with statement for two files

[Diez B. Roggisch]

braver schrieb:
> Can open two files in a with statement:
> 
> with open(src) as readin, open(dst,"w") as writin:   # WRONG: comma
> doesn't work
>   ...
> 
> -- so that you have transactional safety for two file descriptors?
> The comma syntax doesn't work, but is there a way, except for
> 
> with open(src) as readin:
>   with open(dst,"w) as writin:
>     ...

I'm not aware of any pre-defined context manager which does that.
But you can write your own context manager to do that, even a 
generalized combinator for taking two managers and create one, like this:

with context_creator(open, [src], open, [dst, "w"]) as readin, writin:
      ...

A fundamental problem though would be that the semantics become 
difficult. If closing the "outer" file fails, it's impossible to 
"rollback" the inner one.

So I think it really is better to use the nested with, which makes it 
crystal clear that the inner block might succeed independently from the 
outer one.

Diez