GSoC ’17 Post #2: Enhancing the coala testing API

About the bear testing framework

coala has a wonderful testing API that can be used to write tests for bears with ease. Software testing is a process of executing a program or application with the intent of finding software bugs, preventing them from entering your codebase, and making sure that everything works as expected.

coala supports 3 documented methods for testing bears, namely :

  • check_validity(...)

    check_validity asserts if your bear yields any results for a particular check with a list of strings.

  • check_results(...)

    check_results asserts if your bear results match the actual results on execution on the command line interface.

  • verify_local_bears(...)

    verify_local_bear asserts that a check of the given lines with the given local bear either yields or does not yield any results.

The tests should have 100% coverage and zero redundancy and it is advised to use check_results since developers can verify the bear output and any upstream changes can easily be caught via the given method.

The Problem


The current testing API doesn’t provide useful results as a traceback if a given test fails. It becomes a strenuous and a time consuming job to follow the traceback searching for errors.

Eg. For the check_results(...) method, I received a strange output when I had a minor difference of line=5 instead of line=6 in the given code snippet :


The given output below appear to be strange as both the Result objects contain the same number of characters (632) due to which testing transforms to a challenging task. Hit and trial methods or the usage of a debugger are the only possible ways to overcome these bugs.

E           AssertionError: Lists differ: [<Result object(id=0xce0383d576a84f34848d66d0ddc87479, origin=[632 chars]710>] != [<Result object(id=0x7e58527926a345f8a2bb36305c58f9f6, origin=[632 chars]400>]
E           First differing element 0:
E           <Result object(id=0xce0383d576a84f34848d66d0ddc87479, origin=[631 chars]0710>
E           <Result object(id=0x7e58527926a345f8a2bb36305c58f9f6, origin=[631 chars]0400>
E           Diff is 2804 characters long. Set self.maxDiff to None to see it. : The local bear 'ElmLintBear' doesn't yield the right results. Or the order may be wrong.
E           Running bear ElmLintBear...
E           Running 'elm-format /tmp/tmpzu0n4474 --yes'


The ugly message is generated due to the check_results(...) method of the LocalBearTestHelper class. The following code snippet is responsible for the origin of the bug :

if not check_order:
   self.assertEqual(sorted(bear_output), sorted(results), msg=msg)
   self.assertEqual(bear_output, results, msg=msg)

The expected (ie. bear_output) and observed (ie. results) Result objects are compared by the assertEqual(...) method, which on finding an inequality yields a bewildering message on the console.

Proposed Solution

The idea is to compare all fields of the Result object, one at a time, and complain accordingly for dissimilarity of values. Once assertEqual() fails, the attribute mismatch can be precisely and explicitly mentioned. The following can be achieved by annotating every class that uses @generate_eq(...) or @generate_ordering(...) with a special field (eg. __compare_fields__) which contains the fields of the Result object that we use for comparison.

cls.__comparable_fields__ = tuple(member for member in members)

We then write a generic assertion function detecting this field and apply the principle of comparing them in a dynamic manner with less code duplication.

def assertFieldsEqual(self, object1, object2):
   attributes = Result.__comparable_fields__
   for attribute in attributes:
       self.assertEqual(getattr(object1[0], attribute), getattr(object2[0], attribute), msg='{} mismatch'.format(attribute))

After registering this function as an assertEqual function, we call the above function implicitly.

The sample output in case of an origin mismatch would look like :

E   AssertionError: 'StylintBear' != 'StylintBea'
E   - StylintBear
E   ?           -
E   + StylintBea
E    : origin mismatch

The above message is field specific and quite helpful and will prevent bear writers from scratching their heads and applying trial and error techniques to get their tests passed.

Another sample output containing a message_base mismatch would look like :

E   AssertionError: 'prefer alphabetical when sorting properties  sortOrder' != 'prefer alphabetical when sorting properties'
E   - prefer alphabetical when sorting properties  sortOrder
E   ?                                            -----------
E   + prefer alphabetical when sorting properties
E    : message_base mismatch


The coding phase has begun, work is going on in full swing and I’m thankful my mentor @Makman2 for dropping in subtle hints whenever I seem to get stuck. We’re enjoying so far and lets see what the first coding phase brings for us. I’ll keep things updated on my blog so stick around.

” Invariably, you’ll find that if the language is any good, your users are going to take it to places where you never thought it would be taken. “ – Guido Van Rossum, creator of Python