code: def _find_by_sha1(self, sha1):
          
          for image_part in self:
              # ---skip unknown/unsupported image types, like SVG---
              if not hasattr(image_part, 'sha1'):
                  continue
              if image_part.sha1 == sha1:
                  return image_part
          return None
docs: Return an |ImagePart| object belonging to this package or |None| if
          no matching image part is found. The image part is identified by the
          SHA1 hash digest of the image binary it contains.