Planet Python
Last update: June 29, 2025 10:42 AM UTC
June 27, 2025
Hugo van Kemenade
Run coverage on tests
I recommend running coverage on your tests.
Here’s a couple of reasons why, from the past couple of months.
Example one #
When writing tests, it’s common to copy and paste test functions, but sometimes you forget to rename the new one (see also: the Last Line Effect).
For example:
def test_get_install_to_run_with_platform(patched_installs):
i = installs.get_install_to_run("<none>", None, "1.0-32")
assert i["id"] == "PythonCore-1.0-32"
assert i["executable"].match("python.exe")
i = installs.get_install_to_run("<none>", None, "2.0-arm64")
assert i["id"] == "PythonCore-2.0-arm64"
assert i["executable"].match("python.exe")
def test_get_install_to_run_with_platform(patched_installs):
i = installs.get_install_to_run("<none>", None, "1.0-32", windowed=True)
assert i["id"] == "PythonCore-1.0-32"
assert i["executable"].match("pythonw.exe")
i = installs.get_install_to_run("<none>", None, "2.0-arm64", windowed=True)
assert i["id"] == "PythonCore-2.0-arm64"
assert i["executable"].match("pythonw.exe")
The tests pass, but the first one is never run because its name is redefined. This clearly shows up as a non-run test in the coverage report. In this case, we only need to rename one of them, and both are covered and pass.
But sometimes there’s a bug in the test which would cause it to fail, but we just don’t know because it’s not run.
Example two #
This is more subtle:
im = Image.new("RGB", (1, 1))
for colors in (("#f00",), ("#f00", "#0f0")):
append_images = (Image.new("RGB", (1, 1), color) for color in colors)
im_reloaded = roundtrip(im, save_all=True, append_images=append_images)
assert_image_equal(im, im_reloaded)
assert isinstance(im_reloaded, MpoImagePlugin.MpoImageFile)
assert im_reloaded.mpinfo is not None
assert im_reloaded.mpinfo[45056] == b"0100"
for im_expected in append_images:
im_reloaded.seek(im_reloaded.tell() + 1)
assert_image_similar(im_reloaded, im_expected, 1)
It’s not so obvious when looking at the code, but Codecov highlights a problem:
The append_images generator is being consumed inside roundtrip(), so we have nothing
to iterate over in the for loop – hence no coverage. The
fix is to
use a list instead of a generator.
Header photo: Misplaced manhole cover (CC BY-NC-SA 2.0 Hugo van Kemenade).
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If you have benefited from my work in the past, consider sponsoring me. Whatever you feel comfortable with.
If your organization has extracted any value from my work, then consider picking one of the higher tiers to return the favor. I may be able to assist you with your project, wether it is terminal related or not. There is a lot of knowledge in my fleshy mammalian brain. Even in the age of AI, it could still benefit you.
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