with statement for two files

[braver]

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:
...

Cheers,
Alexy

 

[Diez B. Roggisch]

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

 

[Paul Rubin]

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?

use contextlib.nexted().

 

[Diez B. Roggisch]

use contextlib.nexted().

You mean contextlib.nested I guess. Didn't know about that module, cool!

However, the fundamental problem stays: rolling back only works if the
innermost context fails.

Diez

 

[greg]

Paul Rubin wrote:
>

You mean contextlib.nested I guess.

Although "nexted" is an intriguing-sounding word. I wonder
what it could mean?