Merge pull request #8227 from encukou/defensive-get_source

Make code.FormattedExcinfo.get_source more defensive

src/_pytest/_code/code.py

@@ -734,11 +734,11 @@ class FormattedExcinfo:
     ) -> List[str]:
         """Return formatted and marked up source lines."""
         lines = []
-        if source is None or line_index >= len(source.lines):
+        if source is not None and line_index < 0:
+            line_index += len(source.lines)
+        if source is None or line_index >= len(source.lines) or line_index < 0:
             source = Source("???")
             line_index = 0
-        if line_index < 0:
-            line_index += len(source)
         space_prefix = "    "
         if short:
             lines.append(space_prefix + source.lines[line_index].strip())

testing/code/test_excinfo.py

@@ -1397,6 +1397,29 @@ def test_cwd_deleted(pytester: Pytester) -> None:
     result.stderr.no_fnmatch_line("*INTERNALERROR*")
 
 
+def test_regression_nagative_line_index(pytester: Pytester) -> None:
+    """
+    With Python 3.10 alphas, there was an INTERNALERROR reported in
+    https://github.com/pytest-dev/pytest/pull/8227
+    This test ensures it does not regress.
+    """
+    pytester.makepyfile(
+        """
+        import ast
+        import pytest
+
+
+        def test_literal_eval():
+            with pytest.raises(ValueError, match="^$"):
+                ast.literal_eval("pytest")
+    """
+    )
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines(["* 1 failed in *"])
+    result.stdout.no_fnmatch_line("*INTERNALERROR*")
+    result.stderr.no_fnmatch_line("*INTERNALERROR*")
+
+
 @pytest.mark.usefixtures("limited_recursion_depth")
 def test_exception_repr_extraction_error_on_recursion():
     """