In python reading items in a list one by one is iteration but when iteration on a list can be done once then iteration is known as generators. Generators does not allow all the iterable list values to be stored in memory therefore that list can be iterated only once. Generator list is to be enclosed within (...).
Program in python to demonstrate Generators:
mygenerator = (x*x for x in range(3))
for i in mygenerator: //loop over generator
Now writing below code many times will generate the list only once because generators does not allow all the iterable list values to be stored in memory i.e values generated by generators on the fly.