[Newby question] List comprehension

[Eelco Hoekema]

I'm trying to get a list of tuples, with each tuple consisting of a
directory, and a list of files. I only want a tuple if and only if the
filtered list of files is not empty. And, i want the list of files in the
tuples to be filtered. For this, i came up with the following code:

<code>

# song filter: will return true if the file seems to be an mp3 file.
# (may not be the best way to do this)
def song(f):
(name, ext) = os.path.splitex t(f)
return ext.lower() == '.mp3'

# list comprehension walking through a directory tree
[(root, filter(song, files)) for (root, dir, files) in os.walk(os.path .abspath('.')) if filter(song, files)]


</code>

Now, this will work. However, it seems kind of silly to call the filter
twice. Is there a way to keep this in one list comprehension, but with
just filtering once?

eelco

[Tom B.]

Re: [Newby question] List comprehension


"Eelco Hoekema" <eelco.usenet@x s4all.nl> wrote in message
news:pan.2004.0 8.06.13.22.50.7 5041@xs4all.nl. ..[color=blue]
>
> I'm trying to get a list of tuples, with each tuple consisting of a
> directory, and a list of files. I only want a tuple if and only if the
> filtered list of files is not empty. And, i want the list of files in the
> tuples to be filtered. For this, i came up with the following code:
>
> <code>
>
> # song filter: will return true if the file seems to be an mp3 file.
> # (may not be the best way to do this)
> def song(f):
> (name, ext) = os.path.splitex t(f)
> return ext.lower() == '.mp3'
>
> # list comprehension walking through a directory tree
> [(root, filter(song, files)) for (root, dir, files) in[/color]
os.walk(os.path .abspath('.')) if filter(song, files)][color=blue]
>
>
> </code>
>
> Now, this will work. However, it seems kind of silly to call the filter
> twice. Is there a way to keep this in one list comprehension, but with
> just filtering once?
>
> eelco
>
>[/color]
How about,

fltres = filter(song, files)
[(root, fltres ) for (root, dir, files) in os.walk(os.path .abspath('.')) if
fltres]

Tom

[Larry Bates]

Re: [Newby question] List comprehension

Actually I think (??) this is better done in a loop:
(not tested)

toc=[]
for root, dir, files in os.walk(os.path .abspath('.')):
mp3files=[f for f in files if f.lower().endsw ith('.mp3')]
if mp3files: toc.append((roo t, mp3files))


HTH,
Larry Bates
Syscon, Inc.

"Eelco Hoekema" <eelco.usenet@x s4all.nl> wrote in message
news:pan.2004.0 8.06.13.22.50.7 5041@xs4all.nl. ..[color=blue]
>
> I'm trying to get a list of tuples, with each tuple consisting of a
> directory, and a list of files. I only want a tuple if and only if the
> filtered list of files is not empty. And, i want the list of files in the
> tuples to be filtered. For this, i came up with the following code:
>
> <code>
>
> # song filter: will return true if the file seems to be an mp3 file.
> # (may not be the best way to do this)
> def song(f):
> (name, ext) = os.path.splitex t(f)
> return ext.lower() == '.mp3'
>
> # list comprehension walking through a directory tree
> [(root, filter(song, files)) for (root, dir, files) in[/color]
os.walk(os.path .abspath('.')) if filter(song, files)][color=blue]
>
>
> </code>
>
> Now, this will work. However, it seems kind of silly to call the filter
> twice. Is there a way to keep this in one list comprehension, but with
> just filtering once?
>
> eelco
>
>[/color]

[Eelco Hoekema]

Re: [Newby question] List comprehension

Larry Bates schreef:
[color=blue]
> Actually I think (??) this is better done in a loop:
> (not tested)
>
> toc=[]
> for root, dir, files in os.walk(os.path .abspath('.')):
> mp3files=[f for f in files if f.lower().endsw ith('.mp3')]
> if mp3files: toc.append((roo t, mp3files))[/color]

That is about the same as Facundo Bastida said. But then, i like this
better:

tmp = [(root, files) for (root, dir, files) in os.walk(os.path .abspath('.')) if files]
toc = [(root, files) for (root, files) tmp if filter(song, files)]

But that means 2 list comprehensions. Ans i'm just wondering if it can be
done in one, without filtering twice.

eelco

[Christopher T King]

Re: [Newby question] List comprehension

On Fri, 6 Aug 2004, Eelco Hoekema wrote:
[color=blue]
> [(root, filter(song, files)) for (root, dir, files) in
> os.walk(os.path .abspath('.')) if filter(song, files)]
>
> Now, this will work. However, it seems kind of silly to call the filter
> twice. Is there a way to keep this in one list comprehension, but with
> just filtering once?[/color]

You may do best to split this into two LCs:

temp = [(root, filter(song,fil es)) for (root, dir, files) in
os.walk(os.path .abspath('.'))]
temp = [(root, songs) for (root, songs) in temp if songs]

Or if you prefer, replace the latter with:
temp = filter(temp, lambda x: x[1])

Or even, in 2.4:
temp = filter(temp, itemgetter(1))

In 2.4, you will also be able to replace the first LC with a generator
expression, saving a bit of both memory and processor time (the change
would consist of replacing the brackets with parentheses).

Hope this helps.

[Eelco Hoekema]

Re: [Newby question] List comprehension

Eelco Hoekema schreef:
[color=blue]
> That is about the same as Facundo Bastida said. But then, i like this
> better:[/color]
[color=blue]
> tmp = [(root, files) for (root, dir, files) in os.walk(os.path .abspath('.')) if files]
> toc = [(root, files) for (root, files) tmp if filter(song, files)][/color]

Hmm. That doesn't work. It's the other way around, like Christoffer
King showed:

tmp = [(root, files) for (root, dir, files) in os.walk(os.path .abspath('.')) if filter(song, files)]
toc = [(root, files) for (root, files) tmp if files]

eelco

[Skip Montanaro]

Re: [Newby question] List comprehension

Eelco> # song filter: will return true if the file seems to be an mp3 file.
Eelco> # (may not be the best way to do this)
Eelco> def song(f):
Eelco> (name, ext) = os.path.splitex t(f)
Eelco> return ext.lower() == '.mp3'

Eelco> # list comprehension walking through a directory tree
Eelco> [(root, filter(song, files)) for (root, dir, files) in os.walk(os.path .abspath('.')) if filter(song, files)]

In this particular case, since song() only returns True or False, you could
use

[(root, True) for (root, dir, files) in os.walk(os.path .abspath('.'))
if filter(song, files)]

Skip

[Eelco Hoekema]

Re: [Newby question] List comprehension

Skip Montanaro schreef:
[color=blue]
> Eelco> # list comprehension walking through a directory tree
> Eelco> [(root, filter(song, files)) for (root, dir, files) in os.walk(os.path .abspath('.')) if filter(song, files)][/color]
[color=blue]
> In this particular case, since song() only returns True or False,[/color]

song() does return True or False, but the return value is used in the
filter, which returns a list.
[color=blue]
> you could use[/color]
[color=blue]
> [(root, True) for (root, dir, files) in os.walk(os.path .abspath('.'))
> if filter(song, files)][/color]

That would yield a list of tuples of directories that contain at least one
song. Could still be of use.

eelco