python convert timezone.py

from datetime import datetime, timezone, timedelta


def convert_timezone(dtm, z='utc'):
    t_utc = (dtm - (dtm.utcoffset() or timedelta()))
    t_utc = t_utc.replace(tzinfo=timezone.utc)
    z = z.lower().replace('utc', '').replace(' ', '').replace(':', '')
    if not z:
        return t_utc
    if len(z) == 3:
        z += '00'
    t_z = datetime.strptime(z, '%z')
    t = (t_utc + t_z.utcoffset()).replace(tzinfo=t_z.tzinfo)
    return t

s = 'Fri, 18 Sep 2015 20:32:59 +0200'
f = '%a, %d %b %Y %H:%M:%S %z'
t = datetime.strptime(s, f)

for (s, zz) in sorted({
    '2015-09-18T18:32:59+00:00': ',UTC,+0000,utc,UtC',
    '2015-09-18T19:32:59+01:00': 'UTC+0100,+0100,+01',
    '2015-09-18T16:32:59-02:00': 'UTC-0200,-0200,-02',
}.items()):
    for z in sorted(set(zz.split(','))):
        x = convert_timezone(t, z).isoformat()
        assert s == x, (z, s, x)
        print(repr(z))