[Python-Dev] Handle errors in cleanup code (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Mon Jun 12 04:07:18 EDT 2017

On 12 June 2017 at 17:31, Martin (gzlist) via Python-Dev <python-dev at python.org> wrote:

On 12/06/2017, Serhiy Storchaka <storchaka at gmail.com> wrote:

But if an error is raised when execute undosomething(), it replaces the original exception which become chaining as the context attribute. The problem is that this can change the type of the exception. If dosomethingother() raises SystemExit and undosomething() raises KeyError, the final exception has type KeyError. For Python 2.7, I've used the following idiom, which always masks errors from the undo: dosomething() try: dosomethingother() except: try: undosomething() finally: raise Unfortunately, that breaks down on Python 3, as the new exception propogates with the original as context..

Relevant tracker issue for that problem: https://bugs.python.org/issue18861

Does it mean that we should rewrite every chunk of code similar to the above? And if such cumbersome code is necessary and become common, maybe it needs syntax support in the language? Or changing the semantic of exceptions raised in error handlers and finally blocks? What I want is a way to chain exceptions in the other direction, raising the original error, but attaching a related one. Unfortunately neither cause or context really help.

Aye, agreed. The key challenge we have is that we're trying to represent the exception state as a linked list, when what we really have once we start taking cleanup errors into account is an exception tree.

The thing that finally clicked for me in this thread is that if we add contextlib.ResourceSet, and have it add a new attribute to the original exception (e.g. "cleanup_errors") with a list of exceptions raised while attempt to cleanup after the earlier exception, then that lets us actually start modelling that tree at runtime.

Once we understand the behaviour we want, then we can consider whether we might want to add a context manager to have any raised exceptions be attached to the currently active exception as cleanup errors rather than as new exceptions in their own right.

For example:

do_something()
try:
    do_something_other()
except BaseException as exc:
    with contextlib.cleanup(exc) as reraise:
        # Exceptions raised in here would be added to
        # exc.__cleanup_errors__, rather than being
        # propagated normally
        undo_something()
    reraise()

The need for the "as reraise:"/"reraise()" trick arises from the need to special case the situation where the original exception inherits from Exception, but one of the raised exceptions doesn't - we want SystemExit/KeyboardInterrupt/etc to take precedence in that case, and a bare raise statement won't handle that for us (it could in a hypothetical future version of Python that's natively __cleanup_errors__ aware, but that's not going to be useful for existing versions).

Since I don't see anything in the discussion so far that requires changes to the standard library (aside from "we may want to use this ourselves"), the right place to thrash out the design details is probably contextlib2: https://github.com/jazzband/contextlib2

That's where contextlib.ExitStack was born, and I prefer using it to iterate on context management design concepts, since we can push updates out faster, and if we make bad choices anywhere along the way, they can just sit around in contextlib2, rather than polluting the standard library indefinitely.

Cheers, Nick.

P.S. trio's MultiError is also likely worth looking into in this context

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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