Nick Coghlan <[email protected]> added the comment:
Sorry, I wasn't clear on what I meant by "chained correctly", and that's the
part that makes this trickier than the way contextlib.nested did it. I'm
referring to the __context__ attribute on exceptions that is set automatically
when an exception occurs in another exception handler, which makes error
displays like the following possible:
>>> from contextlib import ExitStack
>>> with ExitStack() as stack:
... @stack.callback
... def f():
... 1/0
... @stack.callback
... def f():
... {}[1]
...
Traceback (most recent call last):
File "/home/ncoghlan/devel/py3k/Lib/contextlib.py", line 243, in
_invoke_next_callback
suppress_exc = _invoke_next_callback(exc_details)
File "/home/ncoghlan/devel/py3k/Lib/contextlib.py", line 240, in
_invoke_next_callback
return cb(*exc_details)
File "/home/ncoghlan/devel/py3k/Lib/contextlib.py", line 200, in _exit_wrapper
callback(*args, **kwds)
File "<stdin>", line 7, in f
KeyError: 1
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
File "/home/ncoghlan/devel/py3k/Lib/contextlib.py", line 256, in __exit__
return _invoke_next_callback(exc_details)
File "/home/ncoghlan/devel/py3k/Lib/contextlib.py", line 245, in
_invoke_next_callback
suppress_exc = cb(*sys.exc_info())
File "/home/ncoghlan/devel/py3k/Lib/contextlib.py", line 200, in _exit_wrapper
callback(*args, **kwds)
File "<stdin>", line 4, in f
ZeroDivisionError: division by zero
The recursive approach maintains that behaviour automatically because it really
does create a nested set of exception handlers. With the iterative approach, we
leave the exception handler before invoking the next callback, so we end up
bypassing the native chaining machinery and will need to recreate it manually.
If you can make that work, then we'd end up with the best of both worlds: the
individual exceptions would be clean (since they wouldn't be cluttered with the
recursive call stack created by the unwinding process), but exception chaining
would still keep track of things if multiple exceptions are encountered in
cleanup operations.
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<http://bugs.python.org/issue14963>
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