unittest.TestResult fails with exception deriving from frozen dataclass #117211
Labels
type-bug
An unexpected behavior, bug, or error
Comments
sobolevn
changed the title
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Since there's no reproducer right now, I am not fully convinced that this is a bug. Exceptions are expected to be mutable. If you drop some parts of an API from the core CPython object, it will raise an expected error. Do you agree? :) |
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I managed to reproduce it now, run with from dataclasses import dataclass
import unittest
class TestFrozenDataclassException(unittest.IsolatedAsyncioTestCase):
async def test_frozen_dataclass_exception(self):
@dataclass(frozen=True)
class FrozenException(Exception):
value: int
try:
raise FrozenException(value=123)
except:
raise ValueError("foo")Which results in: |
I was trying to find if that was documented anywhere but could not find any definitive documentation, maybe I'm not looking at the right place though. The python3.11+ exception notes feature certainly relies on mutability as well but I could not find anything in the PEP describing what happens if an exception overrides Regardless, I would expect a test framework to handle that in a way that only fails the offending test and not the entire test run. |
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Well, if something is not documented as immutable, it is mutable by default :) However, I think that this part:
makes sense. Probably error reporting here can be better (for all cases, not just this one). |
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Same in main, but with much nicer colored output :) |
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I'm going to close this as a duplicate of #99856, as it's the same underlying issue that's being discussed (many places in the stdlib assume that exceptions are mutable; using |
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This small patch avoids this problem, though of course I'm not sure of any other implications here: thomas@thomas-xps13:~/Projects/cpython$ git diff
diff --git a/Lib/unittest/result.py b/Lib/unittest/result.py
index 3ace0a5..da32563 100644
--- a/Lib/unittest/result.py
+++ b/Lib/unittest/result.py
@@ -223,7 +223,11 @@ def _clean_tracebacks(self, exctype, value, tb, test):
ret = tb
first = False
else:
- value.__traceback__ = tb
+ try:
+ value.__traceback__ = tb
+ except AttributeError:
+ # Attribute is not settable in exception class
+ pass
if value is not None:
for c in (value.__cause__, value.__context__): |
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I'm not sure this is exactly the same as the other case ticket since we are speaking about a test runner here. Would it make sense to add a note either in dataclasses or Exception documentation to explicitly mention that Exception subclasses are expected to be mutable and should thus not be frozen (from the dataclass perspective). |
The context in which the issue is arising is different, but it's the same underlying issue: many places in the stdlib and third-party libraries (including the test runners provided by the stdlib) depend on being able to mutate attributes of exception instances; you'll break a lot of assumptions in a lot of places if you try to create a frozen exception class.
I definitely agree that we need to document this better! Let's discuss on the other issue where it would be best to add that documentation, though 👍 |
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Bug report
Bug description:
The unittest.TestResult class assumes that it can assign to the
__traceback__attribute of the exception.This will not work with an exception that is also a frozen dataclass.
I have not been able to create a simple reproducer for this but as far as I can see the offending code is still in main.
See https://github.com/python/cpython/blob/v3.13.0a5/Lib/unittest/result.py#L226
For example:
Here is the traceback using python3.10.12 in an ubuntu based docker container.
CPython versions tested on:
3.10
Operating systems tested on:
Linux
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