I made it like this, you'll probably have to iterate through all posts until data
is empty. Note this is Python 2.x version.
from facepy import GraphAPI
import json
group_id = "YOUR_GROUP_ID"
access_token = "YOUR_ACCESS_TOKEN"
graph = GraphAPI(access_token)
# https://facepy.readthedocs.org/en/latest/usage/graph-api.html
data = graph.get(group_id + "/feed", page=False, retry=3, limit=800)
with open('content.json', 'w') as outfile:
json.dump(data, outfile, indent = 4)