Significance of "start" parameter to string method "endswith"

[John Machin]

On Apr 20, 6:08 am, Larry Bates <larry.ba... at websafe.com> wrote:
> Boris Dušek wrote:
> > Hello,
>
> > what is the use-case of parameter "start" in string's "endswith"
> > method? Consider the following minimal example:
>
> > a = "testing"
> > suffix="ing"
> > a.endswith(suffix, 2)
>
> > Significance of "end" is obvious. But not so for "start".
>
> > Let's assume the "end" parameter is not used - then the function
> > should simple check that the last "len(suffix)" characters of "a" are
> > equal to "ing", no matter where we start (the function does not *scan*
> > the string from the "start", does it?)
> > Only case where it would make difference is if we had start +
> > len(suffix) < len(a)     (excuse possible "of-by-one" error :-)
> > Then the function would never return True. But is there a real use
> > case when we would test for endswith like this? (knowing that it must
> > return false?)

Any or all of a, start and suffix can be variable. I can't see how we
could know that it must return false (except of course in the trivial
and useless case that they are all constant). IMHO it's much better in
general to let a string method do checks for corner cases  (very
efficiently) than to waste brain cells trying to work out how to write
correct (but relatively very slow) Python code to avoid calling the
method.

>
> Seems like a convenience I've never used.
>
> >>> a="abcdef"
> >>> a.endswith('cd',2,4)
> True
> >>> a[2:4].endswith('cd')
>
> True
>

It's not just a convenience, and not just with endswith. In general:
astring[start:end].amethod(anarg) manifests the slice as a separate
temporary object, whereas astring.amethod(anarg, start, stop) does
not, and should be more efficient when one is looping over a long
string

Granted endswith's start arg is not wildly useful, but it's orthogonal
-- IMHO all string-processing functions should have a pair of start/
end args as a matter of course, unless neither start nor end is
useful.

Cheers,
John