[Python-ideas] with-except-finally blocks

[Andrew Barnert]

On Apr 16, 2015, at 06:09, Steven D'Aprano <steve at pearwood.info> wrote:

>> Second and more
>> conceptually, it makes sense to think about exception handling in the context of
>> a with-block.
> 
> No more than any other block of code. There is nothing special about 
> with blocks that go with exception handling, any more than (say) 
> indexing into a list, processing a dict, or importing a module.

I think the key insight behind this is that a context manager is in some sense equivalent to a try/finally--and can in fact be implemented that way using @contextmanager. That isn't true for any of the other cases.

Which also implies that there is "one true answer" to your initial question; the implicit try-equivalent comes after the __enter__.

Nevertheless, I see your point, and I think you're right. This interpretation is clearly not obvious and universal (given that you didn't see it that way...). And anyone who _doesn't_ intuitively equate the context manager to the equivalent a try/finally is just as likely to expect either semantics as the other, and therefore it will be a bug magnet and/or a readability issue. (To use it safely, it's not sufficient for _you_ to understand the intended semantics, but for all of your possible readers, after all.)