[Tutor] list comprehensions

[Christer Edwards]

I've been studying python now for a few weeks and I've recently come
into list comprehensions. Some of the examples that I've found make
sense, and I find them readable and concise. In particular I'm
referring to the python docs on the topic
(http://docs.python.org/tutorial/datastructures.html#list-comprehensions).
Those make sense to me. The way I understand them is:

do something to x for each x in list, with an optional qualifier.

On the other hand I've seen a few examples that look similar, but
there is no action done to the first x, which confuses me. An example:

print sum(x for x in xrange(1,1000) if x%3==0 or x%5==0)

In this case is sum() acting as the "action" on the first x? Can
anyone shed some light on what it is I'm not quite grasping between
the two?

Christer