conditional normalization using oneof

Used Cerberus version / latest commit: current master

• I have the capacity to improve the docs when my problem is solved.
• I have the capacity to submit a patch when a bug is identified.
• My question does not concern a practical use-case that I can’t figure out to solve.

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Use-case abstract

So far Cerberus does not allow for conditional default values which is supported by other validators (e.g. in Schema using Or ). Here pledge for a new feature that adds the possibility to define more complex defaults i.g. where the default value depends on another field. This feature can be added without any new rule simply by considering oneofduring normalization.

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Feature request

It would be nice to consider oneof also for normalization. While other of-rules (anyof, noneof, alloff) are not unambiguous in the context of normalization oneof instead could be used to insert conditional default values.

e.g.

    schema = {
        'foo': {'type': 'string'},
        'bar' :{'oneof':[{'dependencies':{"foo":"B"}, 'default':"B"}, {'dependencies':{"foo":"A"}, 'default':"C"}]}
    }
    document = {'foo': 'A'}
    expected_normalization_result = {'foo': 'A','bar':"C"}

[linupi]

I am not aware of any progress on this, For the moment we are using a patched Validator that does oneof normalization that is somehow customized to our project but of cause it would be great to have this sort of possibility in cerberus itself.

In case you want to have a look, here is what we use: https://gitlab.esrf.fr/bliss/bliss/-/blob/master/bliss/common/validator.py https://gitlab.esrf.fr/bliss/bliss/-/blob/master/tests/common/test_validator.py

[linupi]

To me Cerberus is already quite a strong tool that manages to deal with rather complex problems using a simple grammar, that is one of the reasons why I really like Cerberus.

Looking at my proposal I, currently define a schema like this:

schema = {
    'foo': {'type': 'string'},
    'bar': {
        'oneof': [
            {'dependencies': {"foo": "B"}, 'default': "B"},
            {'dependencies': {"foo": "A"}, 'default': "C"},
        ]
    },
}

how would you like if the normalization role of oneof would be more explicit e.g. by defining a default_setter like this:

schema = {
    'foo': {'type': 'string'},
    'bar': {
        'oneof': [
            {'dependencies': {"foo": "B"}, 'default': "B"},
            {'dependencies': {"foo": "A"}, 'default': "C"},
        ]
    },
    default_setter = 'oneof'           # explicitly defining that oneof is setting the default
}

like this the there is a clear cut between the role of ‘oneof’ in the validation phase and in the normalization phase. I actually would have liked to go this way, but unfortunately the signature of the function called by _normalize_default_setter does not allow for the transmission of a schema. Would it be imaginable to extend the signature of the setter called in _normalize_default_setter to something like mapping, schema, field instead of just mapping (at least in the case where the setter is provided as extension of the validator and not as an external callable)? In that case I could rewrite my proposal … if it is considered as potentially sustainable…