Python: Essential Reference, Third Edition

Generator Expressions

A generator expression is an object that performs the same kind of function as a list comprehension. The syntax is the same as for list comprehensions except that you use parentheses instead of square brackets. For example:

						
(expression for item1 in iterable1
            for item2 in iterable2
            ...
            for itemN in iterableN
            if condition )

Unlike a list comprehension, a generator expression does not actually create a list or immediately evaluate the expression inside the parentheses. Instead, it creates a generator object that produces the values on demand via iteration. For example:

>>> a = [1, 2, 3, 4]
>>> b = (10*i for i in a)
>>> b
<generator object at 0x590a8>
>>> b.next()
10
>>> b.next()
20
...

The difference between list and generator ...