Misuse of list comprehensions?

[Paul McGuire]

On May 20, 8:13 am, "John Salerno" <johnj... at NOSPAMgmail.com> wrote:
> 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.?

Why not make the list comp the actual list you are trying to build?

def compress(s):
    seen = set()
    new = [c for c in s if c not in seen and (seen.add(c) or True)]
    return ''.join(new)

or just:

def compress(s):
    seen = set()
    return ''.join(c for c in s if c not in seen and (seen.add(c) or
True))

Using the set also gets rid of that nasty quadratic performance
thingy.

-- Paul