New submission from Mario Corchero <[email protected]>:
There are multiple places in the standard library where the code captures an
exception and reraises it later (outside of the original except).
This is known to cause a cycle as saving the exception has a traceback that
eventually points back to the exception, but it is especially problematic as it
keeps alive the objects that the user has as well.
This can be reproduced with the following example:
```
import weakref
class A: pass
ref = None
def x():
global ref
cool_var = A()
ref = weakref.ref(cool_var)
assert ref()
try:
1/0
except Exception as e:
ee = e
try:
x()
except Exception:
pass
print(ref())
assert ref() is None
```
There are places in the standard library that try to get around this. See
https://github.com/python/cpython/commit/acb9fa79fa6453c2bbe3ccfc9cad2837feb90093
as an example. This change did not include the exception path though, when the
`err` variable is raised, the issue is still present.
This issue is to fix the instances found in socket.py, codeop.py and dyld.py.
By either setting the variable to None to remove the name or if it needs to be
reraised by using a finally to delete it. This is basically the same that a try
except would do, which automagically adds a finally to delete the local name
used in the `except X as y`.
There was a discussion with twouters,pablogsal,barry about potentially adding
something extra to the exception handling to try to clean this up but it was
suggested the best approach is for the user to handle it and maybe a linter to
potentially flag it, as there might also be multiple references to the
exception being captured. It was also proposed to just change the situations
where this happens in the standard library to remove the name if possible.
Note that eventually those objects will just be collected by the gc, it is just
an improvement.
I'm preparing a PR for those three modules.
----------
components: Library (Lib)
messages: 341620
nosy: mariocj89, pablogsal, twouters
priority: low
severity: normal
status: open
title: Captured exceptions are keeping user objects alive unnecessarily in the
stdlib
type: enhancement
versions: Python 3.8
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue36820>
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