def formfield(self, **kwargs):
        defaults = {'form_class': BlockField, 'block': self.stream_block}
        defaults.update(kwargs)
        return super(StreamField, self).formfield(**defaults)
Override formfield to use a plain forms.Field so that we do no transformation on the value
dataset/ETHPy150Open torchbox/wagtail/wagtail/wagtailcore/fields.py/StreamField.formfield