[Tutor] Removing Null Elements from a List

[Daniel Yoo]

On Fri, 9 Mar 2001, Britt Green wrote:

> If I have a list that looks like:
> 
> [None, None, None, None, None, None, None, None, None, None, 
> ['/atl/horseradishgrill_1'], None, None, None, None, None, None, None, None, 
> None, None, None, None, None, None, None, None, None, None, 
> ['/boston/archive'], None, None, None, ['/boston/cafe_fleuri'], None, None, 
> None, None, None, None, None, None, None, None, ['/boston/rplace'], 
> ['/boston/salamander'], None, ['/boston/test'], ['/boston/the_federalist'], 
> None, ['/boston/yanks']]
> 
> How can I remove those None elements in it? The code that I'm getting that 
> from is, in part, this:

One way to do this is to use the functional tool filter():

###
def isTrue(x):
    if x: return 1
    else: return 0

cleanFiles = filter(isTrue, files)
###

That's one way of removing all the None's out of there: isTrue() is a
function that says 1 only when the filename is either nonempty or not
None.


Hmmm... In more Python 2.0 terms, it's doing something like this:

    cleanfiles = [f for f in files if f != None]

Sorry, I'm a stickler for functional stuff sometimes... The list
comprehension way is probably a lot easier to read: it creates for us a
list of all files that aren't None.


We can extend this method to take care of both the None's and the badfiles
in one swoop:

    chosen = [f for f in files if pick.search(f) and f != None]

should do it.  (I renamed 'elements' to 'f' to save some space.)

Hope this helps!