changedetection.io/changedetectionio/conditions/form.py

# Condition Rule Form (for each rule row)
from wtforms import Form, SelectField, StringField, validators
from wtforms import validators
from flask_babel import lazy_gettext as _l

from changedetectionio.conditions import get_field_choices, get_operator_choices


class ConditionFormRow(Form):

    # Callables, so the plugin-provided choices are collected each time a form is built
    field = SelectField(
        _l("Field"),
        choices=get_field_choices,
        validators=[validators.Optional()]
    )

    operator = SelectField(
        _l("Operator"),
        choices=get_operator_choices,
        validators=[validators.Optional()]
    )

    value = StringField(_l("Value"), validators=[validators.Optional()], render_kw={"placeholder": _l("A value")})

    def validate(self, extra_validators=None):
        # First, run the default validators
        if not super().validate(extra_validators):
            return False

        # Custom validation logic
        # If any of the operator/field/value is set, then they must be all set
        if any(value not in ("", False, "None", None) for value in [self.operator.data, self.field.data, self.value.data]):
            if not self.operator.data or self.operator.data == 'None':
                self.operator.errors.append(_l("Operator is required."))
                return False

            if not self.field.data or self.field.data == 'None':
                self.field.errors.append(_l("Field is required."))
                return False

            if not self.value.data:
                self.value.errors.append(_l("Value is required."))
                return False

        return True  # Only return True if all conditions pass