Travis, Mock, different Python versions, and an afternoon of frustration

I have been tinkering for a while with the idea of setting up Travis CI for Subvenv. I mean, automatically running tests on pull requests sounded like a great idea and the getting started guide seemed pretty straightforward, so why not use a bit of my today's free-for-coding time for setting Travis up?

Armed with enthusiasm, I set up the .travis.yml configuration file, granted the necessary GitHub permissions, enabled Travis for Subvenv, pushed, and... build failure!

There's always a first time (for a failure)

Ok, you know the old adage: if it compiles the first time, there must be something wrong. And in a similar fashion, this first failure didn't come at all as a surprise. Not a big deal. Also, the build for Python 3.4 succeeded, which is good.

I checked out the error messages and - of course - it complains because it couldn't find the unittest.mock module in Python 2. Since I'd rather not add dependencies that are not strictly necessary for setting up the development environment, I was quite happy to read that Travis CI automatically installs mock in its testing environments. So, I wrapped my import statement in a try-except block as such:

    from unittest.mock import patch, mock_open
except ImportError:
    from mock import patch, mock_open

To be honest, I am not super glad about an import that relies on a non-mandatory dependency, but let's play along for now and let's get Travis up and running - I can always come up with something better on a later moment.

Ok so, commit my hack, push, and hold my breath for a handful of seconds. Does it work? Nope.

Builtins, __builtin__, and better mocking decisions

Since Subvenv relies on I/O operations for doing its job, I made sure to mock them away when writing my tests. However, since I use Python 3 as my default, I had initially mocked the open function like this:

patch('', m, create=True)

That of course didn't work in Python 2, since there the module is called __builtin__. So, after a hacky and unsuccessful attempt, I ended up in rewriting my mock in a more specific and robust way. Instead of patching the function within the Builtins module, I patched the function call made inside my module:

patch('', m, create=True)

In this way, there's no need to worry about where the function comes from, since all it matters is that it gets called within my module's namespace.

Python 2.6 and Unittest, Python 3.2 and Pip

In the meanwhile, Python 2.6 was throwing its own kind of errors. That wasn't a big problem either, since I had added it to list of interpreters mostly out of curiosity. Nevertheless, I did a check on the errors and investigated its causes a bit and it came out that before 2.7 it wasn't possible to use self.AssertRaises as a context manager (see here and here).

Supporting Python 2.6 has never been on my roadmap and adapting my tests to it didn't seem worth the effort, so I didn't think twice about ditching it from the interpreters list.

Python 3.2 ended up sharing the same fate as Python 2.6, since I discovered that Pip no longer supports it. Thus, due to the impossibility to install the necessary dependencies, I decided to simply follow Pip's example and remove also Python 3.2 from my Travis configuration file.

Which mock?

Ok, so back to my mocks! Now they get imported and patch what they need to patch, but still don't seem to work as expected. Or rather, it looks like there's something off with the mock_open helper, since for some reason it returns a MagicMock instance instead of the specified read_data.

After a bit of head scratching and googling, I found a feature request to Travis that shaded light over my own problem. Thanks to it I got to discover that Travis runs an old version of Mock (I haven't figured out which one exactly, but for sure older than 1.3.0) which does not implement mock_open as later versions do. Thankfully the same issue suggested also a workaround for it, i.e. to add an explicit install command for the desired version to the configuration file:

  - pip install .
  - pip install mock==2.0
  - pip install -r requirements.txt

Mock and Python 3.3

Ok, after the latest changes, I managed to add Python 2.7 to the list of successful builds. However, Python 3.3 shows up still in red and the error message is basically the same one I got when I add the Mock version issue in Python 2.7. That's weird!

Just to be sure, I double checked: yes, Mock is already part of the Unittest module in Python 3.3 and yes, its implementation of the mock_open does take read_data as an argument. However, as the documentation specifies, the readline and readlines method have been added in version 3.4, hence my error.

Ok, let's flip the imports then, so that the external library gets tried first:

    from mock import patch, mock_open
except ImportError:
    from unittest.mock import patch, mock_open

And finally, all green!

All in all

Phew! That was quite an amount of trials and errors - especially considering that I was expecting to be done with it in half an hour or so!

It is a bit sad to think that if I would have coded Subvenv in Python 2.7 I would have spared myself quite some headaches. Also, I am still not super happy to have to rely on an external backport of a built-in module in order to continuously integrate my code.

But hey, the bright side is that I got to learn a lot from it! While there have been moments when the thought of giving up has surfaced my mind, the whole process has been a super interesting travel through how different things are implemented in different versions of Python, as well as how to write code that is compatible with them. Also, code gets now automatically tested when opening a pull request, which is great. So yeah, I guess I can call it a long but profitable afternoon!

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