display single contained exception in excgroups in test summary

changelog/12943.improvement.rst

@@ -0,0 +1 @@
+If a test fails with an exceptiongroup with a single exception, the contained exception will now be displayed in the short test summary info.

src/_pytest/_code/code.py

@@ -589,6 +589,31 @@
         representation is returned (so 'AssertionError: ' is removed from
         the beginning).
         """
+
+        def _get_single_subexc(
+            eg: BaseExceptionGroup[BaseException],
+        ) -> BaseException | None:
+            res: BaseException | None = None
+            for subexc in eg.exceptions:
+                if res is not None:
+                    return None
+
+                if isinstance(subexc, BaseExceptionGroup):
+                    res = _get_single_subexc(subexc)
+                    if res is None:
+                        # there were multiple exceptions in the subgroup
+                        return None
+                else:
+                    res = subexc
+            return res
+
+        if (
+            tryshort
+            and isinstance(self.value, BaseExceptionGroup)
+            and (subexc := _get_single_subexc(self.value)) is not None
+        ):
+            return f"[in {type(self.value).__name__}] {subexc!r}"
+
         lines = format_exception_only(self.type, self.value)
         text = "".join(lines)
         text = text.rstrip()

testing/code/test_excinfo.py

@@ -1703,6 +1703,83 @@ def test_exceptiongroup(pytester: Pytester, outer_chain, inner_chain) -> None:
     _exceptiongroup_common(pytester, outer_chain, inner_chain, native=False)
 
 
+def test_exceptiongroup_short_summary_info(pytester: Pytester):
+    pytester.makepyfile(
+        """
+        import sys
+
+        if sys.version_info < (3, 11):
+            from exceptiongroup import BaseExceptionGroup, ExceptionGroup
+
+        def test_base() -> None:
+            raise BaseExceptionGroup("NOT IN SUMMARY", [SystemExit("a" * 10)])
+
+        def test_nonbase() -> None:
+            raise ExceptionGroup("NOT IN SUMMARY", [ValueError("a" * 10)])
+
+        def test_nested() -> None:
+            raise ExceptionGroup(
+                "NOT DISPLAYED", [
+                    ExceptionGroup("NOT IN SUMMARY", [ValueError("a" * 10)])
+                ]
+            )
+
+        def test_multiple() -> None:
+            raise ExceptionGroup(
+                "b" * 10,
+                [
+                    ValueError("NOT IN SUMMARY"),
+                    TypeError("NOT IN SUMMARY"),
+                ]
+            )
+
+        def test_nested_multiple() -> None:
+            raise ExceptionGroup(
+                "b" * 10,
+                [
+                    ExceptionGroup(
+                        "c" * 10,
+                        [
+                            ValueError("NOT IN SUMMARY"),
+                            TypeError("NOT IN SUMMARY"),
+                        ]
+                    )
+                ]
+            )
+        """
+    )
+    # run with -vv to not truncate summary info, default width in tests is very low
+    result = pytester.runpytest("-vv")
+    assert result.ret == 1
+    backport_str = "exceptiongroup." if sys.version_info < (3, 11) else ""
+    result.stdout.fnmatch_lines(
+        [
+            "*= short test summary info =*",
+            (
+                "FAILED test_exceptiongroup_short_summary_info.py::test_base - "
+                "[in BaseExceptionGroup] SystemExit('aaaaaaaaaa')"
+            ),
+            (
+                "FAILED test_exceptiongroup_short_summary_info.py::test_nonbase - "
+                "[in ExceptionGroup] ValueError('aaaaaaaaaa')"
+            ),
+            (
+                "FAILED test_exceptiongroup_short_summary_info.py::test_nested - "
+                "[in ExceptionGroup] ValueError('aaaaaaaaaa')"
+            ),
+            (
+                "FAILED test_exceptiongroup_short_summary_info.py::test_multiple - "
+                f"{backport_str}ExceptionGroup: bbbbbbbbbb (2 sub-exceptions)"
+            ),
+            (
+                "FAILED test_exceptiongroup_short_summary_info.py::test_nested_multiple - "
+                f"{backport_str}ExceptionGroup: bbbbbbbbbb (1 sub-exception)"
+            ),
+            "*= 5 failed in *",
+        ]
+    )
+
+
 @pytest.mark.parametrize("tbstyle", ("long", "short", "auto", "line", "native"))
 def test_all_entries_hidden(pytester: Pytester, tbstyle: str) -> None:
     """Regression test for #10903."""