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Merge pull request #165 from njsmith/save-the-__context__
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Correctly preserve exception __context__ in MultiError.catch
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njsmith authored May 19, 2017
2 parents 0cac9f6 + a2af710 commit f64200c
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Showing 2 changed files with 35 additions and 10 deletions.
16 changes: 11 additions & 5 deletions trio/_core/_multierror.py
Original file line number Diff line number Diff line change
Expand Up @@ -134,11 +134,17 @@ def __exit__(self, etype, exc, tb):
if filtered_exc is None:
# Swallow the exception
return True
# We can't stop Python from setting __context__, but we can
# hide it. (Unfortunately Python *will* wipe out any existing
# __context__. Nothing we can do about it :-(.)
filtered_exc.__suppress_context__ = True
raise filtered_exc
# When we raise filtered_exc, Python will unconditionally blow
# away its __context__ attribute and replace it with the original
# exc we caught. So after we raise it, we have to pause it while
# it's in flight to put the correct __context__ back.
old_context = filtered_exc.__context__
try:
raise filtered_exc
finally:
_, value, _ = sys.exc_info()
assert value is filtered_exc
value.__context__ = old_context

class MultiError(BaseException):
"""An exception that contains other exceptions; also known as an
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29 changes: 24 additions & 5 deletions trio/_core/tests/test_multierror.py
Original file line number Diff line number Diff line change
Expand Up @@ -228,9 +228,10 @@ def simple_filter(exc):
# ValueError disappeared & KeyError became RuntimeError, so now:
assert isinstance(new_m.exceptions[0], RuntimeError)
assert isinstance(new_m.exceptions[1], NameError)
# we can't stop Python from attaching the original MultiError to this as a
# __context__, but we can hide it:
assert new_m.__suppress_context__
# Make sure that Python did not successfully attach the old MultiError to
# our new MultiError's __context__
assert not new_m.__suppress_context__
assert new_m.__context__ is None

# check preservation of __cause__ and __context__
v = ValueError()
Expand All @@ -241,13 +242,31 @@ def simple_filter(exc):
assert isinstance(excinfo.value.__cause__, KeyError)

v = ValueError()
v.__context__ = KeyError()
context = KeyError()
v.__context__ = context
with pytest.raises(ValueError) as excinfo:
with MultiError.catch(lambda exc: exc):
raise v
assert isinstance(excinfo.value.__context__, KeyError)
assert excinfo.value.__context__ is context
assert not excinfo.value.__suppress_context__

for suppress_context in [True, False]:
v = ValueError()
context = KeyError()
v.__context__ = context
v.__suppress_context__ = suppress_context
distractor = RuntimeError()
with pytest.raises(ValueError) as excinfo:
def catch_RuntimeError(exc):
if isinstance(exc, RuntimeError):
return None
else:
return exc
with MultiError.catch(catch_RuntimeError):
raise MultiError([v, distractor])
assert excinfo.value.__context__ is context
assert excinfo.value.__suppress_context__ == suppress_context


def assert_match_in_seq(pattern_list, string):
offset = 0
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