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Re: RAII vs gc (was fortran lib which provide python like data type)

References <0b8db0f6-75c5-4a8f-a9a5-51fa936b306e@googlegroups.com> <8ff4c8aa-a858-481f-8964-7c794f848de2@googlegroups.com> <mailman.18322.1422653315.18130.python-list@python.org> <54cd96b5$0$12994$c3e8da3$5496439d@news.astraweb.com>
Date 2015-02-01 14:21 +1100
Subject Re: RAII vs gc (was fortran lib which provide python like data type)
From Chris Angelico <rosuav@gmail.com>
Newsgroups comp.lang.python
Message-ID <mailman.18346.1422760885.18130.python-list@python.org> (permalink)

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On Sun, Feb 1, 2015 at 2:00 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
>> A C++ statement with RAII like
>>
>> {
>>    Foo bar();
>>    // suite
>> }
>>
>> is not equivalent to
>>
>>     bar = Foo()
>>
>> in Python. It actually corresponds to
>>
>> with Foo() as bar:
>>     <suite>
>
>
> Nice answer! I'm not qualified to tell whether you are right or not, but
> what you say has the ring of truth about it.

I would say that this is indeed correct, with the caveat that RAII is
most often used for memory allocation, in which case the correct
transformation into Python _would_ be a constructor call. But when you
use this kind of thing to set up state and clean up afterwards, then
yes, Python's equivalent would be a context manager.

So a C++ way to release and reacquire the GIL might look like this
(borrowing from https://docs.python.org/3.5/c-api/init.html):

class ReleaseGIL
{
    //Internal state
    PyThreadState *_save;
public:
    //Constructor
    ReleaseGIL() {_save = PyEval_SaveThread();}
    //Destructor
    ~ReleaseGIL() {PyEval_RestoreThread(_save);}
};

//Usage example:
int do_work()
{
    while (1)
    {
        //use the Python C API to figure out what we need to do
        ReleaseGIL heavy_processing_coming;
        //do the heavy computational work
    } //GIL is reacquired here
}


The Python equivalent would be something like:

@contextlib.contextmanager
def release_gil():
    """Why the <bleep> are we manipulating the
    GIL from Python code anyway???"""
    _save = PyEval_SaveThread()
    yield
    PyEval_RestoreThread(_save)

def do_work()
    while True:
        # get some work
        with release_gil() as heavy_processing_coming:
            # do the heavy computational work
        # GIL is reacquired here

In each case, you have a guarantee that code following the suite will
not be executed without first performing the appropriate cleanup.

ChrisA

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[OT] fortran lib which provide python like data type 1989lzhh <1989lzhh@gmail.com> - 2015-01-29 21:39 +0800
  Re: [OT] fortran lib which provide python like data type beliavsky@aol.com - 2015-01-29 14:39 -0800
    Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-01-29 17:40 -0800
      Re: [OT] fortran lib which provide python like data type Christian Gollwitzer <auriocus@gmx.de> - 2015-01-30 08:32 +0100
        Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-01-30 08:27 -0800
          Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-01-30 10:08 -0700
            Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-01-30 09:31 -0800
              RAII vs gc (was fortran lib which provide python like data type) Rustom Mody <rustompmody@gmail.com> - 2015-01-30 10:02 -0800
                Re: RAII vs gc (was fortran lib which provide python like data type) Sturla Molden <sturla.molden@gmail.com> - 2015-01-30 21:28 +0000
                Re: RAII vs gc (was fortran lib which provide python like data type) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-01 14:00 +1100
                Re: RAII vs gc (was fortran lib which provide python like data type) Chris Angelico <rosuav@gmail.com> - 2015-02-01 14:21 +1100
                Re: RAII vs gc (was fortran lib which provide python like data type) Devin Jeanpierre <jeanpierreda@gmail.com> - 2015-01-31 19:27 -0800
              Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-01-30 11:15 -0700
                Re: [OT] fortran lib which provide python like data type Paul Rubin <no.email@nospam.invalid> - 2015-01-30 10:23 -0800
                Re: [OT] fortran lib which provide python like data type Christian Gollwitzer <auriocus@gmx.de> - 2015-01-31 09:27 +0100
                Re: [OT] fortran lib which provide python like data type Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-01-31 11:51 +1300
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-31 10:49 +1100
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-01-31 14:22 +0200
                Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-01-31 07:50 -0800
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-01-31 19:43 +0200
                Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-01-31 18:30 -0800
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-01 09:38 +0200
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-02 03:49 +1100
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-01 20:57 +0200
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-02 12:59 +1100
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-02 13:05 +1100
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-02 10:51 +0200
                Re: [OT] fortran lib which provide python like data type Chris Angelico <rosuav@gmail.com> - 2015-02-02 13:04 +1100
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-02 18:01 +1100
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-02 10:52 +0200
                Re: [OT] fortran lib which provide python like data type Paul Rudin <paul.nospam@rudin.co.uk> - 2015-02-02 12:56 +0000
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-02 15:27 +0200
                Re: [OT] fortran lib which provide python like data type Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-02-02 16:08 +0000
                Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-02-02 08:21 -0800
                Re: [OT] fortran lib which provide python like data type Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-02-02 16:52 +0000
                Re: [OT] fortran lib which provide python like data type Chris Angelico <rosuav@gmail.com> - 2015-02-03 04:25 +1100
                Re: [OT] fortran lib which provide python like data type Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-02-02 18:34 +0000
                Re: [OT] fortran lib which provide python like data type Chris Angelico <rosuav@gmail.com> - 2015-02-03 10:15 +1100
                Re: [OT] fortran lib which provide python like data type Matthew Barnett <auxlang@mrabarnett.plus.com> - 2015-02-02 02:35 +0000
                Re: [OT] fortran lib which provide python like data type Chris Angelico <rosuav@gmail.com> - 2015-02-02 13:45 +1100
                Re: [OT] fortran lib which provide python like data type Roy Smith <roy@panix.com> - 2015-02-01 23:12 -0500
                Re: [OT] fortran lib which provide python like data type Roy Smith <roy@panix.com> - 2015-02-01 23:14 -0500
                Re: [OT] fortran lib which provide python like data type Chris Angelico <rosuav@gmail.com> - 2015-02-02 15:47 +1100
                Re: [OT] fortran lib which provide python like data type Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-02-02 16:06 +0000
                Re: [OT] fortran lib which provide python like data type Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-01-31 17:59 +0000
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-02-01 13:27 +1100
                Re: [OT] fortran lib which provide python like data type Paul Rubin <no.email@nospam.invalid> - 2015-01-31 22:26 -0800
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-01 09:58 +0200
                Re: [OT] fortran lib which provide python like data type Christian Gollwitzer <auriocus@gmx.de> - 2015-02-01 09:14 +0100
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-01 21:12 +0200
                Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-02-01 17:50 -0700
                Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-02-01 18:02 -0700
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-02 09:39 +0200
                Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-02-02 09:25 -0700
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-02 19:57 +0200
                Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-02-02 11:23 -0700
                Re: [OT] fortran lib which provide python like data type Marko Rauhamaa <marko@pacujo.net> - 2015-02-02 20:54 +0200
                Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-02-01 18:02 -0700
                Re: [OT] fortran lib which provide python like data type beliavsky@aol.com - 2015-02-02 05:30 -0800
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-31 10:50 +1100
                Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-01-30 20:49 -0700
                Re: [OT] fortran lib which provide python like data type Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-01-31 22:04 +1100
                Re: [OT] fortran lib which provide python like data type Rustom Mody <rustompmody@gmail.com> - 2015-01-31 08:18 -0800
          Re: [OT] fortran lib which provide python like data type Sturla Molden <sturla.molden@gmail.com> - 2015-01-30 23:12 +0000
          Re: [OT] fortran lib which provide python like data type Michael Torrie <torriem@gmail.com> - 2015-01-30 20:35 -0700

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