From serge.guelton at quiet-oceans.com  Fri Jul  5 09:57:30 2013
From: serge.guelton at quiet-oceans.com (serge Guelton)
Date: Fri, 5 Jul 2013 09:57:30 +0200
Subject: [C++-sig] boosst.python: Unregistering builtins
Message-ID: <20130705075730.GA17513@lakota>

Hi there,

I have a small Python function that takes many arguments, perform a few
computations then returns. It is called a lot of time and almost all my
computations are spent there. Turing it into a C++ function wrapped with
boost.python make the whole program run much faster, and a profiling
pass informs me that all my time is spent in
`rvalue_from_python_stage1'.

Digging through the code, it appears that unregistering some intrinsics
would make the whole stuff run faster, as the linear search for the
right converter would be (hopefully) faster. From what I understand of
`registry.cpp` there is no way to undo, control or prevent the registrations made by `initialize_builtin_converters()`.

Any tip & trick?

cheers

From ashish.sadanandan at gmail.com  Fri Jul 26 22:08:22 2013
From: ashish.sadanandan at gmail.com (Ashish Sadanandan)
Date: Fri, 26 Jul 2013 20:08:22 +0000 (UTC)
Subject: [C++-sig] Exposing a C-style array data member to Python via
	Boost.Python
Message-ID: <loom.20130726T220713-964@post.gmane.org>

I have a struct that contains a C-style array data member. I'd like to have
this struct exposed to Python, and this data member be accessible as a tuple
in Python.

struct S
{
  char arr[10];
};

void foo( S const * )
{}

BOOST_PYTHON_MODULE( test )
{
  using namespace boost::python;

  class_<S>( "S" )
    .def_readwrite( "arr", &S::arr )
    ;

  def( "foo", foo );
}

The code above fails to build

error C2440: '=' : cannot convert from 'const char [10]' to 'char [10]'

C-style arrays are not assignable, so the error makes sense. The code
compiles if I change the data member to a plain char instead of an array.

I cannot replace the array with an std::array or some other container
because the structure is being consumed by a C API. The only solution I can
think of is to write a couple of wrappers and do the following

    1. A struct S1 that duplicates S except it'll have an std::array instead
of a C-style array
    2. A foo_wrapper that accepts a S1 const *, copies the contents over to
an S instance and calls foo
    3. Register a to_python_converter to convert the std::array to a Python
tuple

This should work, and I'm not too concerned about the data copying at this
point, but it'd be nice if I could avoid it and expose the array directly
without having to jump through all these hoops.

So the question is, how can I expose that C-style array data member to
Python as a tuple?


From ashish.sadanandan at gmail.com  Fri Jul 26 23:16:21 2013
From: ashish.sadanandan at gmail.com (Ashish Sadanandan)
Date: Fri, 26 Jul 2013 21:16:21 +0000 (UTC)
Subject: [C++-sig]
	=?utf-8?q?Exposing_a_C-style_array_data_member_to_Pytho?=
	=?utf-8?q?n_via=09Boost=2EPython?=
References: <loom.20130726T220713-964@post.gmane.org>
Message-ID: <loom.20130726T231419-469@post.gmane.org>

Ashish Sadanandan <ashish.sadanandan <at> gmail.com> writes:

> 
> I have a struct that contains a C-style array data member. I'd like to have
> this struct exposed to Python, and this data member be accessible as a tuple
> in Python.
> 
> struct S
> {
>   char arr[10];
> };
> 
> 
> So the question is, how can I expose that C-style array data member to
> Python as a tuple?
> 

I screwed up the question. I actually need a list, not a tuple, since it
needs to be modifiable on the Python side. The rest of the question remains
the same; except I should also mention that the array in question is 4128
chars long, not just 10. So copying it is not as trivial as the question
first implies.



From brandsmeier at gmx.de  Sat Jul 27 09:09:45 2013
From: brandsmeier at gmx.de (Holger Brandsmeier)
Date: Sat, 27 Jul 2013 09:09:45 +0200
Subject: [C++-sig] Exposing a C-style array data member to Python via
	Boost.Python
In-Reply-To: <loom.20130726T231419-469@post.gmane.org>
References: <loom.20130726T220713-964@post.gmane.org>
	<loom.20130726T231419-469@post.gmane.org>
Message-ID: <CAFP7zVsV3EK3BZFng_HdR3rT0LmgsuNgViK5hugMjJdj+Vy+vA@mail.gmail.com>

Ashish,

if you really need it to be a `list` in python, then there is no way
around it, you have to copy it. If you just need it to behave like a
list, then that is possible. Just export your struct to python and
implement methods such as __get__(),  __set__(), __len()__, __eq__(),
...

You may also read up on boost python indexing suite (optionally
version 2) and see if that helps you to expose all these methods. If
you write you class very much like std::vector, then the indexing
suite can be of great help. Otherwise, just export those functions by
hand.

-Holger


On Fri, Jul 26, 2013 at 11:16 PM, Ashish Sadanandan
<ashish.sadanandan at gmail.com> wrote:
> Ashish Sadanandan <ashish.sadanandan <at> gmail.com> writes:
>
>>
>> I have a struct that contains a C-style array data member. I'd like to have
>> this struct exposed to Python, and this data member be accessible as a tuple
>> in Python.
>>
>> struct S
>> {
>>   char arr[10];
>> };
>>
>>
>> So the question is, how can I expose that C-style array data member to
>> Python as a tuple?
>>
>
> I screwed up the question. I actually need a list, not a tuple, since it
> needs to be modifiable on the Python side. The rest of the question remains
> the same; except I should also mention that the array in question is 4128
> chars long, not just 10. So copying it is not as trivial as the question
> first implies.
>
>
> _______________________________________________
> Cplusplus-sig mailing list
> Cplusplus-sig at python.org
> http://mail.python.org/mailman/listinfo/cplusplus-sig

From ashish.sadanandan at gmail.com  Mon Jul 29 16:22:30 2013
From: ashish.sadanandan at gmail.com (Ashish Sadanandan)
Date: Mon, 29 Jul 2013 14:22:30 +0000 (UTC)
Subject: [C++-sig]
	=?utf-8?q?Exposing_a_C-style_array_data_member_to_Pytho?=
	=?utf-8?q?n_via=09Boost=2EPython?=
References: <loom.20130726T220713-964@post.gmane.org>
	<loom.20130726T231419-469@post.gmane.org>
	<CAFP7zVsV3EK3BZFng_HdR3rT0LmgsuNgViK5hugMjJdj+Vy+vA@mail.gmail.com>
Message-ID: <loom.20130729T161934-220@post.gmane.org>

Holger Brandsmeier <brandsmeier <at> gmx.de> writes:

> 
> Ashish,
> 
> if you really need it to be a `list` in python, then there is no way
> around it, you have to copy it. If you just need it to behave like a
> list, then that is possible. Just export your struct to python and
> implement methods such as __get__(),  __set__(), __len()__, __eq__(),
> ...
> 
> You may also read up on boost python indexing suite (optionally
> version 2) and see if that helps you to expose all these methods. If
> you write you class very much like std::vector, then the indexing
> suite can be of great help. Otherwise, just export those functions by
> hand.
> 
> -Holger
> 

Holger,
Thanks for the response. I ended up doing what you suggested. Created an
array_ref class that presents a non-owning view of an array; this has all
the member functions that vector does, except for the ones that mutate the
size of the container. The getter for the char array member wraps it in an
array_ref, and then another class, array_indexing_suite, copied from Boost's
vector_indexing_suite takes care of the rest of the interop.

Thanks again,
Ashish.