For example you have a piece of code that runs through and performs an operation on each item in a list.
items = [ 'hello', 'there', 'blah' ]
for i in range(len(items)):
items[i] = items[i].replace('e', 'i')
You can turn it into a 1 line wonder by changing it to:
items = map(lambda x: x.replace('e', 'i'), items)
Or you could just use what triclops mentioned which is MUCH simpler...
items = [word.replace('e', 'i') for word in items]