"""
    Provides a django form field to handle a signature capture field with
    with jSignature jQuery plugin
"""
import json
from django.forms.fields import Field
from django.core import validators
from django.core.exceptions import ValidationError
from .widgets import JSignatureWidget

JSIGNATURE_EMPTY_VALUES = validators.EMPTY_VALUES + ('[]', )


class JSignatureField(Field):
    """
    A field handling a signature capture field with with jSignature
    """
    widget = JSignatureWidget()

    def to_python(self, value):
        """
        Validates that the input can be red as a JSON object.
        Returns a Python list (JSON object unserialized).
        """
        if value in JSIGNATURE_EMPTY_VALUES:
            return None
        try:
            return json.loads(value)
        except ValueError:
            raise ValidationError('Invalid JSON format.')