code: def find(cls, key=None, **kwargs):
          
          if not key:
              return super(Asset, cls).find(**kwargs)
  
          params = {"asset[key]": key}
          params.update(kwargs)
          theme_id = params.get("theme_id")
          path_prefix = "%s/themes/%s" % (cls.site, theme_id) if theme_id else cls.site
  
          resource = cls.find_one("%s/assets.%s" % (path_prefix, cls.format.extension), **params)
  
          if theme_id and resource:
              resource._prefix_options["theme_id"] = theme_id
          return resource
docs: Find an asset by key
          E.g.
              shopify.Asset.find('layout/theme.liquid', theme_id=99)