Python Summit 2022: Never Write Scripts Again

As a DevOps engineer, when you write Python code, do you also write tests? If you're like the majority of the folks out there, chances are you don't. Why would you? It's usually just a script, even if it gets longer over time and starts feeling like Bash.

This talk explains why it makes sense to stop hacking glue code and start developing serious CLI applications, test-driven, with automated tests. Even if you have some experience with writing tests, it's not immediately obvious how to get started. You'll get to know the cli-test-helpers package and see a hands-on demonstration of developing a CLI application from scratch, TDD-style.

We'll scratch the surface of some popular CLI frameworks (argparse, click, docopt), and you'll take home working code samples that will help you refuse the temptation of writing code without tests, in future.

Original slides at: https://slides.com/bittner

Transcript

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4. Source: , 2015
   Technology and Friends, Episode 354
   

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5. from application import cli
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   from cli_test_helpers import shell
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   def test_cli_entrypoint():
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   result = shell("python-summit --help")
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   assert result.exit_code == 0
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   1. What's wrong with scripts?
   2. Coding example #1 (refactoring)
   3. Why CLI applications?
   4. Challenges with writing tests
   5. Coding example #2 (cli & tests)
   

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6. print("This is important code")
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   for index, arg in enumerate(sys.argv):
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   print(f"[{index}]: {arg}")
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   What's Wrong with Scripts?
   Easy to get started
   Limited possibilities for structure
   Hard to (unit) test
   No dependency management
   Deployment may require care
   Custom user experience
   

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7. $ cowsay -e ~~ WHAT IS WRONG WITH SCRIPTS? | lolcat
   _____________________________
   < WHAT IS WRONG WITH SCRIPTS? >
   -----------------------------
   \ ^__^
   \ (~~)\_______
   (__)\ )\/\
   ||----w |
   || ||
   Coding Example #1
   

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8. print("Usage: foo --baz")
   def main():
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   if __name__ == "__main__":
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   main()
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   Why CLI Applications?
   Standardized user experience
   More possibilities for structure
   Possibilities for all kinds of testing
   Dependency management
   Packaging & distribution
   

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9. import argparse
   from . import __version__
   def parse_arguments():
   parser = argparse.ArgumentParser(description=__doc__)
   parser.add_argument('--version', action='version',
   version=__version__)
   parser.add_argument('filename')
   args = parser.parse_args()
   return args
   def main():
   args = parse_arguments()
   ...
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   Argparse
   

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10. import click
    @click.command()
    @click.version_option()
    @click.argument('filename')
    def main(filename):
    click.echo(filename)
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    Click
    import click
    @click.command()
    @click.version_option()
    @click.argument('filename', type=click.Path(exists=True))
    def main(filename):
    click.echo(filename)
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    import click
    @click.command()
    @click.version_option()
    @click.argument('infile', type=click.File())
    def main(infile):
    click.echo(infile.read())
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11. """Foobar
    Usage:
    foobar (-h | --help | --version)
    foobar [-s | --silent]
    foobar [-v | --verbose]
    Positional arguments:
    file target file path name
    Optional arguments:
    -h, --help show this help message and exit
    -s, --silent don't show progress output
    -v, --verbose explain progress verbosely
    --version show program's version number and exit
    """
    from docopt import docopt
    from . import __version__
    def parse_arguments():
    args = docopt(__doc__, version=__version__)
    return dict(
    file=args[''],
    silent=args['-s'] or args['--silent'],
    verbose=args['-v'] or args['--verbose'],
    )
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    Docopt
    

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12. with pytest.raises(SystemExit):
    foobar.cli.main()
    pytest.fail("CLI doesn't abort")
    def test_cli():
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    Challenges with Writing Tests
    How test our CLI configuration?
    Control taken away from us
    Package features require deployment
    

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13. result = shell('foobar')
    def a_functional_test():
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    assert result.exit_code != 0, result.stdout
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    CLI Test Strategy
    1. Start from the user interface with functional tests.
    2. Work down towards unit tests.
    1. Start from the user interface with functional tests.
    result = shell('foobar')
    with ArgvContext('foobar', 'myfile', '--verbose'):
    args = foobar.cli.parse_arguments()
    def a_functional_test():
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    assert result.exit_code != 0, result.stdout
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    def a_unit_test():
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    assert args['verbose'] == True
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14. result = shell('foobar --help')
    assert result.exit_code == 0
    def test_entrypoint():
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    """Is entrypoint script installed? (setup.py)"""
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    Functional Tests (User Interface)
    

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15. @patch('foobar.command.process')
    with ArgvContext('foobar', 'myfile', '-v'):
    foobar.cli.main()
    import foobar
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    from cli_test_helpers import ArgvContext
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    from unittest.mock import patch
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    def test_process_is_called(mock_command):
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    assert mock_command.called
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    assert mock_command.call_args.kwargs == dict(
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    file='myfile', silent=False, verbose=True)
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    Unit Tests
    

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16. [tox]
    [testenv]
    cli-test-helpers
    coverage[toml]
    pytest
    coverage run -m pytest {posargs}
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    envlist = py{38,39,310}
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    description = Unit tests
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    deps =
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    commands =
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    coverage xml
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    coverage report
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    Tox
    

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17. $ cowsay MOO! I ♥ CLI-TEST-HELPERS | lolcat
    ___________________________
    < MOO! I ♥ CLI-TEST-HELPERS >
    ---------------------------
    \ ^__^
    \ (oo)\_______
    (__)\ )\/\
    ||----w |
    || ||
    Coding Example #2
    

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18. Thank you!
    for your precious time
    Painless Software
    Less pain, more fun.
    

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