Python - storing persistance objects in file with shelve

# The shelve module is used to store objects in a file. 
# You use the file like a glorified dict with key, value 
# pairs. 
import shelve
# shelve python doc
objList = []
filename = 'shelveFile.shelve' 
 
# open and or create the file 
file = shelve.open(filename)
 
# Here is an example class we'll create 
# instances of and then store in the file 
class ExampleClass(object):
    def __init__(self):
        self.a = 0
        self.b = 1
        self.c = 2
        self.k = 0
    def getTotal(self):
        return self.a + self.b + self.c
 
# create several instances 
for i in xrange(3):
    obj = ExampleClass()
    obj.k = i
    obj.a = i+1
    obj.b = i+2
    obj.c = i+3
    objList.append(obj)
 
# now add the objects to file object 
for i in objList:
    # keys are strings 
    file[str(i.k)] = i
 
# The sync command will explicitly 
# write changes to file 
file.sync()
 
# Closing the object will also execute 
# the sync command 
file.close()
 
# The file (and the 3 objects in it 
# are now saved. 
# Now we'll reopen and verify the data is there 
file2 = shelve.open(filename)
 
# Iterate through and print out 
# the object attributes (to verify 
# they are the values we assigned previously) 
for i in file2.keys():
    j = file2[str(i)]
    print "a,b,c,k = ", j.a, j.b, j.c, j.k
#output: 
#a,b,c,k =  1 2 3 0 
#a,b,c,k =  3 4 5 2 
#a,b,c,k =  2 3 4 1 
 
# You can edit these values. 
# Here will change all 'a' attributes to 7 
for i in file2.keys():
    # Take note of how these changes were made. 
    # You cannot merely alter an attribute 
    # like file2[str(i)].a = 7 (this will 
    # not work). 
    j = file2[str(i)]
    j.a = 7
    file2[str(j.k)] = j
 
# And verify that changes are made: 
for i in file2.keys():
    j = file2[str(i)]
    print "a,b,c,k = ", j.a, j.b, j.c, j.k
#output: 
#a,b,c,k =  7 2 3 0 
#a,b,c,k =  7 4 5 2 
#a,b,c,k =  7 3 4 1 
 
 
 
# now close the shelve file so you can 
# use the data objects another day. 
file2.close()