How to use the sgp4.earth_gravity function in sgp4

# satrec.epoch_datetime = TLE.epoch_datetime # Python datetime

    # Pass the source tle_id through the SGP4 class variable, for TLE genealogy
    satrec.parent_tle_id = 0 # int(TLE.tle_id)

    # SGP4 mode variables
    satrec.operationmode  = u'i' # Unicode for cython
    satrec.error          = 0

    if (gravconst == "wgs72old"):
        whichconst = earth_gravity.wgs72old
    elif (gravconst == "wgs84"):
        whichconst = earth_gravity.wgs84
    else:
        # Most popular const used by TLEs
        whichconst = earth_gravity.wgs72
    # satrec.whichconst     = whichconst  # Python extension: remembers its consts
    # satrec.whichconst = gravconst
    rtn_code = sgp4init("wgs72", satrec.operationmode, satrec.satnum, 
             satrec.jdSGP4epoch, # epoch time in days from jan 0, 1950. 0 hr
             satrec.bstar, satrec.ndot, satrec.nddot, satrec.ecco, satrec.argpo, 
             satrec.inclo, satrec.mo, satrec.no_kozai, satrec.nodeo, satrec)
    if (rtn_code is not True):
        if (satrec.error == 1):
            log.error("sgp4init error {}".format(satrec.error))
            log.error("mean elements, ecc >= 1.0 or ecc < -0.001 or a < 0.95 er")
            return False
        elif (satrec.error == 2):
            log.error("sgp4init error {}".format(satrec.error))
            log.error("mean motion less than 0.0")
            return False
        elif (satrec.error == 3):

# TODO: Once mean_motion_radians_per_minute is added to the DB, use it directly here
    satrec.no_kozai       = TLE.mean_motion_orbits_per_day * nocon # rad/min
    satrec.revnum         = TLE.orbit_num

    # Derived quantities
    satrec.jdsatepoch     = TLE.jdsatepoch  # Julian date
    satrec.epoch          = TLE.epoch       # Python datetime

    # SGP4 mode variables
    satrec.operationmode  = False
    satrec.error          = 0

    if (gravconst == "wgs72old"):
        whichconst = earth_gravity.wgs72old
    elif (gravconst == "wgs84"):
        whichconst = earth_gravity.wgs84
    else:
        # Most popular const used by TLEs
        whichconst = earth_gravity.wgs72
    satrec.whichconst     = whichconst  # Python extension: remembers its consts

    # FIXME: Can use jdSGP4epoch here directly with no subctraction, but need to trace all uses  (like the datetime comment above)
    rtn_code = sgp4init(satrec.whichconst, satrec.operationmode, satrec.satnum, 
             satrec.jdsatepoch-2433281.5, # epoch time in days from jan 0, 1950. 0 hr
             satrec.bstar, satrec.ndot, satrec.nddot, satrec.ecco, satrec.argpo, 
             satrec.inclo, satrec.mo, satrec.no_kozai, satrec.nodeo, satrec)
    if (rtn_code is not True):
        if (satrec.error == 1):
            log.error("sgp4init error {}".format(satrec.error))
            log.error("mean elements, ecc >= 1.0 or ecc < -0.001 or a < 0.95 er")
            return False
        elif (satrec.error == 2):