[Python-3000] Draft pre-PEP: function annotations

Collin Winter collinw at gmail.com
Sat Aug 12 01:49:32 CEST 2006

I'll combine my replies to Josian and Talin:

On 8/11/06, Josiah Carlson <jcarlson at uci.edu> wrote:
> Let us say that I have two metadata interpters.  One that believes that
> the metadata is types and wants to verify type on function call.  The
> other believes that the metadata is documentation.  Both were written
> without regards to the other.  Please describe to me (in code preferably)
> how I would be able to use both of them without having a defined
> metadata interpretation chaining semantic.

On 8/11/06, Talin <talin at acm.org> wrote:
> Say I want to annotate a specific argument with two pieces of
> information, a type and a docstring. I have two metadata interpreters,
> one which uses the type information to restrict the kinds of arguments
> that can be passed in, and another which uses the docstring to enhance
> the generated documentation.

[snipped: the rise of a defacto annotation-sharing standard]

> What this means is that, despite the statements that annotations have no
> defined format or meaning, the fact is that they now do: The defacto A&B
> sharing convention. The sharing convention tells metadata interpreters
> how to distinguish between metadata that they can interpret, and how to
> skip over other metadata.

What Josiah is hinting at -- and what Talin describes more explicitly
-- is the problem of how exactly "chaining" annotation interpreters
will work.

The case I've thought out the most completely is that of using
decorators to analyse/utilise the annotations:

1) Each decorator should be written with the assumption that it is the
only decorator that will be applied to a given function (with respect
to annotations).

2) Chaining will be accomplished by maintaining this illusion for each
decorator. For example, if our annotation-sharing convention is that
annotations will be n-tuples (n == number of annotation-interpreting
decorators), where t[i] is the annotation the i-th decorator should
care about, the following chain() function will do the trick (a full
demo script is attached):

>>> def chain(*decorators):
>>>     assert len(decorators) >= 2
>>>     def decorate(function):
>>>         sig = function.__signature__
>>>         original = sig.annotations
>>>         for i, dec in enumerate(decorators):
>>>             fake = dict((p, original[p][i]) for p in original)
>>>             function.__signature__.annotations = fake
>>>             function = dec(function)
>>>         function.__signature__.annotations = original
>>>         return function
>>>     return decorate

A similar function can be worked out for using dictionaries to specify
multiple annotations.

I'll update the PEP draft to include a section on guidelines for
writing such decorators.

Collin Winter
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