TDD-ing a toy sized project

Posted on September 21, 2016

Just recently I was porting a toy sized parser combinator library (a proof of concept) from Haskell to Python. You know, for educational purposes. It turns out I’m not smart enough to keep the complicated types (and explicit laziness) in my head even for such a small project. So my solution was to do TDD. To clarify: I wanted to test happy paths through my functions to make sure at least types fit together.

So tests I will write. But that means I need another file for tests (yes, it was I single file project, that’s what I mean with “toy-sized”), some actual structure, a test runner, probably a package description for dependencies… How about no? I remembered seeing a thing called doctest once. Turns out it fit perfectly!.

This is how using doctest looks in Python:

def parseString(target):
  """
  >>> parseString('foo')('foobar')
  [('foo', 'bar')]
  """
  def p(s):
    if s.startswith(target):
      return [(target, s[len(target):])]
    else:
      return []
  return p


if __name__ == '__main__':
  import doctest
  doctest.testmod()

And then run with python myfile.py. It is that simple (and built-int). You just put examples in the docstring and they will be machine checked.

But I’m lazy and don’t want to run tests every time by hand. I want a poor-man’s runner with --watch capability. And I can have it as a bash one-liner (given that inotify-tools package is installed on my system).

while true; do
  inotifywait myfile.py
  clear
  python myfile.py
done

This will automatically clear the screen and re-run tests every time I change the file - save it from my editor. Now I can finally develop my toy projects in split screen with terminal and vim using only my editor’s save function to run tests :)

Bonus section

Doctest is ported to Haskell, so I can use it there as well. Just need to stack install it globally and it will be available for my one-file projects.

-- |
-- >>> parseNumber "123"
-- [(Number 123,""),(Number 12,"3"),(Number 1,"23")]
parseNumber :: Parser Expr
parseNumber = mapParser (Number . read . concat) $ plus parseDigit

And run with

while true; do
  inotifywait myfile.py
  clear
  doctest myfile.py
done
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