JSON field for WTForms that converts between the form string data and a dictionary representation, with validation

json_field.py

from wtforms import fields
import json


class JSONField(fields.StringField):
    def _value(self):
        return json.dumps(self.data) if self.data else ''

    def process_formdata(self, valuelist):
        if valuelist:
            try:
                self.data = json.loads(valuelist[0])
            except ValueError:
                raise ValueError('This field contains invalid JSON')
        else:
            self.data = None

    def pre_validate(self, form):
        super().pre_validate(form)
        if self.data:
            try:
                json.dumps(self.data)
            except TypeError:
                raise ValueError('This field contains invalid JSON')