Scan and imageset slicing inconsistent

[rjgildea]

def test_scan_imageset_slice_consistency(dials_data):
    files = dials_data("centroid_test_data").listdir("*.cbf", sort=True)[1:]
    expt = ExperimentListFactory.from_filenames(f.strpath for f in files)[0]
    assert expt.scan[0:8] == expt.scan
    # The following doesn't work, and expects expt.imageset[1:9]
    assert expt.imageset[0:8] == expt.imageset

gives the following error:

self = <dxtbx_imageset_ext.ImageSequence object at 0x114481810>, item = slice(0, 8, None)

    def __getitem__(self, item):
        """Get an item from the sequence stream.
    
        If the item is an index, read and return the image at the given index.
        Otherwise, if the item is a slice, then create a new Sequence object
        with the given number of array indices from the slice.
    
        Params:
            item The index or slice
    
        Returns:
            An image or new Sequence object
    
        """
        if isinstance(item, slice):
            offset = self.get_scan().get_batch_offset()
            if item.step is not None:
                raise IndexError("Sequences must be sequential")
    
            # nasty workaround for https://github.com/dials/dials/issues/1153
            # slices with -1 in them are meaningful :-/ so grab the original
            # constructor arguments of the slice object.
            # item.start and item.stop may have been compromised at this point.
            if offset < 0:
                start, stop, step = item.__reduce__()[1]
                if start is None:
                    start = 0
                else:
                    start -= offset
                if stop is None:
                    stop = len(self)
                else:
                    stop -= offset
                return self.partial_set(start, stop)
            else:
                start = item.start or 0
                stop = item.stop or (len(self) + offset)
>               return self.partial_set(start - offset, stop - offset)
E               OverflowError: can't convert negative value to unsigned int

imageset.py:264: OverflowError

This issue is the root cause of dials/dials#1382

[dagewa]

Following #634 the inconsistency has been dealt with. Scans and imagesets now slice in the same way (see also dials/dials#2472 (review))

[ndevenish]

So, it is probably safe to remove this XPASS now? 3fad1e5