On 3/8/06, Christopher Spears <[EMAIL PROTECTED]> wrote:
Hi Christopher,
Others have ably deciphered this particular list comprehension (LC) for you. I'm curious though, are you having trouble because this is a nested list comprehension or do you not have a good grasp on LCs in general? They can be a little tricky to grok at first.
In case you're not completely clear on LCs in general, here's a bit more description. If you are clear, just skip to the next email. ;-)
First off - a list comprehension creates a new list. It does so by pulling items from an existing list or other iterable object. So, for example:
newlist = [item for item in oldlist]
print newlist
This example wasn't terribly useful - it just made a new list. You can, however, *do* stuff to the item as you're passing it to the new list. For example, if you want the squares of the items in the old list, you coudl do:
oldlist = [1,2,3]
sqlist = [item*item for item in oldlist]
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?
Hi Christopher,
Others have ably deciphered this particular list comprehension (LC) for you. I'm curious though, are you having trouble because this is a nested list comprehension or do you not have a good grasp on LCs in general? They can be a little tricky to grok at first.
In case you're not completely clear on LCs in general, here's a bit more description. If you are clear, just skip to the next email. ;-)
First off - a list comprehension creates a new list. It does so by pulling items from an existing list or other iterable object. So, for example:
print newlist
This example wasn't terribly useful - it just made a new list. You can, however, *do* stuff to the item as you're passing it to the new list. For example, if you want the squares of the items in the old list, you coudl do:
oldlist = [1,2,3]
sqlist = [item*item for item in oldlist]
Another thing you can do is filter the items in the old list in some way. So, if you only want the even numbers you could do:
elist = [item for item in oldlist if item%2==0] # using modulo, which returns the remainder
You can use LCs with files, with dicts, with strings, with any iterable object. It's really slick, really easy, pretty fast. LCs can be a little tricky, like I mentioned (especially when nested) but are quite handy and fun once you get to know them. Just don't get so tricky that it's hard to read your code. If it's getting hard to read, use a for loop.
HTH,
Anna
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