[Python-checkins] bpo-37261: Document sys.unraisablehook corner cases (GH-14059)
Victor Stinner
webhook-mailer at python.org
Fri Jun 14 12:03:32 EDT 2019
https://github.com/python/cpython/commit/212646cae6b7c4ddc8d98c8b9b6d39a5f259e864
commit: 212646cae6b7c4ddc8d98c8b9b6d39a5f259e864
branch: master
author: Victor Stinner <vstinner at redhat.com>
committer: GitHub <noreply at github.com>
date: 2019-06-14T18:03:22+02:00
summary:
bpo-37261: Document sys.unraisablehook corner cases (GH-14059)
Document reference cycle and resurrected objects issues in
sys.unraisablehook() and threading.excepthook() documentation.
Fix test.support.catch_unraisable_exception(): __exit__() no longer
ignores unraisable exceptions.
Fix test_io test_writer_close_error_on_close(): use a second
catch_unraisable_exception() to catch the BufferedWriter unraisable
exception.
files:
M Doc/library/sys.rst
M Doc/library/test.rst
M Doc/library/threading.rst
M Lib/test/support/__init__.py
M Lib/test/test_io.py
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 5bde6870717c..817c3f1e56f9 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1514,13 +1514,21 @@ always available.
* *err_msg*: Error message, can be ``None``.
* *object*: Object causing the exception, can be ``None``.
- :func:`sys.unraisablehook` can be overridden to control how unraisable
- exceptions are handled.
-
The default hook formats *err_msg* and *object* as:
``f'{err_msg}: {object!r}'``; use "Exception ignored in" error message
if *err_msg* is ``None``.
+ :func:`sys.unraisablehook` can be overridden to control how unraisable
+ exceptions are handled.
+
+ Storing *exc_value* using a custom hook can create a reference cycle. It
+ should be cleared explicitly to break the reference cycle when the
+ exception is no longer needed.
+
+ Storing *object* using a custom hook can resurrect it if it is set to an
+ object which is being finalized. Avoid storing *object* after the custom
+ hook completes to avoid resurrecting objects.
+
See also :func:`excepthook` which handles uncaught exceptions.
.. versionadded:: 3.8
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 0a98c882465d..920c018084b8 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -1086,17 +1086,13 @@ The :mod:`test.support` module defines the following functions:
Context manager catching unraisable exception using
:func:`sys.unraisablehook`.
- If the *object* attribute of the unraisable hook is set and the object is
- being finalized, the object is resurrected because the context manager
- stores a strong reference to it (``cm.unraisable.object``).
-
Storing the exception value (``cm.unraisable.exc_value``) creates a
reference cycle. The reference cycle is broken explicitly when the context
manager exits.
- Exiting the context manager clears the stored unraisable exception. It can
- trigger a new unraisable exception (ex: the resurrected object is finalized
- again and raises the same exception): it is silently ignored in this case.
+ Storing the object (``cm.unraisable.object``) can resurrect it if it is set
+ to an object which is being finalized. Exiting the context manager clears
+ the stored object.
Usage::
diff --git a/Doc/library/threading.rst b/Doc/library/threading.rst
index 2907b65f5bca..9ffd5cd58179 100644
--- a/Doc/library/threading.rst
+++ b/Doc/library/threading.rst
@@ -58,6 +58,14 @@ This module defines the following functions:
:func:`threading.excepthook` can be overridden to control how uncaught
exceptions raised by :func:`Thread.run` are handled.
+ Storing *exc_value* using a custom hook can create a reference cycle. It
+ should be cleared explicitly to break the reference cycle when the
+ exception is no longer needed.
+
+ Storing *object* using a custom hook can resurrect it if it is set to an
+ object which is being finalized. Avoid storing *object* after the custom
+ hook completes to avoid resurrecting objects.
+
.. seealso::
:func:`sys.excepthook` handles uncaught exceptions.
diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 174e0456dc71..7c0efc783edb 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -3040,17 +3040,13 @@ class catch_unraisable_exception:
"""
Context manager catching unraisable exception using sys.unraisablehook.
- If the *object* attribute of the unraisable hook is set and the object is
- being finalized, the object is resurrected because the context manager
- stores a strong reference to it (cm.unraisable.object).
-
Storing the exception value (cm.unraisable.exc_value) creates a reference
cycle. The reference cycle is broken explicitly when the context manager
exits.
- Exiting the context manager clears the stored unraisable exception. It can
- trigger a new unraisable exception (ex: the resurrected object is finalized
- again and raises the same exception): it is silently ignored in this case.
+ Storing the object (cm.unraisable.object) can resurrect it if it is set to
+ an object which is being finalized. Exiting the context manager clears the
+ stored object.
Usage:
@@ -3080,10 +3076,5 @@ def __enter__(self):
return self
def __exit__(self, *exc_info):
- # Clear the unraisable exception to explicitly break a reference cycle.
- # It can call _hook() again: ignore the new unraisable exception in
- # this case.
- self.unraisable = None
-
sys.unraisablehook = self._old_hook
del self.unraisable
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 55686d743983..fc474c99053d 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2072,8 +2072,12 @@ def writer_close():
writer.close = lambda: None
writer = None
+ # Ignore BufferedWriter (of the BufferedRWPair) unraisable exception
with support.catch_unraisable_exception():
- pair = None
+ # Ignore BufferedRWPair unraisable exception
+ with support.catch_unraisable_exception():
+ pair = None
+ support.gc_collect()
support.gc_collect()
def test_reader_writer_close_error_on_close(self):
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