No viewers found for '%s' output format.

Raise code

 candidate in candidate_viewers:
                path = shutil.which(candidate)
                if path is not None:
                    viewer = path
                    break
            else:
                raise OSError(
                    "No viewers found for '%s' output format." % output)
    else:
        if viewer == "file":
            if filename is None:
                raise ValueError("filename has to be specified if viewer=\"file\"")
        elif viewer == "BytesIO":
            if outputbuffer is None: