ImageSequence slicing: handle negative offset (#485)

When image files are numbered from 0, the resulting ImageSequence has batch_offset -1. This situation was handled incorrectly after #438; this fixes the bug and adds a test.

Closes dials/dials#2011.

newsfragments/485.bugfix

@@ -0,0 +1 @@
+Correctly handle slicing ImageSequences made from images starting with 0

src/dxtbx/imageset.py

@@ -297,7 +297,9 @@ def __getitem__(self, item):
             An image or new Sequence object
 
         """
-        if isinstance(item, slice):
+        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")
@@ -316,18 +318,16 @@ def __getitem__(self, item):
                     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)
-                if self.data().has_single_file_reader():
-                    reader = self.reader().copy(self.reader().paths(), stop - start)
-                else:
-                    reader = self.reader().copy(self.reader().paths())
-                return self.partial_set(reader, start - offset, stop - offset)
-        else:
-            return self.get_corrected_data(item)
+                start -= offset
+                stop -= offset
+            if self.data().has_single_file_reader():
+                reader = self.reader().copy(self.reader().paths(), stop - start)
+            else:
+                reader = self.reader().copy(self.reader().paths())
+            return self.partial_set(reader, start, stop)
 
     def get_template(self):
         """Return the template"""

tests/test_imageset.py

@@ -2,6 +2,7 @@
 import pickle
 import shutil
 from unittest import mock
+import copy
 
 import pytest
 
@@ -393,6 +394,12 @@ def tst_get_item(self, sequence):
         with pytest.raises(IndexError):
             _ = sequence[3:7:2]
 
+        # 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)
+
     @staticmethod
     def tst_paths(sequence, filenames1):
         filenames2 = sequence.paths()