RFC: Remove `fields` attribute from ValidationError

Currently, ValidationError stores the field name and field instances associated with the error.

from marshmallow import Schema, fields

class MySchema(Schema):
    foo = fields.Int()

schema = MySchema()
try:
    schema.load({'foo': 'invalid'})
except Exception as error:
    print(error.fields)  # [<fields.Integer(...)>]

I propose removing the fields because it unnecessarily complicates the validation logic.

fields and field_names were added at a time (version 1.1.0 😓 ) when it was impossible to access all the error messages for a schema (see #63). It wasn’t until 2.0 that it became possible to get the full error messages dict when using strict mode.

At this point, I don’t see a use case for accessing ValidationError.fields.

[sloria]

Good catch, @lafrech . I’ll get your webargs patch merged and released.

Once that’s in, I think @decaz 's use case can be met with something like

from flask import Flask, jsonify
from webargs.flaskparser import use_kwargs
from marshmallow import fields


app = Flask(__name__)


@app.errorhandler(422)
def handle_422(error):
    errors = []
    schema = error.data['schema']
    client_keys_to_fields = {
        field.data_key or field_name: field
        for field_name, field in schema.fields.items()
        if not field.dump_only
    }
    for field_name in error.exc.messages:
        field = client_keys_to_fields[field_name]
        error_msg = f'Invalid field {field.name}'
        location = field.metadata.get('location')
        if location:
            error_msg += f' at {location}'
        errors.append(error_msg)
    return jsonify(errors)


@app.route('/')
@use_kwargs({'x': fields.Int(location='query')})
def index(x):
    return jsonify(x)


if __name__ == "__main__":
    app.debug = True
    app.run()

Edit: clean up

[sloria]

@decaz It’s released now.