create a test file

Choose a name for your test file. We'll use test_fname.py:

touch src/allmydata/test/test_fname.py

./bin/tahoe debug trial allmydata.test.test_fname

Okay, so it didn't work, because there is no file by that name. Create such a file, with these contents:

rom twisted.trial import unittest

class T(unittest.TestCase):
    def test_a(self):
        pass

Now install Ned Batchelder's "​coverage" tool and run your with code coverage, like this:

./bin/tahoe @coverage run --branch --include='src/allmydata/*' @tahoe debug trial allmydata.test.test_fname

If you installed coverage from Debian or Ubuntu then you have to name it python-coverage, like this:

./bin/tahoe @python-coverage run --branch --include='src/allmydata/*' @tahoe debug trial allmydata.test.test_fname

This does the same as running the tests without coverage -- print a list of what happened when each test was run. It also writes out a file named .coverage into the current directory. Run the following command to read that file and produce nice HTML pages:

./bin/tahoe @coverage html

That will product a directory named htmlcov. View its contents with a web browser.