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

Serhiy Storchaka storchaka at gmail.com
Mon Jun 12 03:10:28 EDT 2017

There is an idiomatic Python code:

 do_something()
 try:
     do_something_other()
 except:
     undo_something()
     raise

If an error is raised when execute do_something_other(), then we should first restore the state that was before calling do_something(), and then reraise the exception. It is important that the bare "except" (or "except BaseException") is used, because undo_something() should be executed on any exception. And this is one of few cases where using the bare "except" is correct.

But if an error is raised when execute undo_something(), 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 do_something_other() raises SystemExit and undo_something() raises KeyError, the final exception has type KeyError.

Yury in the comment for PR 2108 [1] suggested more complicated code:

 do_something()
 try:
     do_something_other()
 except BaseException as ex:
     try:
         undo_something()
     finally:
         raise ex

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?

[1] https://github.com/python/cpython/pull/2108

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