Converting altaz to gal

How to convert altitude-azimuth coordinates to Galactic coordinates

import astropy
import astropy.coordinates as ac
import astropy.time as at
import astropy.units as u

### Set the location of antenna

antenna_location=ac.EarthLocation(lon=120*u.deg,lat=31*u.deg,height=4*u.m)

### Set observation time

obstime=at.Time(['2000-01-01 20:00:00'], scale='utc')

### Set local altaz coordinate frame

altaz_frame = ac.AltAz(alt=0*u.deg, az=0*u.deg, obstime=obstime,
                            location=antenna_location)

### converting altaz to galactic coordinates

g=altaz_frame.transform_to(ac.Galactic)
print(g.l, g.b)

[107d03m41.7999s] [0d08m07.4045s]

### Convert galactic coordinates to local altaz coordinates

g=ac.Galactic(l=0*u.deg, b=0*u.deg)
a=g.transform_to(altaz_frame)
print(a.alt, a.az)

[-26d48m29.0849s] [109d11m38.6094s]