[Python-ideas] revisit pep 377: good use case?

[Steven D'Aprano]

Craig Yoshioka wrote:

> with uncached('file') as file:
>     if not file: return
> 
> which isn't so bad, except it is overloading the meaning of file a bit, and

No, I don't agree with that. It is normal Pythonic idiom for objects to be 
interpreted in a boolean concept. The only difference here is that sometimes 
file will be (presumably) a file-like object, and sometimes it needs to be a 
sentinel like None.

But this shouldn't disturb you: you have already suggested one possible 
implementation would be for __enter__ to return a special value, 
SkipWithBlock, to force skipping the block. That's fine: you can have this 
functionality *right now*. All you need do is change the spelling from 
SkipWithBlock to None, and use an explicit test inside the block rather than 
an implicit test, and you're done.

Best of all, the use of an explicit test means you can do this:

with whatever('file') as file:
     if file is None:
         log('error')
     else:
         normal_processing()


which you can't do with your suggested implicit test-and-skip.

The fact that with blocks are not flow-control is a feature, not a bug.



> why shouldn't the with block be skippable?

It is skippable. Like every other code block, it is skippable by wrapping it 
in flow-control code to decide whether or not to skip it.


-- 
Steven