Avoid IndexError when the pytest traceback cut has a single entry

src/sybil/integration/pytest.py

@@ -80,9 +80,12 @@ def runtest(self) -> None:
     def _traceback_filter(self, excinfo: ExceptionInfo[BaseException]) -> Traceback:
         traceback = excinfo.traceback
         tb = traceback.cut(path=example_module_path)
-        tb_entry = tb[1]
-        if getattr(tb_entry, '_rawentry', None) is not None:
-            traceback = Traceback(tb_entry._rawentry)
+        # ``tb`` has a single entry when the example frame is the deepest in
+        # the traceback, leaving no frame below it to re-root at.
+        if len(tb) > 1:
+            tb_entry = tb[1]
+            if getattr(tb_entry, '_rawentry', None) is not None:
+                traceback = Traceback(tb_entry._rawentry)
         return traceback
 
     def repr_failure(

tests/test_functional.py

@@ -1,11 +1,15 @@
 import sys
+from inspect import getsourcefile
+from os.path import abspath
 from pathlib import Path
 
 import pytest
-from pytest import CaptureFixture
+from pytest import CaptureFixture, ExceptionInfo
 from testfixtures import Replacer, compare, not_there
 
-from sybil import Sybil
+from sybil import Sybil, example as example_module
+from sybil.document import Document
+from sybil.integration.pytest import SybilItem
 from sybil.parsers.rest import PythonCodeBlockParser, DocTestParser
 from sybil.python import import_cleanup
 from .helpers import (
@@ -131,6 +135,32 @@ def test_pytest(capsys: CaptureFixture[str]):
     assert results.total == 10
 
 
+def test_traceback_filter_single_entry_cut() -> None:
+    # A failure whose deepest frame is the example frame cuts to a single
+    # traceback entry; filtering it must not raise IndexError.
+    source = getsourcefile(example_module)
+    assert source is not None
+    example_path = abspath(source)
+
+    document = Document(".. code-block:: python\n\n    raise ValueError('boom')\n", "x.rst")
+    for region in PythonCodeBlockParser()(document):
+        document.add(region)
+
+    with pytest.raises(ValueError) as excinfo:
+        next(iter(document.examples())).evaluate()
+
+    # Truncate so the example frame is the deepest, collapsing the cut to one.
+    raw = excinfo.tb
+    while raw is not None and raw.tb_frame.f_code.co_filename != example_path:
+        raw = raw.tb_next
+    assert raw is not None
+    raw.tb_next = None
+
+    truncated = ExceptionInfo.from_exc_info((excinfo.type, excinfo.value, raw))
+    result = SybilItem.__new__(SybilItem)._traceback_filter(truncated)
+    assert len(result) == 1
+
+
 def test_pytest_output_immune_to_color_env(capsys: CaptureFixture[str]):
     with Replacer() as replace:
         replace.in_environ('PY_COLORS', not_there)