Public API to check a declared attribute type and to check if it is mandatory/optional

[smarie]

Hello everyone,

First of all thanks a lot for this great library ! I am sad I found it only a couple days ago, otherwise I would probably not have spent time writing classtools-autocode. Indeed its main idea was to remove some of the boilerplate and link with PyContracts for validation.

Anyway, today I make this pull request because in another project of mine currently in development mode, parsyfiles, I perform some type introspection to understand what is required to parse an object. I use PEP484 type hints for that, but I wanted to support attrs too so I implemented something that does the job, and now I am proposing it to you.

The proposal is made of two functions : 'guess_type_from_validators', and 'is_mandatory'. Feel free to change the naming, I am not particularly good at finding great names.

For 'is_mandatory': it is simply checking if the validator is an instance of 'optional'.

For 'guess_type_from_validators': type information is extracted from the 'instance_of' validator. In order to help users rely on this validator instead of doing the same type checking in their custom validators, I introduce the 'chain' validator. Note that this might now be useless, since I saw in the source that you allow lists of validators.
In the future if issue 97 is resolved, the 'guess_type_from_validators' method should be updated to also use the new python 3.6 type hints as an additional source of type hint.

I added some unit tests for all the code, and updated the doc in api.rst.

Let me know what you think.

[hynek]

So the good news is that chain is not necessary anymore: as of the (unreleased) 17.1, you can pass a list of validators and it just works.

guess_type_from_validators is certainly very useful but I’m afraid a bit too hacky to be part of an official API (I do encourage anyone to release their own validators and other attrs-related tools tho! I just try to keep attrs slim and opinionated. I prefer no API in core over an API that contains the word guess :)).

is_mandatory opens two interesting questions:

1. optional() should be able to consume lists like validator= does. (filed as validators.optional should take lists like validator= #161)
2. where to put it? It’s more introspection than a validator…

[smarie]

Thanks for the quick review @hynek,
Agreed that you can get rid of chain.. if optional() is able to consume lists.
I will keep my 'hacky guess' method in the parsyfiles project, no problem about it not being part of the public attr API - indeed that was borderline.

[Tinche]

Pulling metadata from validators is something I've played around with and ultimately I think is just too hacky. It won't work with other validators, what about using converters instead of validators, etc.

@smarie I would advise using metadata (the proper, attrs metadata available on Attribute instances).

[hynek]

Yeah so what Tin said. Thanks for sharing your work but this is nothing for attrs proper.