Python: jsonschema package to validate schema and custom error messages

Question

Given this schema:

{
    "type": "object",
    "properties": {
        "account": {
            "type": "object",
            "required": ["id"],
            "properties": {
                "id": {"type": "number"}
            }
        },
        "name": {"type": "string"},
        "trigger": {
            "type": "object",
            "required": ["type"],
            "properties": {
                "type": {"type": "string"}
            }
        },
        "content": {
            "type": "object",
            "properties": {
                "emails": {
                    "type": "array",
                    "minItems": 1,
                    "items": {
                        "type": "object",
                        "required": ["fromEmail","subject"],
                        "properties": {
                            "fromEmail": {"type": "string", "format": "email"},
                            "subject": {"type": "string"}
                        }
                    }
                }
            }
        }
    }
}

I am trying to use jsonschema.Draft4Validator to validate a POSTed JSON object to check for validity, but I am having some issues trying to come up with better human readable messages from the returned errors.

Here is how I am validating:

from jsonschema import Draft4Validator

v = Draft4Validator(self.schema)
errors = sorted(v.iter_errors(autoresponder_workflow), key=lambda e: e.path)

if errors:
    print(', '.join(
        '%s %s %s' % (error.path.popleft(), error.path.pop(), error.message) for error in errors
    ))

The error message looks like:

content emails [] is too short, trigger type None is not of type u'string'

Im trying to create an error message that looks a little more like Please add at least one email to your workflow," "Please ensure that all of your emails contain subject lines,", etc

Answer 1

I had the need of displaying a more detailed custom message to the user when an error occurred. I did this by adding a field to the property in the schema and then looking it up on ValidationError.

import json
import jsonschema


def validate(_data, _schema):
    try:
        jsonschema.validate(_data, _schema)
    except jsonschema.ValidationError as e:
        return e.schema["error_msg"] 


schema = {
    "title": "index",
    "type": "object",
    "required": [
        "author",
        "description"
    ],
    "properties": {
        "author": {
            "type": "string",
            "description": "Name of the Author",
            "minLength": 3,
            "default": "",
            "error_msg": "Please provide a Author name"
        },
        "description": {
            "type": "string",
            "description": "Short description",
            "minLength": 10,
            "default": "",
            "error_msg": "Please add a short description"
        }
    }
}

data = {
    "author": "Jerakin",
    "description": ""
}

Answer 2

You could catch ValidationError exception and build a custom messages for the specific cases you want from ValidationError metadata. In this object you have:

info of the failed validator: its value and schema path info of the failed instance: its path and value at the time of the failed validation possible errors in subschemas cause (errors caused by a non-validation error)