Hi All, Some of you may recall, during a hangout a month or so ago, that I spoke enthusiastically about a testing harness for Python-based layered charms that we were working on in the Big Data team.
Unfortunately, that project ran into some technical blockers (details below my sig, for the curious). As a result, we've moved away from the idea of doing a testing harness, and instead put together a short list of "best practices" for unit testing layered charms. We currently have a PR open against the Juju docs here: https://github.com/juju/docs/pull/1317, and we welcome feedback! If anything needs clarification, if you disagree with anything in particular actually being a "best practice", or you have anything to else to add, please drop a note on that PR. ~ PeteVG The details follow ... The problem to be addressed: we'd like to run unit tests before building a layered charm, as unit tests should execute in seconds (or less), and `charm build` can take multiple seconds to run. Unfortunately, if you run tests before Python libraries from other layers have been added by `charm build`, you may wind up with ImportErrors in your test code. The harness: I wrote a test harness that temporarily patches sys.modules with mock objects in place of the missing libraries. This fixes the ImportErrors, but it creates other problems, some of which proved to be intractable ... The problems with the harness: patching sys.modules leads to a catch-22: if we don't leave the mocks in place, we still get import errors when using the mock library's mock.patch method, but if we do leave them in place, tests that set properties on them can end up interfering with each other. There are workarounds, but they're not intuitive, and they don't generate friendly error messages when they fail. We felt it best to leave the harness behind, and provide some advice on more straightforward things that you can do to work around the import errors. Thus the PR referenced above.
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