[Python-ideas] Conventions for function annotations

[Jasper St. Pierre]

Indeed. I've looked at annotations before, but I never understood the
purpose. It seemed like a feature that was designed and implemented without
some goal in mind, and where the community was supposed to discover the
goal themselves.

So, if I may ask, what was the original goal of annotations? The PEP gives
some suggestions, but doesn't leave anything concrete. Was it designed to
be an aid to IDEs, or static analysis tools that inspect source code?
Something for applications themselves to munge through to provide special
behaviors, like a command line parser, or runtime static checker?

The local decorator influence might work, but that has the problem of only
being able to be used once before we fall back to the old method. Would you
rather:

    @tab_expand(filename=glob('*.txt'))
    @types
    def read_from_filename(filename:str, num_bytes:int) -> bytes:
        pass

or

    @tab_expand(filename=glob('*.txt'))
    @types(filename=str, num_bytes=int, return_=bytes)
    def read_from_filename(filename, num_bytes):
        pass

For consistency's sake, I'd prefer the latter.

Note that we could take a convention, like Thomas suggests, and adopt both:

    @tab_expand
    @types
    def read_from_filename(filename:(str, glob('*.txt')), num_bytes:int) ->
bytes:
        pass

But that's a "worst of both worlds" approach: we lose the locality of which
argument applies to which decorator (unless we make up rules about
positioning in the tuple or something), and we gunk up the function
signature, all to use a fancy new Python 3 feature.

With a restricted and narrow focus, I could see them gaining adoption, but
for now, it seems like extra syntax was introduced simply for the point of
having extra syntax.



On Sun, Dec 2, 2012 at 5:23 PM, Steven D'Aprano <steve at pearwood.info> wrote:

> On 02/12/12 22:43, Nick Coghlan wrote:
>
>  Last time it came up, the collective opinion on python-dev was still to
>> leave PEP 8 officially neutral on the topic so that people could
>> experiment
>> more freely with annotations and the community could help figure out what
>> worked well and what didn't. Admittedly this was long enough ago that I
>> don't remember the details, just the obvious consequence that PEP 8
>> remains
>> largely silent on the matter, aside from declaring that function
>> annotations are off-limits for standard library modules: "The Python
>> standard library will not use function annotations as that would result in
>> a premature commitment to a particular annotation style. Instead, the
>> annotations are left for users to discover and experiment with useful
>> annotation styles."
>>
>
> I fear that this was a strategic mistake. The result, it seems to me, is
> that
> annotations have been badly neglected.
>
> I can't speak for others, but I heavily use the standard library as a guide
> to what counts as good practice in Python. I'm not a big user of third
> party
> libraries, and most of those are for 2.x, so with the lack of annotations
> in
> the std lib I've had no guidance as to what sort of things annotations
> could
> be used for apart from "type checking".
>
> I'm sure that I'm not the only one.
>
>
>
> --
> Steven
>
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-- 
  Jasper
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