class VariableForm(Form):

    def __new__(cls, questions, **kwargs):
        for index, question in enumerate(questions):
            field_name = 'question_{}'.format(index)
field = RadioField(question, validators=[Required()], choices=CHOICES)
setattr(cls, field_name, field)
return super(VariableForm, cls).__new__(cls, **kwargs)
WTForms создает переменное количество полей