fix(config): report a malformed ini as a handled error instead of a traceback (#4027)

Author: VXNCXNX

docs/changelog/4027.bugfix.rst

@@ -0,0 +1,2 @@
+Report a malformed ``tox.ini`` or ``setup.cfg`` as a handled error during config discovery instead of raising an
+unhandled :class:`configparser.Error` traceback - by :user:`VXNCXNX`

src/tox/config/source/ini.py

@@ -4,6 +4,7 @@
 
 from collections import defaultdict
 from configparser import ConfigParser
+from configparser import Error as ConfigParserError
 from itertools import chain
 from typing import TYPE_CHECKING
 
@@ -34,7 +35,12 @@ def __init__(self, path: Path, content: str | None = None) -> None:
             if not path.exists():
                 raise ValueError
             content = path.read_text(encoding="utf-8")
-        self._parser.read_string(content, str(path))
+        try:
+            self._parser.read_string(content, str(path))
+        except ConfigParserError as exc:
+            # configparser errors do not derive from ValueError, unlike tomllib ones, so translate them to let
+            # config discovery report them as a handled error instead of leaking a traceback
+            raise ValueError(exc) from exc
         self._section_mapping: defaultdict[str, list[str]] = defaultdict(list)
 
     def transform_section(self, section: Section) -> Section:  # ruff:ignore[no-self-use]

tests/config/source/test_discover.py

@@ -67,6 +67,17 @@ def test_malformed_toml_in_dir_reports_error(tox_project: ToxProjectCreator) ->
     assert "failed loading" in outcome.out
 
 
+def test_malformed_ini_in_dir_reports_error(tox_project: ToxProjectCreator) -> None:
+    """Config discovery in a directory should report ini parse errors instead of raising a traceback."""
+    project = tox_project({})
+    # Write a tox.ini with an unterminated section header
+    (project.path / "tox.ini").write_text("[tox\nenv_list = a\n", encoding="utf-8")
+    outcome = project.run("l", "-c", str(project.path))
+    outcome.assert_failed()
+    assert "failed loading" in outcome.out
+    assert "File contains no section headers" in outcome.out
+
+
 def test_toml_native_preferred_over_legacy_tox_ini(tox_project: ToxProjectCreator) -> None:
     """When pyproject.toml has both legacy_tox_ini and native TOML config, native TOML should win."""
     pyproject = """\