def _find_by_sha1(self, sha1):
        """
        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.
        """
        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
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.