def sample_ids(file_fp):
    """Return list of sample IDs parsed from alpha diversity file.

    Parameters
    ----------
    file_fp: string
        filepath to alpha diversity file (output of alpha_diversity.py)

    Returns
    -------
    sample_ids: list
        list of sample IDs
    """
    sample_ids = []
    # read alpha div file
    with open(file_fp) as file_f:
        next(file_f)
        for line in file_f:
            line = line.strip().split('\t')
            sample_id = line[0].lower()
            if sample_id not in sample_ids:
                sample_ids.append(sample_id)
            else:
                raise ValueError(
                    "Duplicate sample ID %s in %s" % (sample_id, file_fp))
    return sample_ids