def liftover_bed(
    bed, 
    chain, 
    mapped=None, 
    unmapped=None,
    liftOver_path='liftOver',
):
    """
    Lift over a bed file using a given chain file. 

    Parameters
    ----------
    bed : str or pybedtools.BedTool
        Coordinates to lift over.
        
    chain : str
        Path to chain file to use for lift over.

    mapped : str
        Path for bed file with coordinates that are lifted over correctly.

    unmapped : str
        Path for text file to store coordinates that did not lift over
        correctly. If this is not provided, these are discarded.

    liftOver_path : str
        Path to liftOver executable if not in path.

    Returns
    -------
    new_coords : pandas.DataFrame
        Pandas data frame with lift over results. Index is old coordinates in
        the form chrom:start-end and columns are chrom, start, end and loc
        (chrom:start-end) in new coordinate system.
    """
    import subprocess
    import pybedtools as pbt
    if mapped == None:
        import tempfile
        mapped = tempfile.NamedTemporaryFile()
        mname = mapped.name
    else:
        mname = mapped
    if unmapped == None:
        import tempfile
        unmapped = tempfile.NamedTemporaryFile()
        uname = unmapped.name
    else:
        uname = unmapped
    if type(bed) == str:
        bt = pbt.BedTool(bed)
    elif type(bed) == pbt.bedtool.BedTool:
        bt = bed
    else:
        sys.exit(1)
    bt = bt.sort()
    c = '{} {} {} {} {}'.format(liftOver_path, bt.fn, chain, mname, uname)
    subprocess.check_call(c, shell=True)
    with open(uname) as f:
        missing = pbt.BedTool(''.join([x for x in f.readlines()[1::2]]),
                              from_string=True)
    bt = bt.subtract(missing)
    bt_mapped = pbt.BedTool(mname)
    old_loc = []
    for r in bt:
        old_loc.append('{}:{}-{}'.format(r.chrom, r.start, r.end))
    new_loc = []
    new_chrom = []
    new_start = []
    new_end = []
    for r in bt_mapped:
        new_loc.append('{}:{}-{}'.format(r.chrom, r.start, r.end))
        new_chrom.append(r.chrom)
        new_start.append(r.start)
        new_end.append(r.end)
    new_coords = pd.DataFrame({'loc':new_loc, 'chrom': new_chrom, 
                               'start': new_start, 'end': new_end},
                              index=old_loc)
    for f in [mapped, unmapped]:
        try:
            f.close()
        except AttributeError:
            continue
    return new_coords