What is the best way to delete strings in a string list that that match certain pattern?

[Peng Yu]

On Fri, Nov 6, 2009 at 10:42 AM, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
> On Fri, 6 Nov 2009, Peng Yu wrote:
>
>> On Fri, Nov 6, 2009 at 3:05 AM, Diez B. Roggisch <deets at nospam.web.de> wrote:
>> > Peng Yu schrieb:
>> >>
>> >> Suppose I have a list of strings, A. I want to compute the list (call
>> >> it B) of strings that are elements of A but doesn't match a regex. I
>> >> could use a for loop to do so. In a functional language, there is way
>> >> to do so without using the for loop.
>> >
>> > Nonsense. For processing over each element, you have to loop over them,
>> > either with or without growing a call-stack at the same time.
>> >
>> > FP languages can optimize away the stack-frame-growth (tail recursion) - but
>> > this isn't reducing complexity in any way.
>> >
>> > So use a loop, either directly, or using a list-comprehension.
>>
>> What is a list-comprehension?
>>
>> I tried the following code. The list 'l' will be ['a','b','c'] rather
>> than ['b','c'], which is what I want. It seems 'remove' will disrupt
>> the iterator, right? I am wondering how to make the code correct.
>>
>> l = ['a', 'a', 'b', 'c']
>> for x in l:
>>   if x == 'a':
>>     l.remove(x)
>>
>> print l
>
>  list comprehension seems to be what you want:
>
>  l = [i for i in l if i != 'a']

My problem comes from the context of using os.walk(). Please see the
description of the following webpage. Somehow I have to modify the
list inplace. I have already tried 'dirs = [i for i in l if dirs !=
'a']'. But it seems that it doesn't "prune the search". So I need the
inplace modification of list.

http://docs.python.org/library/os.html

When topdown is True, the caller can modify the dirnames list in-place
(perhaps using del or slice assignment), and walk() will only recurse
into the subdirectories whose names remain in dirnames; this can be
used to prune the search, impose a specific order of visiting, or even
to inform walk() about directories the caller creates or renames
before it resumes walk() again. Modifying dirnames when topdown is
False is ineffective, because in bottom-up mode the directories in
dirnames are generated before dirpath itself is generated.