On Sun, Feb 1, 2015 at 2:00 PM, Steven D'Aprano
<steve+comp.lang.pyt...@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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