.4 to run.
    Two variables that were present on defaults.py were
    migrated to core.py to allow removal.

    Change-Id: I90d3aae867a21417b701e0fd40a75a57d827eb52
    Closes-Bug: #1788610