Making a coverage badge

Monday 19 September 2022

This is a sketch of how to use GitHub actions to get a total combined coverage number, and create a badge for your README. There are other approaches too, but this uses some commonly used tools to get the job done.

We’ll use tox to run tests, and GitHub actions to run tox. A GitHub gist will be used as a scratch file to store parameters for the badge, which will be rendered by

Start with the tox.ini that runs your test suite, and also includes a “coverage” environment that combines, reports, and produces a JSON data file:


envlist = py37,py38,py39,py310,coverage

commands =
    python -m coverage run -p -m pytest

basepython = python3.10
commands =
    python -m coverage combine
    python -m coverage report -m --skip-covered
    python -m coverage json

python =
    3.7: py37
    3.8: py38
    3.9: py39
    3.10: py310

We’ll use a GitHub action to run tox, but before we get to that, we need two bits of infrastructure. Go to and make an empty secret gist. Copy the id of the gist. Here we’ll call it 123abc456def789.

Next we’ll create a personal access token for updating the gist. Go to your GitHub personal access tokens page and click “Generate new token.” Select the “gist” scope, and click “Generate token.” Copy the value displayed, it will look like “ghp_FSfkCeFblahblahblah”. You can’t get the value again, so be careful with it.

In your repo on GitHub, go to Settings - Secrets - Actions, click “New repository secret.” Use “GIST_TOKEN” as the Name, and paste the ghp_etc token as the Secret, then “Add secret.”

Now we’re ready to create the GitHub action. It will run the test suite on many versions of Python, then run the coverage step to combine all the data files. It uses the JSON report to extract a displayable percentage, then uses a third-party GitHub action to create the JSON data in the Gist so that can display the badge.

The badge is automatically colored: 50% or lower is red, 90% or higher is green, with a gradient between the two, like this:

The spectrum of badge colors.

As a bonus, there’s an action job summary with the coverage total. Here’s the workflow file:


# Run tests

name: "Test Suite"


    shell: bash

    name: "Python ${{ matrix.python-version }} on ${{ matrix.os }}"
    runs-on: "${{ matrix.os }}"

      fail-fast: false
          - ubuntu-latest
          - macos-latest
          - windows-latest
          - "3.7"
          - "3.8"
          - "3.9"
          - "3.10"

      - name: "Check out the repo"
        uses: "actions/checkout@v2"

      - name: "Set up Python"
        uses: "actions/setup-python@v2"
          python-version: "${{ matrix.python-version }}"

      - name: "Install dependencies"
        run: |
          python -m pip install tox tox-gh-actions

      - name: "Run tox for ${{ matrix.python-version }}"
        run: |
          python -m tox

      - name: "Upload coverage data"
        uses: actions/upload-artifact@v3
          name: covdata
          path: .coverage.*

    name: Coverage
    needs: tests
    runs-on: ubuntu-latest
      - name: "Check out the repo"
        uses: "actions/checkout@v2"

      - name: "Set up Python"
        uses: "actions/setup-python@v2"
          python-version: "3.10"

      - name: "Install dependencies"
        run: |
          python -m pip install tox tox-gh-actions

      - name: "Download coverage data"
        uses: actions/download-artifact@v3
          name: covdata

      - name: "Combine"
        run: |
          python -m tox -e coverage
          export TOTAL=$(python -c "import json;print(json.load(open('coverage.json'))['totals']['percent_covered_display'])")
          echo "total=$TOTAL" >> $GITHUB_ENV
          echo "### Total coverage: ${TOTAL}%" >> $GITHUB_STEP_SUMMARY

      - name: "Make badge"
        uses: schneegans/dynamic-badges-action@v1.4.0
          # GIST_TOKEN is a GitHub personal access token with scope "gist".
          auth: ${{ secrets.GIST_TOKEN }}
          gistID: 123abc456def789   # replace with your real Gist id.
          filename: covbadge.json
          label: Coverage
          message: ${{ }}%
          minColorRange: 50
          maxColorRange: 90
          valColorRange: ${{ }}

Now the badge can be displayed with a URL like this, but replace YOUR_GITHUB_NAME with your GitHub name, and 123abc456def789 with your real Gist id:

Consult the docs for your markup language of choice for how to use the image URL to display the badge.

BTW: the files here are simplified versions of the action and tox.ini from scriv, if you are interested.


Saturday 27 August 2022

For fun this summer, I implemented part of the PostScript language, using PyCairo for rendering. I call it Stilted. Implementing a language is an interesting exercise. You always learn some things along the way.

Executable bit: All objects in PostScript have a literal/executable bit that can be changed with the cvx (convert to executable) and cvlit (convert to literal) operators. Literal arrays are delimited by square brackets, executable arrays (procedures) are in curly braces. Like in Python and JavaScript, multiple references share storage. But oddly, in PostScript, you can duplicate an object on the stack, and change its executable bit, and now you have two references to the same storage, but with different attributes.

Here’s an example using GhostScript (a third-party conforming implementation):

GS>      [1 2 3] dup         % make an an array and duplicate it
GS<2>    cvx                 % make the top one executable
GS<2>    pstack              % print the stack
{1 2 3}
[1 2 3]
GS<2>    dup 1 99 put        % change the second element
GS<2>    pstack              % both objects share the storage
{1 99 3}
[1 99 3]

The executable attribute is part of the reference, not part of the object!? This doesn’t seem like a planned and desired outcome: it seems like a side-effect of a common C technique: using low bits of a pointer to store flags.

While writing Stilted, I didn’t realize this behavior until I already had made executability part of the object itself, so Stilted produces a different (wrong) result:

|-0>     [1 2 3] dup        % make an array and duplicate it
|-2>     cvx                % make the top one executable
|-2>     pstack             % oops: both are changed!
{1 2 3}
{1 2 3}
|-2>     dup 1 99 put
|-2>     pstack
{1 99 3}
{1 99 3}

Since I don’t think anyone actually depends on having two objects that share storage, but with different executability, I didn’t bother changing it. An advantage of pure-fun side projects: you can do whatever you want!

BTW: the numbers in the prompts are the current depth of the operand stack.

Cutesy string syntax: PostScript strings are made with parentheses, and they nest, so this is one string:

GS> (Hello (there) 1 2 3) pstack
(Hello \(there\) 1 2 3)

Stilted doesn’t nest the parens in strings, because it uses regexes for lexing tokens, and nesting is hard with regexes. This is a syntax error in Stilted:

|-0> (Hello (there) 1 2 3) pstack
Error: syntaxerror in 3
Operand stack (4):
(Hello \(there)

Also, who depends on nested parens in strings? Just escape the closing parens in your strings.

Flexible scope: PostScript is a stack-oriented language. There’s an operand stack that operators pop and push to, and also a dictionary stack where names are defined and looked up. The dictionary stack is explicitly manipulated with the begin and end operators. Instead of procedures starting new scopes implicitly, the programmer decides when to begin and end scopes. This means they don’t have to correspond to procedure invocations at all.

We’re so used to scoping being tied to function calls in our programming languages, it was strange to realize that the two concepts can be completely unrelated.

Surprising gaps: Re-acquainting myself with PostScript, I was surprised at what it didn’t have: no way to sort arrays, no string formatting, and so on. PostScript pre-dated languages like Python, JavaScript, and even Perl. Its model is much more like C than the higher-level languages that we’re used to now. Though C has string formatting, and you’d think that would be a useful thing in a printing programming language.

More: If you aren’t familiar with PostScript, I’ve got more description of its unusual control structure approach, and also other blog posts tagged #postscript.

Stilted has been a lot of fun. Extra fun: I used the Obfuscated PostScript winners as test cases!

Truchet images

Wednesday 17 August 2022

I got interested in Truchet tiles, and did some hacking around to understand them better, and then display some images using them. The code is not clean or documented, and it’s inefficient in dumb ways, but it made some nice pictures. The code is at nedbat/truchet if you want to experiment.

A simple example of Truchet is Smith tiles. The tiles are designed to fit together seamlessly even when placed randomly:

Random orientations of black/white tiles

Christopher Carlson came up with a way to generalize the tiles so they could be placed on top of each other at different sizes. A square can be covered by four half-sized tiles with inverted colors and extra wings, and the pattern will remain seamless.

Here are his tiles:

The 15 Carlson Truchet tiles

It can be hard to see how they overlap, but this is a start. This is three different sizes of tile overlaid randomly, with the grid displayed to help see the edges:

A Carlson tiling at three different sizes

I love the randomness of these images, how shapes emerge that were not in the tiles themselves. I’ve been using them as Zoom backgrounds and desktop wallpapers. But I wondered if they could be used to create images.

The set of gray values in the Carlson set is somewhat limited, so I created a new set of tiles with more opportunities for variation:

A larger set of new multi-scale Truchet tiles

These produced even more chaos and serendipity when used randomly:

Randomly placed N6 Truchet tiles

To make images, I used a photo as source and fit tiles onto it to match the gray levels. Larger squares would be subdivided when their sub-squares’ intensities differed more than some threshold:

Young Marilyn Monroe, photo
Young Marilyn Monroe, with Truchet tiles
Me, photo
Me, with Truchet tiles

The algorithm to pick a tile will try to choose a good orientation, to match the colors within the square. Notice the tiles used for my shoulders. Though, on the flip side, both these images clearly exhibit “the forehead problem” because there’s little color variation there.

Looking around for other high-contrast images, I tried a well-known blogger’s avatar:

Coding Horror, in Truchet

The subdivision algorithm uses a threshold to decide when a square has enough variation within it to deserve subdivision. What happens if we start that threshold very large, and slide it down to very small, animating the result?

Marilyn, emerging from coarse-grained to fine-grained detail

Fall fallout

Saturday 30 July 2022

More about my bike fall since I wrote about it two weeks ago.

  • I saw a neurosurgeon. He explained that I have a cavernoma, which is an anomalous collection of blood vessels with thin walls, which can lead to bleeding. The bleeding leaves behind iron deposits, which can cause a seizure. My cavernoma is located in a spot especially prone to seizures.
  • The neurosurgeon thought it would be a simple operation to remove the cavernoma, despite literally being brain surgery. He actually used the phrase “easy-peasy.” Also, his perspective was that this wasn’t a serious incident, since it wasn’t a stroke or death. I guess in his line of work, a seizure is on the small side.
  • Another perspective on severity: I took my bike to the shop to get it checked out. I was telling the bike dude about the crash. He said, “You didn’t need dental work? Then no big deal!”
  • I had an electroencephalogram (EEG). This involved having 27 wires pasted to my head and chest, then lying in a dark room with my eyes closed while they measured my brain activity. Toward the end, they placed a strobe light over my closed eyes, and flashed it at various frequencies. I realized, this is a black box unit test, and my brain is the system under test: provide some inputs, check the outputs, without being able to see the implementation. The initial report seemed to be, “nothing unusual,” but I have to check in with the neurologist.
  • After a few failed attempts, I managed to get the name of the person who called the police for me after my crash. I wrote to him, and he was very friendly, but didn’t have any more details about what happened. When he first saw me I was already on the ground, so he can’t explain the cause of the crash. Still, it felt good to connect with him and find out what he knew.

My energy level is not what it used to be, probably because of the Keppra (anti-seizure medication). Psychologically, I am not used to the idea that my brain can just shut off with no notice. I guess over time, I’ll just ignore that possibility?

The Fall

Wednesday 13 July 2022

One moment I was riding my bike; the next thing I remember, I was sitting on the ground talking to an EMT from the ambulance parked nearby.

This happened three weeks ago. I went to the emergency room, had a few CT scans and an MRI. The best theory is that I had a seizure. So now I am on anti-seizure medication, and am legally forbidden to drive a car for six months.

I was wearing a helmet, and was on a bike path, not a street. The physical effects were minimal: a sore shoulder and some road rash.

There was no eye-witness, so doctors guess I fell because I blacked out rather than the other way around. During the time I don’t remember, I called my wife and told her where I was, so maybe I was never truly unconscious? No one knows.

I usually have a low heart rate (a resting pulse of 50 bpm is not unusual), so maybe it was cardiac-related? I’m wearing a heart monitor to collect data.

The first week was hard because I felt completely limited in what I could do. All spring I had been feeling strong and capable in physical activities, and now I was finding a short walk difficult.

At first the anti-seizure meds made me tired and a bit fuzzy-headed. But we’ve adjusted them, and/or I’m adjusting to them, and/or my concussion is wearing off, so I feel more like myself. I’ve gotten back on the bike (though not alone), and have been swimming in the ocean, same as every summer.

I have more visits coming up with more doctors to try to understand what happened, and what might happen. I doubt they will be able to completely rule out a seizure, so I may be on meds for quite some time. Their recommendations are quite cautious (“Don’t take a bath without supervision”), so now we are making absurd trade-offs and considerations of risks and possibilities.

It’s unsettling to have lost time without a clear explanation, and especially unsettling to think that it could happen again at any time. I’m not sure what to do with that.