[Python-ideas] Delay evaluation of annotations

[Nick Coghlan]

On 23 September 2016 at 13:06, Steven D'Aprano <steve at pearwood.info> wrote:
> On Thu, Sep 22, 2016 at 07:21:18PM +0000, אלעזר wrote:
>> "Expression" is something that you need its value right
>> now, and "annotation" is something that, well, annotates the code you see
>> right now.
>
> Right. In the case of Python, function annotations **do** have a runtime
> effect: the expressions are evaluated, and the evaluated results are
> assigned in function.__annotations__ and made available for runtime
> introspection.
>
> Don't think that function annotations are **only** for the static type
> checker. Python is a much richer language than that!

If folks are after a simple non-type-checking related example of
annotation usage, the "begins" CLI library is a decent one:
https://pypi.python.org/pypi/begins

That lets you supply command line help for parameters as annotations:

============
In Python3, any function annotations for a parameter become the
command line option help. For example:

  >>> import begin
  >>> @begin.start                                         # doctest: +SKIP
  ... def run(name: 'What, is your name?',
  ...         quest: 'What, is your quest?',
  ...         colour: 'What, is your favourite colour?'):
  ...     pass

Will generate command help like:

usage: holygrail_py3.py [-h] -n NAME -q QUEST -c COLOUR

optional arguments:
  -h, --help            show this help message and exit
  -n NAME, --name NAME  What, is your name?
  -q QUEST, --quest QUEST
                        What, is your quest?
  -c COLOUR, --colour COLOUR
                        What, is your favourite colour?
============

It's not a substitute for something like click or argparse when it
comes to more complex argument parsing, but it's a good example of the
kind of simple pseudo-DSL folks have long been able to create with
annotations independently of the type hinting use case.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia