required, additionalProperties, propertyNames, dependentRequired and any other validators that reference property *names* need a way to indicate the erroring names in ValidationErrors

[martinpengellyphillips]

Trying to figure out if there is a way to retrieve the name of a missing required property from an error (without hacking it out of error.message).

For example:

from jsonschema import Draft4Validator as Validator

schema = {
    'type': 'object',
    'properties': {
        'a': {
            'type': 'string'
        },
        'b': {
            'type': 'string',
            'pattern': '^hello$'
        }
    },
    'required': ['a', 'b']
}

errors = Validator(schema).iter_errors({'b': 'world'})
for error in errors:
    print error.path, error.schema_path, error.message

# deque([]) deque(['required']) 'a' is a required property
# deque(['b']) deque(['properties', 'b', 'pattern']) 'world' does not match '^hello$'

In error.path and error.schema_path there is no mention of 'a'. Is there a way to determine that the property missing was 'a' without using error.message?

[Julian]

Hey.

There isn't unfortunately. There should be. It just doesn't exist because I haven't thought of a nice way to expose it yet.

There's an informal agreement that .path and .schema_path should always refer to things that exist, so there is not the right place.

I've wavered back and forth on adding an extra_info attribute to ValidationError which could store arbitrary extra info for each failure as decided by a validator, and there would be a reasonable place. Say, required would set error.extra_info to something like {"attributes" : {"all", "the", "missing", "ones}}.

I'm willing to live with something like that I think.

[martinpengellyphillips]

Doesn't the property technically exist for .schema_path? As an error occurs for each missing property, it seems to me like that would be an appropriate place. So it would be schema_path == ['required', 'a'].

Adding the information elsewhere feels like it would make calling code more complicated (more code branches to deal with extra_info).

For context, I am building a GUI dynamically from the schemas and then validating the populated values as an object. From the errors I create an error tree that maps back to the GUI component structure and shows error icons (with tooltip) next to the correct field. For required properties I am choosing to display the error next to the missing field rather than at the group level, so this is why I need the property reference.

[Julian]

I see. Yeah, putting it in schema_path would be reasonable.

(as ["required", 0] though – assuming what you had was just a typo, but just to clarify, the guarantee is that someone can always pull out the element that is being referred to in the path attributes).

[martinpengellyphillips]

Ah, yes - I forgot it was an array not an object there.

So then the logic in my system would be to retrieve the associated schema from .schema and use .schema_path to find out that the property name was 'a'? If so, that sounds great.

[gazpachoking]

I agree this would be useful info to have. I'm not so sure about having it in schema_path though, as currently that always points to the json schema keyword, and I like that consistency. I sorta like the extra_info idea, as there are some other validators that could use it, for example, it would be nice to know with the additionalProperties validator which extra properties caused it to fail.

[martinpengellyphillips]

Another idea would be that validator_value becomes the single value being evaluated (validator_value == 'a'). Note sure if that breaks conventions though.

Happy with any solution that makes it easy to get at the appropriate value though.

On that note I have noticed that .schema refers to the original schema whilst .schema_path may be a subschema path so my previous idea of using .schema_path to index into .schema wouldn't work.

[gazpachoking]

Hmm, I think .schema_path should always be traversable in the .schema of any given error. In the instance of a subschema, the .schema of the error should be the subschema. Is that not the case?

[martinpengellyphillips]

I realise I actually meant the other way round, but, yeah, it doesn't seem to work. Have opened a new issue to avoid diverting this one (#120).

[ricobl]

I've came to this issue too, it would be nice to know what property caused an error, not only for the required validator.

Maybe a good approach is to return the JSON Path of the property:
http://goessner.net/articles/JsonPath/

[danmilon]

Bumped into this too. What's the way to go here? I can send a PR with what we decide to do.

[Julian]

I haven't looked at this carefully recently certainly, but I still don't really think there's a good way to do this without just doing ValidationError.extra_info

[alexanderad]

Voting +1 for this issue.

[fre-sch]

Having this issue as well.

I like the idea of adding the property name to error.schema_path.

Additionally I would suggest something like this:

 def required_draft4(validator, required, instance, schema):
     if not validator.is_type(instance, "object"):
         return
     for property in required:
         if property not in instance:
-            yield ValidationError("%r is a required property" % property)
+            yield ValidationError(
+                "%r is a required property" % property,
+                path=[property],
+            )

[martinpengellyphillips]

For reference, I currently use the following workaround:

# Custom validators
def _required(validator, required, instance, schema):
    '''Validate 'required' properties.'''
    if not validator.is_type(instance, 'object'):
        return

    for index, requirement in enumerate(required):
        if requirement not in instance:
            error = jsonschema.ValidationError(
                '{0!r} is a required property'.format(requirement)
            )
            error.schema_path.append(index)
            yield error


# Construct validator as extension of Json Schema Draft 4.
Validator = jsonschema.validators.extend(
    validator=jsonschema.validators.Draft4Validator,
    validators={
        'required': _required
    }
)

[abemusic]

👍 I too would like to pile on this issue. For the time being I will be looking into using the workaround above (thanks for providing that @thesociable)