Convert IUPAC to SMILES and vice versa

convert.py

import urllib.error
import urllib.parse
import urllib.request

SMILES_URL_TEMPLATE = 'http://cactus.nci.nih.gov/chemical/structure/{}/smiles'
IUPAC_URL_TEMPLATE = 'http://cactus.nci.nih.gov/chemical/structure/{}/iupac_name'

def retrieve(url):
    with urllib.request.urlopen(url) as f:
        return f.read()

def iupac2smiles(iupac):
    quoted_iupac = urllib.parse.quote(iupac)
    smiles_url = SMILES_URL_TEMPLATE.format(quoted_iupac)
    return retrieve(smiles_url).decode('utf-8')

def smiles2iupac(smiles):
    quoted_smiles = urllib.parse.quote(smiles)
    iupac_url = IUPAC_URL_TEMPLATE.format(quoted_smiles)
    return retrieve(iupac_url).decode('utf-8')

if __name__ == '__main__':
    import sys
    convert = None
    src = None
    dst = None
    if sys.argv[1] == 'iupac2smiles':
        convert = iupac2smiles
        src = 'iupac'
        dst = 'smiles'
    elif sys.argv[1] == 'smiles2iupac':
        convert = smiles2iupac
        src = 'smiles'
        dst = 'iupac'
    else:
        print('Usage: python3 convert.py <iupac2smiles|smiles2iupac> < <filename>', file=sys.stderr)
        sys.exit(1)
    for name in map(str.strip, sys.stdin):
        try:
            print('[{src}] {original}\n= [{dst}] {converted}'.format(
                src=src, dst=dst, original=name, converted=convert(name)))
        except urllib.error.HTTPError as e:
            print('[{src}] {original}\n: error {error}'.format(
                src=src, original=name, error=e.code))