test slice _to_positive_step

Author: calvinmccarter-at-lightmatter

onnx2pytorch/operations/slice.py

@@ -4,10 +4,12 @@
 
 def _to_positive_step(orig_slice, N):
     """
-    Convert a slice object with a negative step to an equivalent one with a
-    positive step, computed using N, the length of the iterable being sliced.
-    This is because PyTorch currently does not support slicing a tensor with
-    a negative step.
+    Convert a slice object with a negative step to one with a positive step.
+    Accessing an iterable with the positive-stepped slice, followed by flipping
+    the result, should be equivalent to accessing the tensor with the original
+    slice. Computing positive-step slice requires using N, the length of the
+    iterable being sliced. This is because PyTorch currently does not support
+    slicing a tensor with a negative step.
     """
     # Get rid of backward slices
     start, stop, step = orig_slice.indices(N)

tests/onnx2pytorch/operations/test_slice.py

@@ -3,6 +3,7 @@
 import pytest
 
 from onnx2pytorch.operations import Slice
+from onnx2pytorch.operations.slice import _to_positive_step
 
 
 @pytest.fixture
@@ -120,3 +121,13 @@ def test_slice_neg_steps(x, init):
     else:
         op = Slice()
         assert torch.equal(op(x, starts, ends, axes, steps), y)
+
+
+def test_to_positive_step():
+    assert _to_positive_step(slice(-1, None, -1), 8) == slice(0, 8, 1)
+    assert _to_positive_step(slice(-2, None, -1), 8) == slice(0, 7, 1)
+    assert _to_positive_step(slice(None, -1, -1), 8) == slice(0, 0, 1)
+    assert _to_positive_step(slice(None, -2, -1), 8) == slice(7, 8, 1)
+    assert _to_positive_step(slice(None, None, -1), 8) == slice(0, 8, 1)
+    assert _to_positive_step(slice(8, 1, -2), 8) == slice(3, 8, 2)
+    assert _to_positive_step(slice(8, 0, -2), 8) == slice(1, 8, 2)