Modernize ConftestImportFailure to use the exception chain

Give ConftestImportFailure the same shape PluginImportFailure just got:
a bare Exception subclass whose argument is the conftest path, with the
original error carried by `raise ... from` on __cause__ instead of a
hand-rolled `cause` attribute duplicating it. The custom __init__ and
__str__ go away; str(e) is the path, which is exactly what the two
message consumers want.

The `cause` attribute dates to 8.0 (e1074f9), which replaced the old
excinfo triplet without deprecation; the class is private to
_pytest.config and a code search finds no external consumers, only
vendored copies of pytest itself.

No user-visible output changes.

Co-Authored-By: Claude Fable 5 <ai@anthropic.com>
Co-Authored-By: Claude Code <ai@anthropic.com>

Author: RonnyPfannschmidt

changelog/14943.misc.rst

@@ -0,0 +1 @@
+``ConftestImportFailure`` no longer carries a ``cause`` attribute; the original error is available as ``__cause__`` via the regular exception chain.

src/_pytest/config/__init__.py

@@ -129,17 +129,11 @@ class ExitCode(enum.IntEnum):
 
 
 class ConftestImportFailure(Exception):
-    def __init__(
-        self,
-        path: pathlib.Path,
-        *,
-        cause: Exception,
-    ) -> None:
-        self.path = path
-        self.cause = cause
+    """A conftest.py raised while being imported.
 
-    def __str__(self) -> str:
-        return f"{type(self.cause).__name__}: {self.cause} (from {self.path})"
+    The path of the failing conftest is the exception argument; the original
+    error is chained as ``__cause__``.
+    """
 
 
 class PluginImportFailure(Exception):
@@ -179,9 +173,8 @@ def _print_import_error(header: str, cause: BaseException, file: TextIO) -> None
 
 
 def print_conftest_import_error(e: ConftestImportFailure, file: TextIO) -> None:
-    _print_import_error(
-        f"ImportError while loading conftest '{e.path}'.", e.cause, file
-    )
+    assert e.__cause__ is not None, f"{e!r} must be raised `from` the original error"
+    _print_import_error(f"ImportError while loading conftest '{e}'.", e.__cause__, file)
 
 
 def print_plugin_import_error(e: PluginImportFailure, file: TextIO) -> None:
@@ -793,7 +786,7 @@ def _importconftest(
             )
         except Exception as e:
             assert e.__traceback__ is not None
-            raise ConftestImportFailure(conftestpath, cause=e) from e
+            raise ConftestImportFailure(conftestpath) from e
 
         self._check_non_top_pytest_plugins(mod, conftestpath)
 
@@ -1748,7 +1741,7 @@ def parse(self, args: list[str], addopts: bool = True) -> None:
                 # we don't want to prevent --help/--version to work
                 # so just let it pass and print a warning at the end
                 self.issue_config_time_warning(
-                    PytestConfigWarning(f"could not load initial conftests: {e.path}"),
+                    PytestConfigWarning(f"could not load initial conftests: {e}"),
                     stacklevel=2,
                 )
             else:

src/_pytest/debugging.py

@@ -382,7 +382,8 @@ def _postmortem_exc_or_tb(
     elif isinstance(excinfo.value, ConftestImportFailure):
         # A config.ConftestImportFailure is not useful for post_mortem.
         # Use the underlying exception instead:
-        cause = excinfo.value.cause
+        cause = excinfo.value.__cause__
+        assert cause is not None
         if get_exc:
             return cause
 

src/_pytest/nodes.py

@@ -412,7 +412,8 @@ def _repr_failure_py(
         from _pytest.fixtures import FixtureLookupError
 
         if isinstance(excinfo.value, ConftestImportFailure):
-            excinfo = ExceptionInfo.from_exception(excinfo.value.cause)
+            assert excinfo.value.__cause__ is not None
+            excinfo = ExceptionInfo.from_exception(excinfo.value.__cause__)
         if isinstance(excinfo.value, fail.Exception):
             if not excinfo.value.pytrace:
                 style = "value"

testing/test_config.py

@@ -3038,17 +3038,15 @@ def test_pytest_plugins_in_non_top_level_conftest_unsupported_no_false_positives
 
 
 def test_conftest_import_error_repr(tmp_path: Path) -> None:
-    """`ConftestImportFailure` should use a short error message and readable
-    path to the failed conftest.py file."""
+    """`ConftestImportFailure` carries the failed conftest.py path as its
+    argument and the original error as its cause."""
     path = tmp_path.joinpath("foo/conftest.py")
-    with pytest.raises(
-        ConftestImportFailure,
-        match=re.escape(f"RuntimeError: some error (from {path})"),
-    ):
+    with pytest.raises(ConftestImportFailure, match=re.escape(str(path))) as excinfo:
         try:
             raise RuntimeError("some error")
         except Exception as exc:
-            raise ConftestImportFailure(path, cause=exc) from exc
+            raise ConftestImportFailure(path) from exc
+    assert str(excinfo.value.__cause__) == "some error"
 
 
 def test_strtobool() -> None: