cts is e.g. {"Germany" : "DEU"}. inv_cts is the inverse: {"DEU" : "Germany"}

def make_inv_cts(cts):
    """
    cts is e.g. {"Germany" : "DEU"}. inv_cts is the inverse: {"DEU" : "Germany"}
    """
    inv_ct = {}
    for old_k, old_v in cts.items():
        if old_v not in inv_ct.keys():
            inv_ct.update({old_v : old_k})
    return inv_ct