Attempted to create a new commitlist but one exists already!

Raise code

    def from_existing(path):
        commits = CommitList.read_from_disk(path)
        return CommitList(path, commits)

    @staticmethod
    def create_new(path, base_version, new_version):
        if os.path.exists(path):
            raise ValueError('Attempted to create a new commitlist but one exists already!')
        commits = CommitList.get_commits_between(base_version, new_version)
        return CommitList(path, commits)

    @staticmethod
    def read_from_disk(path):
        with open(path) as csvfile:
            reader = csv.reader(csvfile)