Make Imageset slices consistently index from 0

Without this, slices sometimes needed to be indexed by their original
image index, even inside the slice.

The previous workaround in 67aab2 (cctbx#485) for negative offset was
reacting to the erroneous behaviour introduced by including batch
offsets in imagesequence slicing.

newsfragments/633.bugfix

@@ -0,0 +1 @@
+Slicing of imageset objects is now consistently 0-based, including for the sliced data accessor. Previously, the data accessor had to be accessed with the original index offsets.

src/dxtbx/imageset.py

@@ -300,29 +300,12 @@ def __getitem__(self, item):
  if not isinstance(item, slice):
  return self.get_corrected_data(item)
  else:
- 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
- else:
- start = item.start or 0
- stop = item.stop or (len(self) + offset)
- start -= offset
- stop -= offset
+ start = item.start or 0
+ stop = item.stop or len(self)
+
  if self.data().has_single_file_reader():
  reader = self.reader().copy(self.reader().paths(), stop - start)
  else:

src/dxtbx/model/experiment_list.py

@@ -625,8 +625,7 @@ def from_sequence_and_crystal(imageset, crystal, load_models=True):
  # if imagesequence is still images, make one experiment for each
  # all referencing into the same image set
  if imageset.get_scan().is_still():
- start, end = imageset.get_scan().get_array_range()
- for j in range(start, end):
+ for j in range(len(imageset)):
  subset = imageset[j : j + 1]
  experiments.append(
  Experiment(

tests/test_imageset.py

@@ -387,6 +387,11 @@ def tst_get_item(self, sequence):
  with pytest.raises(RuntimeError):
  _ = sequence2[5]
 
+ # Check data access matches expected images from the slice
+ panel_data1 = sequence[3][0]
+ panel_data2 = sequence2[0][0]
+ assert panel_data1.all_eq(panel_data2)
+
  assert len(sequence2) == 4
  assert_can_get_detectorbase(sequence2, range(0, 4), 5)
  self.tst_get_models(sequence2, range(0, 4), 5)
@@ -396,11 +401,12 @@ def tst_get_item(self, sequence):
  with pytest.raises(IndexError):
  _ = sequence[3:7:2]
 
+ # Batch offset should not affect slicing of imagesequence
  # Simulate a scan starting from image 0
  sequence_ = copy.deepcopy(sequence)
  sequence_.get_scan().set_batch_offset(-1)
  sequence3 = sequence_[3:7]
- assert sequence3.get_array_range() == (4, 8)
+ assert sequence3.get_array_range() == (3, 7)
 
  @staticmethod
  def tst_paths(sequence, filenames1):