[Tutor] scratching my head - still

Cameron Simpson cs at zip.com.au
Wed Aug 5 07:53:09 CEST 2015

On 05Aug2015 12:46, Steven D'Aprano <steve at pearwood.info> wrote:
>On Tue, Aug 04, 2015 at 05:52:15PM -0700, Clayton Kirkwood wrote:
>> As seen below (closely), some filenames are not being removed while others
>> are, such as in the first stanza, some pdfs are removed, some aren't. In the
>> second stanza, Thumbs.db makes it through, but was caught in the first
>> stanza. (Thanks for those who have proffered solutions to date!)
>> I see no logic in the results. What am I missing???
>You are modifying the list of files while iterating over it, which plays
>all sorts of hell with the process. Watch this:
[... detailed explaination ...]
>The lesson here is that you should never modify a list while iterating
>over it. Instead, make a copy, and modify the copy.

What Steven said. Yes indeed.

Untested example suggestion:

  all_filenames = set(filenames)
  for filename in filenames:
    if .. test here ...:

You could use a list instead of a set and for small numbers of files be fine.  
With large numbers of files a set is far faster to remove things from.

Cameron Simpson <cs at zip.com.au>

In the desert, you can remember your name,
'cause there ain't no one for to give you no pain.      - America

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