On Wed, Jan 20, 2016 at 05:04:21PM -0800, Guido van Rossum wrote:
On Wed, Jan 20, 2016 at 4:10 PM, Steven D'Aprano <steve@pearwood.info> wrote: [...]
(I'm saving my energy for Eiffel-like require/ensure blocks *wink*).
Now you're making me curious.
Okay, just to satisfy your curiosity, and not as a concrete proposal at this time, here is a sketch of the sort of thing Eiffel uses for Design By Contract. Each function or method has an (optional, but recommended) pre-condition and post-condition. Using a hybrid Eiffel/Python syntax, here is a toy example: class Lunch: def __init__(self, arg): self.meat = self.spam(arg) def spam(self, n:int=5): """Set the lunch meat to n servings of spam.""" require: # Assert the pre-conditions of the method. assert n >= 1 ensure: # Assert the post-conditions of the method. assert self.meat.startswith('Spam') if ' ' in self.meat: assert ' spam' in self.meat # main body of the method, as usual serves = ['spam']*n serves[0] = serves.title() self.meat = ' '.join(serves) The require block runs before the body of the method, and the ensure block runs after the body, but before the method returns to the caller. If either fail their assertions, the method fails and raises an exception. Benefits: - The pre- and post-conditions make up (part of) the method's contract, which is part of the executable documentation of the method. Documentation tools can extract the ensure and require sections as present them as part of the API docs. - The compiler can turn the contract checking on or off as needed, with the ensure/require sections handled independently. - Testing pre- and post-conditions is logically separate from the method's implementation. This allows the implementation to vary while keeping the contract the same. - But at the same time, the contract is right there with the method, not seperated in some potentially distant part of the code base. I'm not going to go into detail about Design By Contract, if anyone wants to learn more you can start here: https://www.eiffel.com/values/design-by-contract/introduction/ https://docs.eiffel.com/book/method/et-design-contract-tm-assertions-and-exc... I've just discovered there's an older PEP for something similar: https://www.python.org/dev/peps/pep-0316/ but that uses docstrings for the contracts. I don't like that. -- Steve