On Aug 20, 11:29 am, Chris Angelico <ros...@gmail.com> wrote:
> If you're using a variable for the stop value, you just need to set it
> to an explicit None if it would fall negative:
>
> >>> a[10:None:-1]
>
That doesn't work if it's set in a loop or if it's calculated as a
formula. For example, this very simple code doesn't work because of
the "-1 problem".
# find the longest substring that reads the same left to right and
right to left
for substr_length in range(len(input),0,-1):
for starting_pos in range(len(input)-substr_length+1):
ending_pos = starting_pos + substr_length - 1
if input[starting_pos:ending_pos+1] == input[ending_pos :
starting_pos-1 : -1]:
print(input[starting_pos:ending_pos+1])
exit(0)
Of course you can rewrite it, but it becomes quite ugly.
(Not to mention, people who learn the language would not always know
this, and will end up with a bug.)
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