Misuse of list comprehensions?

[John Salerno]

I posted this code last night in response to another thread, and after I
posted it I got to wondering if I had misused the list comprehension. Here's
the two examples:

Example 1:
--------------------
def compress(s):
new = []

for c in s:
if c not in new:
new.append(c)
return ''.join(new)
----------------------

Example 2:
------------------------
def compress(s):
new = []
[new.append(c) for c in s if c not in new]
return ''.join(new)
--------------------------

In example 1, the intention to make an in-place change is explicit, and it's
being used as everyone expects it to be used. In example 2, however, I began
to think this might be an abuse of list comprehensions, because I'm not
assigning the result to anything (nor am I even using the result in any
way).

What does everyone think about this? Should list comprehensions be used this
way, or should they only be used to actually create a new list that will
then be assigned to a variable/returned/etc.?

[Ian Kelly]

On Tue, May 27, 2008 at 8:08 PM, Gabriel Genellina
<ga*******@yaho o.com.arwrote:

En Tue, 27 May 2008 14:43:52 -0300, Ian Kelly <ia*********@gm ail.com>
escribió:

>It sounds like the wasteful list creation is the biggest objection to
using a list comprehension. I'm curious what people think of this
alternative, which avoids populating the list by using a generator
expression instead (apart from the fact that this is still quadratic,
which I'm aware of).

def compress(s):
new = []
filter(None, (new.append(c) for c in s if c not in new))
return ''.join(new)

filter returns a newly created list, so this code is as wasteful as a list
comprehension (and harder to read).

Here it returns a newly created *empty* list, which is not nearly as
wasteful as one that's linear in the size of the input. I'll grant
you the second point, though. I very much doubt that I would ever
actually use this myself. I was just curious what others would think
of it.

[Lie]

On May 20, 8:51*pm, "Diez B. Roggisch" <de...@nospam.w eb.dewrote:

bearophileH...@ lycos.com wrote:

John Salerno:

What does everyone think about this?

The Example 2 builds a list, that is then thrown away. It's just a
waste of memory (and time).

No, it doesn't. It uses append because it refers to itself in the
if-expression. So the append(c) is needed - and thus the assignment
possible but essentially useless.

Diez

Yes it does, it build a list of 'None's.

And if list.append is concerned, the example given has no use, since:
x = [c for c in cs]
is essentially the same as
x = []
[x.append(c) for c in cs]

If, you're talking about other function calls, it might be an abuse or
not depending on personal preference.