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bpo-36829: Add test.support.catch_unraisable_exception() #13490

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Merged
vstinner merged 3 commits into from
May 22, 2019
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bpo-36829: Add test.support.catch_unraisable_exception() #13490

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1fc116b
bpo-36829: Add test.support.catch_unraisable_exception()
vstinner May 22, 2019
1ffe98d
catch_unraisable_exception.__exit__ clears unraisable
vstinner May 22, 2019
5c1bf39
Cleanup
vstinner May 22, 2019
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36 changes: 36 additions & 0 deletions Lib/test/support/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3034,3 +3034,39 @@ def collision_stats(nbins, nballs):
collisions = k - occupied
var = dn*(dn-1)*((dn-2)/dn)**k + meanempty * (1 - meanempty)
return float(collisions), float(var.sqrt())


class catch_unraisable_exception:
"""
Context manager catching unraisable exception using sys.unraisablehook.

Usage:

try:
with support.catch_unraisable_exception() as cm:
...

# check the expected unraisable exception
...
finally:

@graingert graingert May 22, 2019 •
edited
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this try/with/finally is somewhat tricky, how about:

try:
    with support.throw_unraisable_exceptions():
        ...
except Exception as e:
    ... # the exception is now here

eg:

@contextlib.contextmanager
def throw_unraisable_exceptions():
    unraisable = None
    old_hook = sys.unraisablehook

    def hook(exc):
        nonlocal unraisable
        unraisable = exc

    sys.unraisablehook = hook
    try:
        yield
        if unraisable is not None:
            raise unraisable
    finally:
        unraisable = None
        sys.unraisablehook = old_hook

@graingert graingert May 22, 2019 •
edited
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or recommend usage like this:

    with support.catch_unraisable_exceptions() as cm:
        ...
        cm.unraisable  # only available here
    assert cm.unraisable is None  # now it's gone

by updating __exit__:

   def __exit__(self, *exc_info):
        self.unraisable = None  # clear unraisable here
        sys.unraisablehook = self._old_hook

@vstinner vstinner May 22, 2019

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Good idea :-) I implemented your __exit__ idea to avoid the need for try/finally.

throw_unraisable_exceptions() might be useful, but modified tests needs to access the 'obj' attribute of the unraisable hook. Later, they might also want to get access to the 'err_msg' attribute: #13488

@graingert graingert May 22, 2019

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There's still a sharp edge here:

how about making cm.unraisable throw if accessed outside the context instead of being None, eg:

def __exit__(self, *exc_info):
    del self.unraisable
    sys.unraisablehook = self._old_hook

@graingert graingert May 22, 2019 •
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and if you want to access additional unraisablehook info using throw_unraisable_exceptions:

class UnraisableException(Exception):
    def __init__(self, unraisable):
        self.unraisable = unraisable
        super().__init__(self, unraisable)


@contextlib.contextmanager
def throw_unraisable_exceptions():
    unraisable = None
    old_hook = sys.unraisablehook

    def hook(unraisable_):
        nonlocal unraisable
        unraisable = unraisable_

    sys.unraisablehook = hook
    try:
        yield
        if unraisable is not None:
            raise UnraisableException(unraisable) from unraisable.exc_value
    finally:
        sys.unraisablehook = old_hook

then you can use it with:

try:
    with throw_unraisable_exceptions():
        ...
except UnraisableException as e:
    print(repr(e.__cause__))
    err_msg = e.unraisable.err_msg

@vstinner vstinner May 24, 2019

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I wrote PR #13554 to implement your "del self.unraisable" idea.

# Explicitly break reference cycle
cm = None

The finally block is required: cm.unraisable contains a traceback object
which indirectly references the 'cm' variable, it's a reference cycle.
"""

def __init__(self):
self.unraisable = None
self._old_hook = None

def _hook(self, unraisable):
self.unraisable = unraisable

def __enter__(self):
self._old_hook = sys.unraisablehook
sys.unraisablehook = self._hook
return self

def __exit__(self, *exc_info):
sys.unraisablehook = self._old_hook
24 changes: 18 additions & 6 deletions Lib/test/test_coroutines.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2342,12 +2342,24 @@ async def corofn():
orig_wuc = warnings._warn_unawaited_coroutine
try:
warnings._warn_unawaited_coroutine = lambda coro: 1/0
with support.captured_stderr() as stream:
corofn()
support.gc_collect()
self.assertIn("Exception ignored in", stream.getvalue())
self.assertIn("ZeroDivisionError", stream.getvalue())
self.assertIn("was never awaited", stream.getvalue())
try:
# Note: catch_unraisable_exception() resurect the coroutine
with support.catch_unraisable_exception() as cm, \
support.captured_stderr() as stream:
# only store repr() to avoid keeping the coroutine alive
coro = corofn()
coro_repr = repr(coro)

# clear reference to the coroutine without awaiting for it
del coro
support.gc_collect()

self.assertEqual(repr(cm.unraisable.object), coro_repr)
self.assertEqual(cm.unraisable.exc_type, ZeroDivisionError)
self.assertIn("was never awaited", stream.getvalue())

@vstinner vstinner May 22, 2019

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It's a little bit strange to have to catch half of the error message from stderr, and the other half from sys.unraisablehook. My PR #13488 will allow to inject the error message in the unraisable exception to catch both at the same time.

finally:
# Explicitly break reference cycle
cm = None

del warnings._warn_unawaited_coroutine
with support.captured_stderr() as stream:
Expand Down
26 changes: 12 additions & 14 deletions Lib/test/test_exceptions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,9 @@
check_warnings, cpython_only, gc_collect, run_unittest,
no_tracing, unlink, import_module, script_helper,
SuppressCrashReport)
from test import support


class NaiveException(Exception):
def __init__(self, x):
self.x = x
Expand Down Expand Up @@ -1190,20 +1193,15 @@ def __del__(self):
for test_class in (BrokenDel, BrokenExceptionDel):
with self.subTest(test_class):
obj = test_class()
with captured_stderr() as stderr:
del obj
report = stderr.getvalue()
self.assertIn("Exception ignored", report)
self.assertIn(test_class.__del__.__qualname__, report)
self.assertIn("test_exceptions.py", report)
self.assertIn("raise exc", report)
if test_class is BrokenExceptionDel:
self.assertIn("BrokenStrException", report)
self.assertIn("<exception str() failed>", report)
else:
self.assertIn("ValueError", report)
self.assertIn("del is broken", report)
self.assertTrue(report.endswith("\n"))
try:
with support.catch_unraisable_exception() as cm:
del obj

self.assertEqual(cm.unraisable.object, test_class.__del__)
self.assertIsNotNone(cm.unraisable.exc_traceback)
finally:
# Explicitly break reference cycle
cm = None

def test_unhandled(self):
# Check for sensible reporting of unhandled exceptions
Expand Down
32 changes: 16 additions & 16 deletions Lib/test/test_generators.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2156,25 +2156,25 @@ def printsolution(self, x):
printing warnings and to doublecheck that we actually tested what we wanted
to test.

>>> import sys, io
>>> old = sys.stderr
>>> from test import support
>>> class Leaker:
... def __del__(self):
... def invoke(message):
... raise RuntimeError(message)
... invoke("del failed")
...
>>> try:
... sys.stderr = io.StringIO()
... class Leaker:
... def __del__(self):
... def invoke(message):
... raise RuntimeError(message)
... invoke("test")
... with support.catch_unraisable_exception() as cm:
... l = Leaker()
... del l
...
... l = Leaker()
... del l
... err = sys.stderr.getvalue().strip()
... "Exception ignored in" in err
... "RuntimeError: test" in err
... "Traceback" in err
... "in invoke" in err
... Leaker.__del__ == cm.unraisable.object
... cm.unraisable.exc_type == RuntimeError
... str(cm.unraisable.exc_value) == "del failed"
... cm.unraisable.exc_traceback is not None
... finally:
... sys.stderr = old
... # Explicitly break reference cycle
... cm = None
True
True
True
Expand Down
41 changes: 41 additions & 0 deletions Lib/test/test_sys.py
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Original file line number Diff line number Diff line change
Expand Up @@ -909,6 +909,47 @@ def test_original_unraisablehook(self):
self.assertIn('Traceback (most recent call last):\n', err)
self.assertIn('ValueError: 42\n', err)

def test_original_unraisablehook_err(self):
# bpo-22836: PyErr_WriteUnraisable() should give sensible reports
class BrokenDel:
def __del__(self):
exc = ValueError("del is broken")
# The following line is included in the traceback report:
raise exc

class BrokenStrException(Exception):
def __str__(self):
raise Exception("str() is broken")

class BrokenExceptionDel:
def __del__(self):
exc = BrokenStrException()
# The following line is included in the traceback report:
raise exc

for test_class in (BrokenDel, BrokenExceptionDel):
with self.subTest(test_class):
obj = test_class()
with test.support.captured_stderr() as stderr, \
test.support.swap_attr(sys, 'unraisablehook',
sys.__unraisablehook__):
# Trigger obj.__del__()
del obj

report = stderr.getvalue()
self.assertIn("Exception ignored", report)
self.assertIn(test_class.__del__.__qualname__, report)
self.assertIn("test_sys.py", report)
self.assertIn("raise exc", report)
if test_class is BrokenExceptionDel:
self.assertIn("BrokenStrException", report)
self.assertIn("<exception str() failed>", report)
else:
self.assertIn("ValueError", report)
self.assertIn("del is broken", report)
self.assertTrue(report.endswith("\n"))


def test_original_unraisablehook_wrong_type(self):
exc = ValueError(42)
with test.support.swap_attr(sys, 'unraisablehook',
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Tests/2019-05-22-12-57-15.bpo-36829.e9mRWC.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Add :func:`test.support.catch_unraisable_exception`: context manager
catching unraisable exception using :func:`sys.unraisablehook`.