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gh-103791: Make contextlib.suppress also act on exceptions within an ExceptionGroup#103792

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ambv merged 6 commits intopython:mainfromambv:gh-103791
Apr 24, 2023
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8 changes: 8 additions & 0 deletionsDoc/library/contextlib.rst
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Expand Up@@ -304,8 +304,16 @@ Functions and classes provided:

This context manager is :ref:`reentrant <reentrant-cms>`.

If the code within the :keyword:`!with` block raises an
:exc:`ExceptionGroup`, suppressed exceptions are removed from the
group. If any others are left, the modified group is re-raised.
Otherwise, the exception group is empty and so nothing is raised.

.. versionadded:: 3.4

.. versionchanged:: 3.12
``suppress`` now supports suppressing exceptions raised as
part of an :exc:`ExceptionGroup`.

.. function:: redirect_stdout(new_target)

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11 changes: 10 additions & 1 deletionLib/contextlib.py
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Expand Up@@ -441,7 +441,16 @@ def __exit__(self, exctype, excinst, exctb):
# exactly reproduce the limitations of the CPython interpreter.
#
# See http://bugs.python.org/issue12029 for more details
return exctype is not None and issubclass(exctype, self._exceptions)
if exctype is None:
return
if issubclass(exctype, self._exceptions):
return True
if issubclass(exctype, ExceptionGroup):
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Was there a specific reason to handleExceptionGroup but notBaseExceptionGroup?

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I doubt it. Want to make a PR?

Zac-HD reacted with thumbs up emoji
match, rest = excinst.split(self._exceptions)
if rest is None:
return True
raise rest
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This isn't ideal as it makes the exception group's own traceback include thedef __exit__(...) frame. For example:

  File "/Volumes/RAMDisk/cpython/Lib/test/test_contextlib.py", line 1220, in test_exception_groups    with suppress(ValueError):  File "/Volumes/RAMDisk/cpython/Lib/contextlib.py", line 454, in __exit__    raise rest from excinst  File "/Volumes/RAMDisk/cpython/Lib/test/test_contextlib.py", line 1221, in test_exception_groups    raise eg_all()

in case of the newly added test.

This isn't a big problem because:

  • the group's member exceptions contain pristine tracebacks, only the group itself contains extra frames; and
  • theraise of the original group is still included in the traceback because we used.split() to create the new object.

Ideally, we wouldn't need this. However, the API of__exit__ makes it impossible to replace theExceptionGroup instance with another one, whileExceptionGroup itself makes its exceptions read-only.

return False


class _BaseExitStack:
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25 changes: 25 additions & 0 deletionsLib/test/support/testcase.py
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@@ -0,0 +1,25 @@
class ExceptionIsLikeMixin:
def assertExceptionIsLike(self, exc, template):
"""
Passes when the provided `exc` matches the structure of `template`.
Individual exceptions don't have to be the same objects or even pass
an equality test: they only need to be the same type and contain equal
`exc_obj.args`.
"""
if exc is None and template is None:
return

if template is None:
self.fail(f"unexpected exception: {exc}")

if exc is None:
self.fail(f"expected an exception like {template!r}, got None")

if not isinstance(exc, ExceptionGroup):
self.assertEqual(exc.__class__, template.__class__)
self.assertEqual(exc.args[0], template.args[0])
else:
self.assertEqual(exc.message, template.message)
self.assertEqual(len(exc.exceptions), len(template.exceptions))
for e, t in zip(exc.exceptions, template.exceptions):
self.assertExceptionIsLike(e, t)
27 changes: 26 additions & 1 deletionLib/test/test_contextlib.py
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Expand Up@@ -10,6 +10,7 @@
from contextlib import * # Tests __all__
from test import support
from test.support import os_helper
from test.support.testcase import ExceptionIsLikeMixin
import weakref


Expand DownExpand Up@@ -1148,7 +1149,7 @@ class TestRedirectStderr(TestRedirectStream, unittest.TestCase):
orig_stream = "stderr"


class TestSuppress(unittest.TestCase):
class TestSuppress(ExceptionIsLikeMixin,unittest.TestCase):

@support.requires_docstrings
def test_instance_docs(self):
Expand DownExpand Up@@ -1202,6 +1203,30 @@ def test_cm_is_reentrant(self):
1/0
self.assertTrue(outer_continued)

def test_exception_groups(self):
eg_ve = lambda: ExceptionGroup(
"EG with ValueErrors only",
[ValueError("ve1"), ValueError("ve2"), ValueError("ve3")],
)
eg_all = lambda: ExceptionGroup(
"EG with many types of exceptions",
[ValueError("ve1"), KeyError("ke1"), ValueError("ve2"), KeyError("ke2")],
)
with suppress(ValueError):
raise eg_ve()
with suppress(ValueError, KeyError):
raise eg_all()
with self.assertRaises(ExceptionGroup) as eg1:
with suppress(ValueError):
raise eg_all()
self.assertExceptionIsLike(
eg1.exception,
ExceptionGroup(
"EG with many types of exceptions",
[KeyError("ke1"), KeyError("ke2")],
),
)


class TestChdir(unittest.TestCase):
def make_relative_path(self, *parts):
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22 changes: 2 additions & 20 deletionsLib/test/test_except_star.py
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@@ -1,6 +1,7 @@
import sys
import unittest
import textwrap
from test.support.testcase import ExceptionIsLikeMixin

class TestInvalidExceptStar(unittest.TestCase):
def test_mixed_except_and_except_star_is_syntax_error(self):
Expand DownExpand Up@@ -169,26 +170,7 @@ def f(x):
self.assertIsInstance(exc, ExceptionGroup)


class ExceptStarTest(unittest.TestCase):
def assertExceptionIsLike(self, exc, template):
if exc is None and template is None:
return

if template is None:
self.fail(f"unexpected exception: {exc}")

if exc is None:
self.fail(f"expected an exception like {template!r}, got None")

if not isinstance(exc, ExceptionGroup):
self.assertEqual(exc.__class__, template.__class__)
self.assertEqual(exc.args[0], template.args[0])
else:
self.assertEqual(exc.message, template.message)
self.assertEqual(len(exc.exceptions), len(template.exceptions))
for e, t in zip(exc.exceptions, template.exceptions):
self.assertExceptionIsLike(e, t)

class ExceptStarTest(ExceptionIsLikeMixin, unittest.TestCase):
def assertMetadataEqual(self, e1, e2):
if e1 is None or e2 is None:
self.assertTrue(e1 is None and e2 is None)
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@@ -0,0 +1,3 @@
:class:`contextlib.suppress` now supports suppressing exceptions raised as
part of an :exc:`ExceptionGroup`. If other exceptions exist on the group, it
is now re-raised without the suppressed ones.
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