On Fri, 17 Nov 2006 12:00:46 -0800, Rares Vernica wrote:
> Problem context:
>
> import os
> dirs_exclude = set(('a', 'b', 'e'))
> for root, dirs, files in os.walk('python/Lib/email'):
> # Task:
> # delete from "dirs" the directory names from "dirs_exclude"
> # case-insensitive
>
> The solution so far is:
>
> for i in xrange(len(dirs), 0, -1):
> if dirs[i-1].lower() in dirs_exclude:
> del dirs[i-1]
>
> I am looking for a nicer solution.
Define "nicer".
First thing I'd do is change the loop:
for i in xrange(len(dirs)-1, -1, -1):
if dirs[i].lower() in dirs_exclude:
del dirs[i]
Second thing I'd do is encapsulate it in a function instead of calling it
in place:
def remove_in_place(source, target):
for i in xrange(len(source)-1, -1, -1):
if source[i].lower() in target:
del source[i]
Third thing I'd do is replace the delete-in-place code away, and build a
new list using the set idiom, finally using list slicing to change the
source in place:
def remove_in_place2(source, target):
target = set(s.lower() for s in target)
source[:] = [x for x in source if x.lower() not in target]
# note the assignment to a slice
And finally, I would test the two versions remove_in_place and
remove_in_place2 to see which is faster.
import timeit
setup = """from __main__ import remove_in_place
target = list("aEIOu")
source = list("AbcdEfghIjklmnOpqrstUvwxyz")
"""
tester = """tmplist = source[:] # make a copy of the list!
remove_in_place(tmplist, target)
"""
timeit.Timer(tester, setup).timer()
You have to make a copy of the list on every iteration because you are
changing it in place; otherwise you change the values you are testing
against, and the second iteration onwards doesn't have to remove anything.
(All code above untested. Use at own risk.)
--
Steven.
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