[Python-ideas] with-except-finally blocks

[Steven D'Aprano]

On Thu, Apr 16, 2015 at 08:35:14AM -0700, Ethan Furman wrote:

> > No more than any other block of code. There is nothing special about 
> > with blocks that go with exception handling [...]
> 
> I have to disagree:  'with' blocks are exactly a condensed try/finally, with the
> added capability of being able to suppress exceptions -- this makes them very
> special with respect to exception handling.

Yes, yes, that's a given that with statements encapsulate a finally 
clause. But that is missing the point. The proposal is not to add a 
finally clause to context managers, because they already have one. 
The proposal is to allow special syntax for the situation where you want 
a separate and independent finally.

with spam():
    try:
        blockA
    except Error:
        blockB
    finally:
        blockC

The finally: blockC has nothing to do with the context manager's 
__exit__ method. We might as well be talking about this:

while spam():
    try:
        blockA
    except Error:
        blockB
    finally:
        blockC

giving us syntactic sugar:

# not a serious proposal
while spam():
    blockA
except Error:
    blockB
finally:
    blockC


In fact, if I were a betting man, I'd venture a small wager that there 
are more while- or for-loops with try blocks inside them than with 
blocks. After all, the whole purpose of with statements is to avoid 
needing to write a try...finally!


-- 
Steve