[Tutor] lazy? vs not lazy? and yielding

[Andreas Kostyrka]

Am Mittwoch, 3. März 2010 21:41:34 schrieb Dave Angel:
> John wrote:
> > Hi,
> >
> > I just read a few pages of tutorial on list comprehenion and generator
> > expression.  From what I gather the difference is "[ ]" and "( )" at the
> > ends, better memory usage and the something the tutorial labeled as "lazy
> > evaluation".  So a generator 'yields'.  But what is it yielding too?
> >
> > John
> 
> A list comprehension builds a whole list at one time.  So if the list
> needed is large enough in size, it'll never finish, and besides, you'll
> run out of memory and crash.  A generator expression builds a function
> instead which *acts* like a list, but actually doesn't build the values

Well, it act like an iterable. A list-like object would probably have 
__getitem__ and friends. Historically (as in I doubt you'll find a version 
that still does it that way in the wild, I looked it up, guess around 2.2 
iterators were introduced), the for loop in python has been doing a "serial" 
__getitem__ call.

Nowadays,

for i in x:
    print i

does something like this:

x_it = iter(x)   # get an iterator for x
try:
    while True:
       i = x_it.next()
       print i
except StopIteration:
    pass

As you can see, x_it (and the underlying x) do not need to provide all items 
in memory at the same time.

Andreas