[salt-users] Accessing existing grains from a custom grain?

I need the ability to access existing grains that are defined by the
core.py grains file as well as those defined in /etc/salt/grains in my
custom grain. This custom grain runs on a Solr node and queries zookeeper
from the local API to return the clusterstate.json and from there parse
whether the node is active or failed or a shard leader, etc. Except that I
need to access grains that are already defined in the core.py as well as
set in /etc/salt/grains in order to properly parse the data in the json.
  here's a forced example, it seems that __grains__ is not available in
custom grains as with custom modules. What lib must I import to have
access to already defined grains? Also, will this custom grain always be
parsed on the fly whenever there's a grains.get/grains.item call? I want
the grain to contain the latest information in the clusterstate.json that
it pulls from Solr/zookeeper. For example, I would want to define a custom
grain for "solrleader" that always reflects in real time whether that
minion is the leader in a SolrCloud shard as defined by grabbing and
parsing the clusterstate.json via the Solr API, and doing so requires
knowing things like the collection name and shard is as defined in
/etc/salt/grains. Is this a chicken and egg problem?

#!/usr/bin/env python

import urllib2
import json
import socket

localhost= __grains__['id']
#localhost = socket.gethostname()

def getsolrcluster():
   grains = {}
   url =
"http://"+localhost+":8080/solr/zookeeper?detail=true&path=%2Fclusterstate.json"
   try:
     response = urllib2.urlopen(url)
   except:
     grains['solrclusterstate']="Exception thrown. No Solr detected at
http://%s:8080/solr"
   raw = response.read()
   data = json.loads(raw)
   grains['solrclusterstate']=data["znode"]["data"]
   return grains


And when I try to import the custom grain:

minion# salt-call saltutil.sync_grains
[WARNING ] Failed to import grain solrgrains, this is due most likely to a
syntax error. Traceback raised:
Traceback (most recent call last):
   File "/usr/lib/python2.6/site-packages/salt/loader.py", line 925, in
load_modules
     ), fn_, path, desc
   File "/var/cache/salt/minion/extmods/grains/solrgrains.py", line 7, in
<module>
     localhost= __grains__['id']
NameError: name '__grains__' is not defined