len_slice: Catch more cases where length is unknown.

jakirkham · Dec 12, 2016 · ab4ab22 · ab4ab22
1 parent 7deb3e8
commit ab4ab22
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Showing 2 changed files with 15 additions and 5 deletions.
diff --git a/kenjutsu/measure.py b/kenjutsu/measure.py
@@ -54,14 +54,21 @@ def len_slice(a_slice, a_length=None):
 
     new_slice_size = 0
     if isinstance(new_slice, slice):
-        if new_slice.stop is None:
-            if new_slice.step > 0:
+        if (new_slice.step > 0 and new_slice.start >= 0 and
+                (new_slice.stop is None or new_slice.stop < 0)):
                 raise UnknownSliceLengthException(
-                    "Cannot determine slice length without a defined end"
+                    "Cannot determine slice length without a defined start"
                     " point. The reformatted slice was %s." % repr(new_slice)
                 )
-            else:
-                new_slice = slice(new_slice.start, -1, new_slice.step)
+        elif (new_slice.step < 0 and new_slice.start < 0 and
+                (new_slice.stop is None or new_slice.stop >= 0)):
+                raise UnknownSliceLengthException(
+                    "Cannot determine slice length without a defined start"
+                    " point. The reformatted slice was %s." % repr(new_slice)
+                )
+
+        if new_slice.step < 0 and new_slice.stop is None:
+            new_slice = slice(new_slice.start, -1, new_slice.step)
 
         new_slice_diff = float(new_slice.stop - new_slice.start)
 

diff --git a/tests/test_measure.py b/tests/test_measure.py
@@ -36,6 +36,9 @@ def test_len_slice(self):
         with self.assertRaises(measure.UnknownSliceLengthException):
             measure.len_slice(slice(None))
 
+        with self.assertRaises(measure.UnknownSliceLengthException):
+            measure.len_slice(slice(None, None, -1))
+
         for size in [10, 11, 12]:
             excess = size + 3
             each_range = range(size)