Of course there is something to replace nested:

...

...

with open("egg.txt","w") as egg, open("spam.txt","w") as spam: egg.write("egg") spam.write("spam")

The nested function was removed because it is broken. E.g. take this:

...

...

with nested(open("egg.txt","w"), open("spam.txt","w")) as egg, spam: egg.write("egg") barspamwrite("spam")

What if opening of spam.txt produces an exception? Then egg.txt will never be closed! The new with syntax takes care of this. It basically rewrites it as:

...

...

with open("egg.txt","w") as egg: with open("spam.txt","w") as spam: egg.write("egg") spam.write("spam")

On 02/03/2012 04:09 PM, Yury Selivanov wrote:

Hello,

With the removal of "contextlib.nested" in python 3.2 nothing was introduced to replace it. However, I found it pretty useful, despite the fact that it had its own quirks. These quirks can (at least partially) be addressed by allowing unpacking syntax in the context manager.

Consider the following snipped of code:

ctxs = () if args.profile: ctxs += (ApplicationProfilerContext(),) if args.logging: ctxs += (ApplicationLoggingContext(),) with *ctxs: Application.run()

As of now, without "nested" we have either option of reimplementing it, or to write lots of ugly code with nested 'try..except's. So the feature was taken out, but nothing replaced it.

What do you think guys?

Thanks, Yury _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

This is not about explicitly writing comma-separated list of context managers in the statement, but rather about ability to compose this list dynamically. With unpacking you won't have the problem of uncaught exception in __new__/__init__, because such exception would propagate on the stage of constructing the list of managers. Exceptions occurred during the with statement execution, i.e. in __enter__ and __exit__ methods will work just fine, or am I missing something? On 2012-02-03, at 11:30 AM, Mathias Panzenböck wrote:

Of course there is something to replace nested:

...

...

...

with open("egg.txt","w") as egg, open("spam.txt","w") as spam: egg.write("egg") spam.write("spam")

The nested function was removed because it is broken. E.g. take this:

...

...

...

with nested(open("egg.txt","w"), open("spam.txt","w")) as egg, spam: egg.write("egg") barspamwrite("spam")

What if opening of spam.txt produces an exception? Then egg.txt will never be closed! The new with syntax takes care of this. It basically rewrites it as:

...

...

...

with open("egg.txt","w") as egg: with open("spam.txt","w") as spam: egg.write("egg") spam.write("spam")

On 02/03/2012 04:09 PM, Yury Selivanov wrote:

...

Hello,

With the removal of "contextlib.nested" in python 3.2 nothing was introduced to replace it. However, I found it pretty useful, despite the fact that it had its own quirks. These quirks can (at least partially) be addressed by allowing unpacking syntax in the context manager.

Consider the following snipped of code:

ctxs = () if args.profile: ctxs += (ApplicationProfilerContext(),) if args.logging: ctxs += (ApplicationLoggingContext(),) with *ctxs: Application.run()

As of now, without "nested" we have either option of reimplementing it, or to write lots of ugly code with nested 'try..except's. So the feature was taken out, but nothing replaced it.

What do you think guys?

Thanks, Yury _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

_______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

On 3 February 2012 15:09, Yury Selivanov wrote:

Hello,

With the removal of "contextlib.nested" in python 3.2 nothing was introduced to replace it. However, I found it pretty useful, despite the fact that it had its own quirks. These quirks can (at least partially) be addressed by allowing unpacking syntax in the context manager.

Consider the following snipped of code:

ctxs = () if args.profile: ctxs += (ApplicationProfilerContext(),) if args.logging: ctxs += (ApplicationLoggingContext(),) with *ctxs: Application.run()

Well, I quite like this syntax and it does allow you to do something not currently easily possible: with *ctxs as tuple_of_results: ... The use case is reasonably obscure however, and should this be possible: with *ctx, other as tuple_of_results, another: ... Michael

As of now, without "nested" we have either option of reimplementing it, or to write lots of ugly code with nested 'try..except's. So the feature was taken out, but nothing replaced it.

What do you think guys?

Thanks, Yury _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

-- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html

On 3 February 2012 18:06, Yury Selivanov wrote:

On 2012-02-03, at 12:50 PM, Michael Foord wrote:

...

with *ctxs as tuple_of_results:

This is not necessary, as 'ctxs' already holds all instances of all context managers; so the 'ctxs' would be equal to 'tuple_of_results'

The results are whatever is returned by ctx.__enter__(), not the context manager itself. Michael

...

with *ctx, other as tuple_of_results, another: ...

Looks useful to me.

- Yury

-- http://www.voidspace.org.uk/ May you do good and not evil May you find forgiveness for yourself and forgive others May you share freely, never taking more than you give. -- the sqlite blessing http://www.sqlite.org/different.html

On Sat, Feb 4, 2012 at 1:09 AM, Yury Selivanov wrote:

As of now, without "nested" we have either option of reimplementing it, or to write lots of ugly code with nested 'try..except's.  So the feature was taken out, but nothing replaced it.

What do you think guys?

I think you should try contextlib2 :) Specifically, ContextStack: http://contextlib2.readthedocs.org/en/latest/index.html#contextlib2.ContextS... Cheers, Nick. -- Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia

Well, I quite like this syntax and it does allow you to do something not currently easily possible:

with *ctxs as tuple_of_results: ...

The use case is reasonably obscure however, and should this be possible:

with *ctx, other as tuple_of_results, another:

wouldn't it be with *ctx, other as *tuple_of_results, another: to allow more general forms like with *ctx, other as first, *some_in_the_middle, last: -Alexander

On Tue, Feb 7, 2012 at 6:08 AM, Yury Selivanov wrote:

Well, native syntax would be much useful, but ContextStack seems like a decent workaround.  Will it be included in the stdlib (py3.3)?

Most likely (I'm the primary maintainer of contextlib, so it's basically my call). Feedback on what it's like to use in practice would definitely help with that - I put it up on PyPI as contextlib2 so people could try it out and help me avoid repeating the mistakes we made with nested() (which was an error prone bug trap). Cheers, Nick. -- Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia