[Python-ideas] Conventions for function annotations

[Andrew Svetlov]

I think code related to annotations is tightly coupled with annotated
function usage context (decorator, metaclass, function caller).
So annotation really can mean anything and it depends from context.
I don't see use case when context need to ignore unexpected
annotation. In my practice annotation is always expected if specified,
absence of annotation for parameter is mark to do nothing with it (it
can be allowed or disabled depending of context requirements).
The same for multiple annotations. If your context allow it — that's
up to you. Exact kind of composition to use depends from context — it
can be tuple, dict, user-defined composition object.

My point is: we dont need to restrict annotations in any way. If some
libraries want to share annotations that means they are tightly enough
coupled and can make rules for itself. All other code can go in the
wild.

On Sat, Dec 1, 2012 at 2:28 PM, Thomas Kluyver <thomas at kluyver.me.uk> wrote:
> Function annotations (PEP 3107) are a very interesting new feature, but so
> far have gone largely unused. The only project I've seen using them is plac,
> a command-line option parser. One reason for this is that because function
> annotations can be used to mean anything, we're wary of doing anything in
> case we interfere with some other use case. A recent thread on ipython-dev
> touched on this [1], and we'd like to suggest some conventions to make
> annotations useful for everyone.
>
> 1. Code inspecting annotations should be prepared to ignore annotations it
> can't understand.
>
> 2. Code creating annotations should use wrapper classes to indicate what the
> annotation means. For instance, we are contemplating a way to specify
> options for a parameter, to be used in tab completion, so we would do
> something like this:
>
> from IPython.core.completer import options
> def my_io(filename, mode: options('read','write') ='read'):
>     ...
>
> 3. There are a couple of important exceptions to 2:
> - Annotations that are simply a string can be used like a docstring, to be
> displayed to the user. Inspecting code should not expect to be able to parse
> any machine-readable information out of these strings.
> - Annotations that are a built-in type (int, str, etc.) indicate that the
> value should always be an instance of that type. Inspecting code may use
> these for type checking, introspection, optimisation, or other such
> purposes. Note that for now, I have limited this to built-in types, so other
> types can be used for other purposes, but this could be extended. For
> instance, the ABCs from collections (collections.Mapping et al.) could well
> be added to this category.
>
> 4. There should be a convention for attaching multiple annotations to one
> value. I propose that all code using annotations expects to handle
> tuples/lists of annotations. (We also considered dictionaries, but the
> result is long and ugly). So in this definition:
>
> def my_io(filename, mode: (options('read','write'), str, 'The mode in which
> to open the file') ='read'):
>     ...
>
> the mode parameter has a set of options (ignored by frameworks that don't
> recognise it), should always be a string, and has a description.
>
> Any thoughts and suggestions are welcome.
>
> As an aside, we may also create a couple of decorators to fill in
> __annotations__ on Python 2, something like:
>
> @return_annotation('A file obect')
> @annotations(mode=(options('read','write'), str, 'The mode in which to open
> the file'))
> def my_io(filename, mode='read'):
>     ...
>
> [1] http://mail.scipy.org/pipermail/ipython-dev/2012-November/010697.html
>
>
> Thanks,
> Thomas
>
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-- 
Thanks,
Andrew Svetlov