counts = {'obs syn':#, 'pos syn':#, 'obs non-syn':#, 'pos non-syn':#}

def calc_PnPs(counts):
    """
    counts = {'obs syn':#, 'pos syn':#, 'obs non-syn':#, 'pos non-syn':#}
    """
    if counts['pos non-syn'] == 0 or counts['pos syn'] == 0:
        return 'n/a'
    NonSyn = float(counts['obs non-syn'])/float(counts['pos non-syn'])
    Syn    = float(counts['obs syn'])    /float(counts['pos syn'])
    if Syn == 0:
        return 'n/a'
    return NonSyn/Syn