How to use RestAPI(cURL)... Function of "LocalLB::Rule::get_description"

You may want to consider using the Python SDK. In that case, 'description' is a property of any object that has one. For example:

!/usr/bin/env python3

from f5.bigip import ManagementRoot
from getpass import getpass
from pprint import pprint

hostname = 'my.f5.ltm.net'
username = 'foo'

mgmt = ManagmentRoot(hostname, username, getpass())

virtual = mgmt.tm.ltm.virtuals.virtual.load(
    partition='Common',
    name='my_virtual_server',
)

if hasattr(virtual, 'description'):
    print(virtual.description)
else:
    print('Virtual server does not have a description.')