Have purge_unknown and allow_unknown play nice together

[audricschiltknecht]

Issue

(Unless I am missing something)
Setting purge_unknown on Validator (or top-level schemas) will ignore specific allow_unknown properties on fields.

The use-case is that while undeclared fields should be purged from the document, there are some of the sub-documents that have been explicitly tagged as permitting unknown fields, and these should not be purged.

Reproduce

• First example

>>> validator = Validator(purge_unknown=True)
>>> schema = {'foo': {'type': 'dict', 'allow_unknown': True}}
>>> document = {'foo': {'bar': True}, 'bar': 'foo'}
>>> validator.validated(document, schema)
{'foo': {}}

Expected:

{'foo': {'bar': True}}

• Second example

validator = Validator(purge_unknown=True)
schema = {'foo': {'type': 'dict', 'schema': {'bar': {'type': 'string'}}, 'allow_unknown': True}}
document = {'foo': {'bar': 'baz', 'corge': False}, 'thud': 'xyzzy'}
>>> validator.validated(document, schema)
{'foo': {'bar': 'baz'}}

Expected:

{'foo': {'bar': 'baz', 'corge': False}}

[audricschiltknecht]

I have implemented a small patch audricschiltknecht@07b61ed
If there is interest, I can create a pull-request for this.

[funkyfuture]

imo, it's reasonable to address and merge that.

this means that allow_unknown and purge_unknown should be declared as mutually exclusive, both in the validator of schemas and the configuration of a validator instance.

[audricschiltknecht]

@funkyfuture thanks for your comment.
However, I do not think that they should be mutually exclusive, but that allow_unknown will have precedence on purge_unknown where defined (this is our use-case). Am I making sense?

[funkyfuture]

yes, it makes sense. my intent is to avoid questions from users who are not aware of that behaviour by telling them upfront what they shouldn't expect to function as they may expect that.

[nicolaiarocci]

@audricschiltknecht please submit a PR for this. Make sure the docs are updated, maybe with the very same example you posted here, so users know what the intended behavior is.

[audricschiltknecht]

Ok, will do

[audricschiltknecht]

I've just created a PR. However, I could not run doc creation nor doctests because of an error raised by sphinx (neither using my virtualenv nor by running the docker script).