[Tutor] if item.find(extension)!= -1: -- what's the -1?

[Kent Johnson]

On Sat, Feb 6, 2010 at 11:35 AM, David <ldl08 at gmx.net> wrote:
> Hello again,
>
> in Knowlton's 2008 book "Python: Create, Modify, Reuse" the author makes
> frequent use of the term -1 in his code, but doesn't tell to what aim. For
> want of search terms Google is not helpful. Could someone please enlighten
> me by means of a one-liner as a reply?
>
> Example Code:
>
> for item in filelist:
>        if item.find(extension)!= -1:
>            snaplist.append(item)

Wayne has explained the -1; I will add that a more idiomatic way to
write this is

for item in filelist:
  if extension in item:
    snaplist.append(item)

or using a list comprehension (assuming snaplist starts out empty):

snaplist = [ item for item in filelist if extension in item ]

Assuming extension is a file extension, I would actually use
endswith() to filter the items:

snaplist = [ item for item in filelist if item.endswith(extension) ]

Kent