Added validator_for to provide direct access to validators

[gonvaled]

Very simple implementation. No test case added since it is covered by TestValidate.

[gazpachoking]

Maybe it should take a default keyword arg rather than have Draft4Validator hard set as the default?

[gonvaled]

Draf4Validator was the default before, and stays as default in this new commit. Anyway: how are you going to provide a default? Do you really want to import Draft4Validator in client code? Or maybe provide a new validator class?

It would not be difficult to do, though:

def validator_for(schema, default_validator=Draft4Validator):
    return meta_schemas.get(schema.get("$schema", ""), default_validator)

I can do that if required.

[gazpachoking]

Yeah, I was just thinking that if we were making validator_for public now maybe it should be an option. I'll defer to whatever @Julian things though.

[Julian]

The reason default is good is because Draft4Validator is an arbitrary default, not one specified by the spec (right now it's just because it's the newest version, but if draft 5 is released tomorrow [not likely] we'd still likely have draft 4 as the default for backwards compat for a while), and clients should be expected to make that decision as well.

There probably is a secondary use case here for "I want the default that jsonschema would have provided", and I think None is unfortunately going to be a thing someone might actually want as a default for "I can't figure it out", so maybe validator_for(schema, default=_unset) would be the thing.

Also it needs at least two tests (one for when a validator is found from $schema, and one for when it isn't, and now I guess one for _unset) :D.

And it'd be nice if even though there isn't any prose there now if you added at least an .. autofunction validator_for:: to the docs in docs/creating.rst, since I think at the moment that will probably be the most likely place for it to be documented.

Thanks for working on this!

[gonvaled]

Maybe you mean something like this: #113