Why does `[:-0]` return an empty string? There is a variable in my code which determines a length of string which have to be cut. The problem is that it returns empty string when there is int(0) in this variable.

Example:

```
def rightPass(word,value):
    substring = word[:-value]
    return substring

for i in range(3,-1,-1):
    print rightPass('word',i)

>>> w
>>> wo
>>> wor
>>> 

```

I do know that it is not a big deal to do this:

```
def rightPass(word,value):
    substring = word[:-value] if value!=0 else word
    return substring

```

But I can't find a reason why is `[:-0]` acting this way.

I though that it is because Python doesn't know `-0` value but I tried it using `0` and the same problem occurred.

Can somebody explain it?