Patch sys.unraisablehook to format exception groups

Fixes #18.

CHANGES.rst

@@ -3,6 +3,10 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Patch ``sys.unraisablehook`` to properly format exceptiongroups
+
 **1.0.0rc8**
 
 - Don't monkey patch anything if ``sys.excepthook`` has been altered

src/exceptiongroup/_formatting.py

@@ -252,6 +252,17 @@ def exceptiongroup_excepthook(
     sys.stderr.write("".join(traceback.format_exception(etype, value, tb)))
 
 
+def exceptiongroup_unraisablehook(__unraisable: sys.UnraisableHookArgs) -> None:
+    err_msg = __unraisable.err_msg or "Exception ignored in"
+    formatted_tb = "".join(traceback.format_tb(__unraisable.exc_traceback))
+    formatted_tb += "".join(
+        traceback.format_exception(
+            __unraisable.exc_type, __unraisable.exc_value, __unraisable.exc_traceback
+        )
+    )
+    sys.stderr.write(f"{err_msg}: {__unraisable.object!r}\n{formatted_tb}")
+
+
 if sys.excepthook is sys.__excepthook__:
     traceback_exception_original_init = traceback.TracebackException.__init__
     traceback.TracebackException.__init__ = (  # type: ignore[assignment]
@@ -271,3 +282,7 @@ def exceptiongroup_excepthook(
         traceback.TracebackException, "_format_syntax_error", None
     )
     sys.excepthook = exceptiongroup_excepthook
+
+    # Patch sys.unraisablehook if it's untouched
+    if sys.version_info >= (3, 8) and sys.unraisablehook is sys.__unraisablehook__:
+        sys.unraisablehook = exceptiongroup_unraisablehook

tests/test_formatting.py

@@ -1,6 +1,10 @@
+import gc
 import sys
 
+import pytest
+
 from exceptiongroup import ExceptionGroup
+from exceptiongroup._formatting import exceptiongroup_unraisablehook
 
 
 def test_formatting(capsys):
@@ -119,3 +123,36 @@ def test_formatting_syntax_error(capsys):
 SyntaxError: invalid syntax
 """
     )
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 8), reason="sys.unraisablehook was added in Python 3.8"
+)
+def test_unraisablehook(capsys, monkeypatch):
+    # Pytest overrides sys.unraisablehook so we temporarily override that here
+    monkeypatch.setattr(sys, "unraisablehook", exceptiongroup_unraisablehook)
+
+    class Foo:
+        def __del__(self):
+            raise ExceptionGroup("the bad", [Exception("critical debug information")])
+
+    Foo()
+    gc.collect()
+
+    lineno = Foo.__del__.__code__.co_firstlineno
+    module_prefix = "" if sys.version_info >= (3, 11) else "exceptiongroup."
+    output = capsys.readouterr().err
+    assert output == (
+        f"""\
+Exception ignored in: {Foo.__del__!r}
+  File "{__file__}", line {lineno + 1}, in __del__
+    raise ExceptionGroup("the bad", [Exception("critical debug information")])
+  + Exception Group Traceback (most recent call last):
+  |   File "{__file__}", line {lineno + 1}, in __del__
+  |     raise ExceptionGroup("the bad", [Exception("critical debug information")])
+  | {module_prefix}ExceptionGroup: the bad (1 sub-exception)
+  +-+---------------- 1 ----------------
+    | Exception: critical debug information
+    +------------------------------------
+"""
+    )