Support for negative ranges

[harry-clarke]

Hey there,

Am I missing something here?
Negative ranges don't seem to be supported, it seems in part due to string formatting issues (-1-1 confuses the parser)

Is there support for negative ranges, and if not, would others find this a useful addition?

[thiell]

@harry-clarke indeed it is not supported, we only test use cases with positive indexes. - is parsed as a range separator, not a minus sign. Could you tell us a bit more about your use case where that would be useful?

[harry-clarke]

Funnily, it's just because I've been making my way through the Advent of Code challenges, and I wanted a RangeSet for intersecting rows and columns with shapes.

In its simplest form, I want to do:

RangeSet(range(-5,7)) - RangeSet(range(1,3))

[harry-clarke]

At least for now, can we provide a ValueError when passing in ranges with negative values?

I don't mind creating a PR

Ideally, it would be good to remove the dependency on string parsing for interpreting ranges, which should reduce complexity a bit.

[thiell]

I checked and it is an unexpected change (or "regression") between 1.8.x and 1.9.

In 1.9, we actually changed the way the inner set handles indices: we now rely on strings instead of integers. This allows RangeSet to support complex cases of zero padding, for things like 01-05,001-002 (see
https://clustershell.readthedocs.io/en/latest/tools/nodeset.html?#zero-padding)

With 1.8.4:

>>> RangeSet(range(-5,7))
-5-6
>>> RangeSet(range(1,3))
1-2
>>> RangeSet(range(-5,7)) - RangeSet(range(1,3))
-5-0,3-6

With 1.9:

>>> RangeSet(range(-5,7))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sthiell/github/thiell/clustershell/lib/ClusterShell/RangeSet.py", line 124, in __init__
    self._parse(pattern)
  File "/home/sthiell/github/thiell/clustershell/lib/ClusterShell/RangeSet.py", line 178, in _parse
    raise RangeSetParseError(subrange, msg)
ClusterShell.RangeSet.RangeSetParseError: cannot convert string to integer : "-5"

For now, you could use 1.8.4, but I will see if we can restore functionality in 1.9.x and officially support negative ranges. I will post updates in this ticket. Thanks for the report!