configuration $=$ AppConfig$($reload$=$True$)$

env.web$2$py$\_$runtime$\_$gae:

A If NOT running on Google App Engine use SQLite or other DB

db $=$ DAL$("$mysql:$//$root:root@localhost$/$test$?$set$\_$encoding$=$utf$8$mb$4",$

migrate$\_$enabled$=$False$)$

else:

$$

# Connect to Google BigTable $($optional 'google:datastore:$//$namespace$\text{'})$

#

$db=$ DAL$(\text{'}$google:datastore$+$ndb$\text{'})$

# store sessions and tickets there

#

session. connect $($ request, response, $db=db)$

# or store session in Memcache; Redis, etc.

# from gluon.contrib.memdb import MEMDB

# from google.appengine. api.memcache import client

# session. connect $($request$,$ response, $db=$ MEMDB$($client $()))$

#

db$.$ define$\_$table$(\text{'}$authors$\text{'},$

Field$(\text{'}$id$\text{'},$ 'integer'$),$

Field$(\text{'}$name$\text{'},$ requires$=$IS$\_$NOT$\_$EMPTY$()),$

Field$(\text{'}$bio$\text{'},$ 'text'$),$

Field$(\text{'}$email$\text{'},$ requires$=$IS$\_$EMAIL$()),$

db$.$ def ine$\_$table$(\text{'}$books$\text{'},$

Field$(\text{'}$id$\text{'},$ 'integer'$),$

Field$(\text{'}$titie$\text{'},$ requires'IS$\_$NOT$\_$EMPTY$()),$

Field$(\text{'}$author$\text{'},$ 'reference authors'$),$

Field$(\text{'}$publication$\_$year', 'integer', requires$=$IS$\_$INT$\_$IN$\_$RANGE$(1000,3000)),$

Field$(\text{'}$summary$\text{'},$ 'text'$)\text{'},$

In web$2$py $,$ what is the purpoes of the field class in table definitions?