def seq_format_from_suffix(suffix):
    
    """
    Guesses input format from suffix
    
    >>> print(seq_format_from_suffix('gb'))
    genbank
    """
    
    suffixes = {'fasta': ['fas','fasta','fa','fna'],
                'genbank': ['gb','genbank'],
                'embl': ['embl']}
    found = False
    for key in suffixes.keys():
        if suffix in suffixes[key]:
            found = True
            return key
    if not found:
        raise RuntimeError(suffix+' is not a recognised suffix of an unaligned sequence file')