FIX: empty SeqView absolute/relative position returns 0

src/cogent3/core/sequence.py

@@ -2007,6 +2007,8 @@ def absolute_position(self, rel_index: int, include_boundary=False):
         the absolute index with respect to the coordinates of the self
         including offset
         """
+        if not self:
+            return 0
 
         if rel_index < 0:
             raise IndexError("only positive indexing supported!")
@@ -2029,6 +2031,8 @@ def relative_position(self, abs_index, stop=False):
         NOTE: the returned value DOES NOT reflect python indexing. Importantly, negative values represent positions that
         precede the current view.
         """
+        if not self:
+            return 0
 
         if abs_index < 0:
             raise IndexError("Index must be +ve and relative to the + strand")

tests/test_core/test_sequence.py

@@ -2714,3 +2714,9 @@ def test_sequences_propogates_seqid():
 def test_make_seq_assigns_to_seqview():
     seq = cogent3.make_seq("ACGT", name="s1")
     assert seq.name == seq._seq.seqid == "s1"
+
+
+def test_empty_seqview_translate_position():
+    sv = SeqView("")
+    assert sv.absolute_position(0) == 0
+    assert sv.relative_position(0) == 0