GitHub Actions: How to override the default matrix with workflow_dispatch inputs?

Question

If I want to let a user override the default values in the matrix, how do I do that?

Consider the following simplified example:

name: CICD
on:
  push:
    branches:
      - main
  workflow_dispatch:
    inputs:
      py_version:
        description: "Python version"
        default: "3.11"

jobs:
  test:
    runs-on: ubuntu-22.04
    strategy:
      matrix:
        py_version: [3.7, 3.10]
    steps:
      - uses: actions/setup-python@v4
        with:
          python-version: ${{ matrix.py_version }}

I want to let a user override the py_version matrix list with the list provided in the workflow_dispatch input. Is there a way?

I tried asking ChatGPT but it messed things up all the time. I went looking for a way to dynamically build the matrix JSON, but this looks complicated and I failed to find a working solution.

PS: Is it possible to let the user provide a list-like value in the inputs, say, [3.5, 2.7] that would be populated in the matrix? I am fine with a single value as well, though.

Answer 1

You can use a comma-separated list along with format() and fromJSON() functions; and, OR operator || like this:

name: populate_matrix_from_inputs

on:
  push:
    branches: [main]
  workflow_dispatch:
    inputs:
      python_version:
        description: 'Comma-separated list of Python versions e.g. "3.10","3.11"'
        default: '"3.11"'
        required: true

jobs:
  ci:
    runs-on: ubuntu-latest

    strategy:
      matrix:
        python_version: ${{ fromJSON(format('[{0}]', inputs.python_version || '"3.7","3.10"')) }}

    steps:
      - uses: actions/setup-python@v4
        with:
          python-version: ${{ matrix.python_version }}

Inputs

Runs

---

NOTE: Quotes around versions are mandatory to avoid the removal of trailing zeros during conversions. For example, without quotes, 3.10 would become 3.1 which is not a valid version. If such cases are not an issue, quotes may be avoided.