[Tutor] how does this list comprehension work?

[Kent Johnson]

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:]
> 
> What the program does is first print the glob and then
> a list of everything caught by the glob.  For example:
> 
> ['*.py']
> ['showglob.py']
> 
> 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?

A list comprehension is a shortcut for a series of one or more nested 
for loops and if statements with a list append in the middle of it.

The list comp pulls the value to be appended out of the loops but 
otherwise the order is not affected. Looking at

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

item is the thing that will be appended to the list. The nested for 
loops are
   for arg in sys.argv:
     for item in glob.glob(arg):

If you initialize a list outside the loop, and append to it inside the 
loop, you have the equivalent loops:

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

Kent