I am not smart enough to consistently write and debug shell scripts that use any conditional or looping constructs. So as soon as I’m writing something in bash that requires use of those, I switch to python. This works fine when writing scripts, but what to do when writing GitHub Actions workflows? You can only write bash in run: stanzas in your steps, right?

Not at all! You can set the shell parameter to anything you want, and the contents of run will be passed to the shell in the form of a file. This allows you to use not just Python, but basically any langauge when writing your GitHub actions workflows.

Here is an example step that used python.

  steps:
    - name: Something in python!
      # The -u means 'unbuffered', so print() statements in your python code are output correctly
      # otherwise, they might be out of order with stdout from commands your code calls
      # {0} is replaced with the name of the temporary file GitHub Actions creates with
      # the contents of the run:
      shell: python -u {0}
      run: |
        import sys
        import subprocess

        print("Hello, I am python")

        # We have to use string substitution for getting inputs, which is bad and ugly
        # However, it isn't as bad me trying to write conditionals in bash.
        # It might be possible to use environment values here, I haven't explored.
        variable = '${{ inputs.some_input }}'

        # Use subprocess.check_call to call out to external process. stdout is
        # handled correctly
        subprocess.check_call([
          sys.executable,
          '-m',
          'pip', 'install', 'django'
        ])

If you instead use mamba or conda to set up your Python, perhaps with the setup-miniconda action, you need to set shell: bash -l -c "python -u {0}". The bash -l makes sure that the appropriate conda environment is loaded, and then passes off to the ‘correct’ python.