def _urls(repository, commit, mirrors):
    """Compute the urls from which an archive of the provided GitHub
    repository and commit may be downloaded.

     Args:
        repository: GitHub repository name in the form organization/project.
        commit: git revision for which the archive should be downloaded.
        mirrors: dictionary of mirrors, see mirrors.bzl in this directory for
            an example.
    """
    return [
        x.format(
            repository = repository,
            commit = commit,
        )
        for x in mirrors.get("github")
    ]