Junk Drawer

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Adventures with Python unittest

28 Jul 2018 in Work

Some good, some bad, some ugly, as with all tests.

Today I Learned

Since this is a summary of a week’s worth of learning, I’ve left it ‘til now to write and publish.

  1. About test-running scripts
  2. The difference between tearDown and cleanup
  3. How to use tempfile to create entire mock projects
  4. How to use pdb

There’s quite a bit, so let’s dive in. First, we’ll talk shop, then tests, then tools.

Run, tests, run !

I’ve written a fair few run_tests scripts over the course of my projects, but I’m mentioning this one today because it nicely complements the overall theme of python’s unittest module.

I’ve been using it as my TDD impetus whilst working on cake, a neat build-artifact cache I’m developing at my internship. unittest has it’s quirks, but it gets the job done (mostly) and avoids external dependencies. Normally, they aren’t such a bad thing, but for ease of integration I wanted this code to run without any extra setup. So, python2.7 and unittest it was.

Over the course of writing tests, the need quickly arises for a ‘run tests NOW’ button. In vim, of course, we can take care of this with macros or mappings, but what about in the shell? Entrez: scripts.

I’ll even give you the entirety of it.

#! /usr/bin/env python
import unittest
import sys

import tests

def main():
    args = sys.argv
    verbosity = 1
    if '-v' in args or '--verbose' in args:
        verbosity = 2
    elif '-q' in args or '--quiet' in args:
        verbosity = 0

    suite = tests.suite()

    runner = unittest.TextTestRunner(verbosity=verbosity)

if __name__ == '__main__':

Yep, that simple. Sure, the verbosity checks are a little redundant looking, and could possibly be solved with proper data structures. But this is a little script, and that feature is (a) completely inconsequential and (b) fits neatly across a few lines. I’m not worried about it.

Literally the entire script could fit in 4 lines if I felt like it, minus the argument handling.

The one piece of magic is the line suite = tests.suite(). And I’m going to show you how it works.

# tests/__init__.py
def suite():
    import unittest
    import tests.recipe
    import tests.meta
    import tests.pantry
    import tests.chefs_make
    import tests.oven

    loader = unittest.TestLoader()
    loadmodule = loader.loadTestsFromModule
    suite = unittest.TestSuite()

    return suite

Simply import all the modules you need, and load ‘em in a suite. It’s done in my __init__.py to make it very easy: at the interpreter, one import and unittest can run the suite. In my eventual setup.py, it will be just as easy.

So there, auto-test button.

Clean Up, Clean Up, Everybody Do Your Share

Thought you were never going to hear that again, didn’t you?

cake works with files and directories across several locations. The cache has to live somewhere, there’s a project and build location, things get copied around, the whole works. Not unlike git, which inspired it’s model (more on that later if I get permission to open-source the project–otherwise, tough luck).

So you can imagine, then, that I might be doing some setup and cleanup during my testing. The precise mechanism I’m using is the tempfile module, which does exactly what you think it does. I’ll get to how in a moment, but note that it doesn’t clean up after itself in the case of temporary directories. That burden is on me, which is fine. tempfile will never remove something I’m in the middle of using (even if I accidentally might).

Ok, so, setup and cleanup are a pretty standard part of test suites, right? Well, not exactly. They come from the JUnit-style tests, upon which unittest is based. Other frameworks use things like fixtures and function annotations, but we don’t get the yummy goodness of that here. Hell, I can’t even mock the filesystem easily, so I am forced to use disk operations extensively in testing. Which makes me think about efficiency in my code to avoid the disk. Which is good.

… I’m getting a little off track. Setup. Right. In python’s unittest, you can define setUp and tearDown methods on a TestCase. Setup is run before each test, tear down afterwards.


See, if setUp fails for any reason, tearDown won’t run. You can yell at me about poor tests later, but my setUp method needed to invoked methods on the unit under test in order to pre-populate the cache. I didn’t have to do it that way, but I did, and I’m not arguing about it now since it works.

Anyways, that fails sometimes. So I ended up with temporary directories getting created and not removed; the deletion code was in the tearDown method.

A google search and help doc later, I knew the answer. Turns out, ‘cleanups’ run no matter what. Perfect.

I present the trimmed down solution.

class TestCache(unittest.TestCase):

    def __init__(self, methodname='runTest'):
        super(TestCache, self).__init__(methodname)
        # this is a clean up, since it must run even if setUp fails

    def setUp(self):
        # create a 'project' with files in it
        self.project = tempfile.mkdtemp()
        out_dir = join(self.project, 'output')
        for fname in ['art1', 'art2a', 'art2b', art_tree]:
            fullname = join(out_dir, fname)
            dirname = os.path.dirname(fullname)
            if dirname and not exists(dirname):
            open(fullname, 'a').close()
        # configure the cache
        self.cache_dir = tempfile.mkdtemp()
        self.conf = Configuration(self.cache_dir, out_dir, 'make all', [
                    Target('one', ['src1'], ['art1']),
                    Target('two', ['src2a', 'src2b'], ['art2a', 'art2b']),
                    Target('tree', ['tree'], [art_tree])
        self.cache = cake.pantry.Cache(self.conf)
        # we do this here because we it's common setup between all the tests,
        # even though it also assumes add is working correctly
        self.cache.add(artifact='art1', commit='123')
        self.cache.add(artifact=art_tree, commit='123')
        self.cache.add(artifact='art2b', commit='123')
        self.cache.add(artifact='art1', commit='456', ancestor='123')
        self.cache.add(artifact=art_tree, commit='456', ancestor='123')
        self.cache.add(artifact='art2a', commit='789', ancestor='456')

    def remove_temp(self):
        for tmp in [self.project, self.cache_dir]:
            if tmp:

    def test_things(self):
        # .... blah blah blah


Ok, I was going to write about it, but it’s listed up there in the code and I’m about at 1000 words here, so forgive my laziness.

PDB, or, Debugging Python

import pdb ; pdb.set_trace()

That’s all.

Or, well, not all. But that is the only line you need to place in python code to start the debugger. It has all the usual commands, which you can read about using help (? in the provided debugger REPL). You can also evaluate python code, just be careful; if the debugger thinks it’s a debug command, you’ll need the ! prefix.

Finally, a piece of advice. pdb can be customized with a pdbrc file. Google it. There’s examples everywhere, and some of them are quite helpful. But the one you really need to know is n;;l, which is “execute next command, then list the surrounding code.”

That way you don’t get lost.


Categories: Work

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