WritableTestCase: Example Use

Posted on Sun 18 September 2016 in TDDA

In my PyCon UK talk yesterday I promised to update the and document the copy of writabletestcase.WritableTestCase on GitHub.

The version I've put up is not quite as powerful as the example I showed in the talk—that will follow—but has the basic functionality.

I've now added examples to the repository and, below, show how these work.

The library is available with

git clone https://github.com/tdda/tdda.git

WritableTestCase extends unittest.TestCase, from the Python's standard library, in three main ways:

It provides methods for testing strings produced in memory or files written to disk against reference results in files. When a test fails, rather than just showing a hard-to-read difference, it writes the actual result to file (if necessary) and then shows the diff command needed to compare it—something like this:
```
Compare with "diff /path/to/actual-output /path/to/expected-output"
```
Obviously, the diff command can be replaced with a graphical diff tool, an open command or whatever.

Although this shouldn't be necessary (see below), you also have the option, after verification, or replacing diff with cp to copy the actual output as the new reference output.
Secondly, the code supports excluding lines of the output contain nominated strings. This is often handy for excluding things like date stamps, version numbers, copyright notices etc. These often appear in output, and vary, without affecting the semantics.

(The version of the library I showed at PyCon had more powerful variants of this, which I'll add later.)
Thirdly, if you verify that the new output is correct, the library supports re-running with the -w flag to overwrite the expected ("reference") results with the ones generated by the code.

Obviously, if this feature is abused, the value of the tests will be lost, but provided you check the output carefully before re-writing, this is a significant convenience.

The example code is in the examples subdirectory, called test_using_writabletestcase.py. It has two test functions, one of which generates HTML output as a string, and the other of which produces some slightly different HTML output as a file. In each case, the output produced by the function is not identical to the expected "reference" output (in examples/reference), but differs only on lines containing "Copyright" and "Version". Since these are passed into the test as exclusions, the tests should pass.

Here is the example code:

# -*- coding: utf-8 -*-
"""
test_using_writabletestcase.py: A simple example of how to use
tdda.writabletestcase.WritableTestCase.

Source repository: https://github.com/tdda/tdda

License: MIT

Copyright (c) Stochastic Solutions Limited 2016
"""
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

import os
import tempfile

from tdda import writabletestcase
from tdda.examples.generators import generate_string, generate_file


class TestExample(writabletestcase.WritableTestCase):
    def testExampleStringGeneration(self):
        """
        This test uses generate_string() from tdda.examples.generators
        to generate some HTML as a string.

        It is similar to the reference HTML in
        tdda/examples/reference/string_result.html except that the
        Copyright and version lines are slightly different.

        As shipped, the test should pass, because the ignore_patterns
        tell it to ignore those lines.

        Make a change to the generation code in the generate_string
        function in generators.py to change the HTML output.

        The test should then fail and suggest a diff command to run
        to see the difference.

        Rerun with

            python test_using_writabletestcase.py -w

        and it should re-write the reference output to match your
        modified results.
        """
        actual = generate_string()
        this_dir = os.path.abspath(os.path.dirname(__file__))
        expected_file = os.path.join(this_dir, 'reference',
                                     'string_result.html')
        self.check_string_against_file(actual, expected_file,
                                       ignore_patterns=['Copyright',
                                                        'Version'])


    def testExampleFileGeneration(self):
        """
        This test uses generate_file() from tdda.examples.generators
        to generate some HTML as a file.

        It is similar to the reference HTML in
        tdda/examples/reference/file_result.html except that the
        Copyright and version lines are slightly different.

        As shipped, the test should pass, because the ignore_patterns
        tell it to ignore those lines.

        Make a change to the generation code in the generate_file function
        in generators.py to change the HTML output.

        The test should then fail and suggest a diff command to run
        to see the difference.

        Rerun with

            python test_using_writabletestcase.py -w

        and it should re-write the reference output to match your
        modified results.
        """
        outdir = tempfile.gettempdir()
        outpath = os.path.join(outdir, 'file_result.html')
        generate_file(outpath)
        this_dir = os.path.abspath(os.path.dirname(__file__))
        expected_file = os.path.join(this_dir, 'reference',
                                     'file_result.html')
        self.check_file(outpath, expected_file,
                        ignore_patterns=['Copyright', 'Version'])


if __name__ == '__main__':
    writabletestcase.main(argv=writabletestcase.set_write_from_argv())

If you download it, and try running it, you should output similar to the following:

$ python test_using_writabletestcase.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.004s

OK

The reference output files it compares against are:

examples/reference/string_result.html
examples/reference/file_result.html

To see what happens when there's a difference, try editing one or both of the main functions that generate the HTML in generators.py. They're most just using explicit strings, so the simplest thing is just to change a word or something in the output.

If I change It's to It is in the generate_string() function and rerun, I get this output:

$ python test_using_writabletestcase.py
.
File check failed.
Compare with "diff /var/folders/w7/lhtph66x7h33t9pns0616qk00000gn/T/string_result.html /Users/njr/python/tdda/examples/reference/string_result.html".

Note exclusions:
Copyright
Version
F
======================================================================
FAIL: testExampleStringGeneration (__main__.TestExample)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_using_writabletestcase.py", line 55, in testExampleStringGeneration
    'Version'])
  File "/Users/njr/python/tdda/writabletestcase.py", line 294, in check_string_against_file
    self.assertEqual(failures, 0)
AssertionError: 1 != 0

----------------------------------------------------------------------
Ran 2 tests in 0.005s

FAILED (failures=1)
1 godel:$

If I then run the diff command it suggests, the output is:

$ diff /var/folders/w7/lhtph66x7h33t9pns0616qk00000gn/T/string_result.html /Users/njr/python/tdda/examples/reference/string_result.html
5,6c5,6
<     Copyright (c) Stochastic Solutions, 2016
<     Version 1.0.0
—
>     Copyright (c) Stochastic Solutions Limited, 2016
>     Version 0.0.0
29c29
<     It is not terribly exciting.
—
>     It's not terribly exciting.

Here you can see the differences that are excluded, and the change I actually made.

(The version I showed at PyCon has an option to see the only the non-excluded differences, but this version doesn't; that will come!)

If I now run again using -w, to re-write the reference output, it shows:

$ python test_using_writabletestcase.py -w
.Expected file /Users/njr/python/tdda/examples/reference/string_result.html written.
.
----------------------------------------------------------------------
Ran 2 tests in 0.003s

OK

And, of course, if I run a third time, without -w, the test now passes:

$ python test_using_writabletestcase.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.003s

OK

So that's a quick overview of it works.