# sequence multiplied by -1

Thomas Jollans thomas at jollybox.de
Sat Sep 25 13:45:11 CEST 2010

```On Saturday 25 September 2010, it occurred to Yingjie Lan to exclaim:
> Hi,
>
> I noticed that in python3k, multiplying a sequence by a negative integer is
> the same as multiplying it by 0, and the result is an empty sequence. It
> seems to me that there is a more meaningful symantics.

Um...

for every list l and integer n >= 0:
len(l*n) == len(l)*n

Multiplying a list by a negative integer should produce a list of negative
length, which does not exist. IMHO, the only correct behaviour would be to
raise an exception, though one could argue that there are practical benefits
for the operation to succeed for any integer operand.

>
> Simply put, a sequence multiplied by -1 can give a reversed sequence.

For that, we have slicing. A negative step value produces a reverse slice of
the list. You can't argue that this makes sense, can you

>>> [1,2,3,4][::-1]
[4, 3, 2, 1]
>>> [1,2,3,4][::-2]
[4, 2]
>>>

```