'Generate the (hidden) input field. Should be used in __call__.
@param field: the Field
@param values: the parsed value (as dict)
@param classes: standard HTML classes
@param attributes: the widget attributes as passed in to the widget
@return: the INPUT field'

def inputfield(self, field, values, classes, **attributes):

    if isinstance(classes, (tuple, list)):
        _class = '    '.join(classes)
    else:
        _class = classes
    requires = self.postprocess
    fieldname = str(field).replace('.', '_')
    if fieldname.startswith('sub_'):
        from s3forms import SKIP_POST_VALIDATION
        requires = SKIP_POST_VALIDATION(requires)
    defaults = dict(requires=requires, _type='hidden', _class=_class)
    attr = FormWidget._attributes(field, defaults, **attributes)
    return INPUT(_value=self.serialize(values), **attr)
