[Tutor] how does this list comprehension work?

[Danny Yoo]

On Wed, 8 Mar 2006, Christopher Spears wrote:

> I copied this program from Learning Python and got it to work on Windows
> XP:
>
> import sys, glob
> print sys.argv[1:]
> sys.argv = [item for arg in sys.argv for item in
> glob.glob(arg)]
> print sys.argv[1:]
>
> The key to the script is the list comprehension, which I am having
> trouble dissecting.  How do I go about trying to figure out how it
> works?


Hi Chris,


Hmmm.  Let me stare at it again.

> sys.argv = [item for arg in sys.argv for item in
>             glob.glob(arg)]

... Yikes.  to tell the truth, I can't read this either.


But it might help if we translated it as this:

######
expandedArguments = []
for arg in sys.argv:
    for item in glob.glob(arg):
        expandedArguments.append(item)
######

Does this code make more sense?

But I'm not so sure myself that list comprehensions are so good when they
get so deeply nested; the nested structure is deceptively obscured in the
flatness of that list comprehension.

I would have just written the code above instead.  Actually, I'd write a
function to capture that abstraction of a mapping and appending function.
Let's call it "mappend()":

##########################################################################
def mappend(f, L):
    """mappend applies f to each elements in L, and assumes that f returns
    a list.  It collects those results in a single flattened list."""
    results = []
    for x in L:
        results.extend(f(x))
    return results
##########################################################################

(The function name is common to folks who've done Lispy kinds of coding.)


And we can see this mappend() function in action:

######
>>> import glob
>>> mappend(glob.glob, ['*.txt', '*.doc'])
['sequences.txt', 'foo.txt', 'pub2ara-pipeline_20060216.doc',
 'Admission Conditions.doc', 'PhD Offer.doc']
######


If you have more questions, please feel free to ask.  Good luck!