Returns a string of space-separated CSS classes for the wrapping element of this input field.

def css_classes(self, extra_classes=None):
        """
        Returns a string of space-separated CSS classes for the wrapping element of this input field.
        """
        if hasattr(extra_classes, 'split'):
            extra_classes = extra_classes.split()
        extra_classes = set(extra_classes or [])
        # field_css_classes is an optional member of a Form optimized for django-angular
        field_css_classes = getattr(self.form, 'field_css_classes', None)
        if hasattr(field_css_classes, 'split'):
            extra_classes.update(field_css_classes.split())
        elif isinstance(field_css_classes, (list, tuple)):
            extra_classes.update(field_css_classes)
        elif isinstance(field_css_classes, dict):
            extra_field_classes = []
            for key in ('*', self.name):
                css_classes = field_css_classes.get(key)
                if hasattr(css_classes, 'split'):
                    extra_field_classes = css_classes.split()
                elif isinstance(css_classes, (list, tuple)):
                    if '__default__' in css_classes:
                        css_classes.remove('__default__')
                        extra_field_classes.extend(css_classes)
                    else:
                        extra_field_classes = css_classes
            extra_classes.update(extra_field_classes)
        return super(NgBoundField, self).css_classes(extra_classes)