First question on extending Python...

[Redefined Horizons]

I still new to Python, and I've only dabbled in C, but I've got my
first project I need to tackle that involves both languages. I was
hoping to get some advice on how to proceed.

There is a third-party application that I need to work with. It is
closed-source, but it exposes a C API. I want to wrap this C API so
that it is available from Python. I have no ability to modify the C
API of the third part application.

Do I the C functions that I wrap in an extensions module for Python
need to be in a certian format? If so, I will have to write an
intermediate DLL in C that wraps the third-party application and
exports the functions in a form that Python can use.

Or can an extension module for Python wrap any C function? If this is
the case I think I can skip the intermediate C DLL.

Are there any advantages to using the intermediate DLL written in C in
this particular case where I will not have ability to manipulate the C
API of the third party application directly? (For example, if the
third-party application developers are willing to call a "call-back"
function that must be written in C, but not Python. Could the same
extension module export both C functions and serve as a Python
module?)

Thanks,

Scott Huey

[John Machin]

Re: First question on extending Python...

On 10/06/2006 8:32 AM, Redefined Horizons wrote:[color=blue]
> I still new to Python, and I've only dabbled in C, but I've got my
> first project I need to tackle that involves both languages. I was
> hoping to get some advice on how to proceed.
>
> There is a third-party application that I need to work with. It is
> closed-source, but it exposes a C API. I want to wrap this C API so
> that it is available from Python. I have no ability to modify the C
> API of the third part application.
>
> Do I the C functions that I wrap in an extensions module for Python
> need to be in a certian format?[/color]

No.

[color=blue]
> If so, I will have to write an
> intermediate DLL in C that wraps the third-party application and
> exports the functions in a form that Python can use.[/color]

No way. You need only *one* layer of glue.
[color=blue]
>
> Or can an extension module for Python wrap any C function?[/color]

I've never heard of a C function that couldn't be wrapped. If there were
such a function, having more C code in a separate DLL wouldn't make it
wrappable.
[color=blue]
> If this is
> the case I think I can skip the intermediate C DLL.[/color]

I think so too.
[color=blue]
>
> Are there any advantages to using the intermediate DLL written in C in
> this particular case where I will not have ability to manipulate the C
> API of the third party application directly? (For example, if the
> third-party application developers are willing to call a "call-back"
> function that must be written in C, but not Python.[/color]

No advantages that I can see. Your C call-back function could then call
a Python function if you really wanted to -- the 3rd-party devs don't
need to know :-)
[color=blue]
> Could the same
> extension module export both C functions and serve as a Python
> module?)[/color]

Yes.

Cheers,
John

[Larry Bates]

Re: First question on extending Python...

Redefined Horizons wrote:[color=blue]
> I still new to Python, and I've only dabbled in C, but I've got my
> first project I need to tackle that involves both languages. I was
> hoping to get some advice on how to proceed.
>
> There is a third-party application that I need to work with. It is
> closed-source, but it exposes a C API. I want to wrap this C API so
> that it is available from Python. I have no ability to modify the C
> API of the third part application.
>
> Do I the C functions that I wrap in an extensions module for Python
> need to be in a certian format? If so, I will have to write an
> intermediate DLL in C that wraps the third-party application and
> exports the functions in a form that Python can use.
>
> Or can an extension module for Python wrap any C function? If this is
> the case I think I can skip the intermediate C DLL.
>
> Are there any advantages to using the intermediate DLL written in C in
> this particular case where I will not have ability to manipulate the C
> API of the third party application directly? (For example, if the
> third-party application developers are willing to call a "call-back"
> function that must be written in C, but not Python. Could the same
> extension module export both C functions and serve as a Python
> module?)
>
> Thanks,
>
> Scott Huey[/color]

John Machin has answered most of your questions in a separate post. All you
need is to take a look at ctypes module for Python. You can call virtually
any C based API that is stored in a .DLL using ctypes. I've called .DLL
APIs for Castelle's Faxpress, Expervision's OCR toolkit and Softrak's ADS.DLL
with no problems. The trick is creating some functions/classes that help
with the C structures that need to be passed back and forth. For that you
will probably also need to take a look at the Python struct module, but ctypes
has some built-in helper functions also.

ctypes can be located here: http://starship.python.net/crew/theller/ctypes/

-Larry Bates

[Larry Bates]

Re: First question on extending Python...

Redefined Horizons wrote:[color=blue]
> I still new to Python, and I've only dabbled in C, but I've got my
> first project I need to tackle that involves both languages. I was
> hoping to get some advice on how to proceed.
>
> There is a third-party application that I need to work with. It is
> closed-source, but it exposes a C API. I want to wrap this C API so
> that it is available from Python. I have no ability to modify the C
> API of the third part application.
>
> Do I the C functions that I wrap in an extensions module for Python
> need to be in a certian format? If so, I will have to write an
> intermediate DLL in C that wraps the third-party application and
> exports the functions in a form that Python can use.
>
> Or can an extension module for Python wrap any C function? If this is
> the case I think I can skip the intermediate C DLL.
>
> Are there any advantages to using the intermediate DLL written in C in
> this particular case where I will not have ability to manipulate the C
> API of the third party application directly? (For example, if the
> third-party application developers are willing to call a "call-back"
> function that must be written in C, but not Python. Could the same
> extension module export both C functions and serve as a Python
> module?)
>
> Thanks,
>
> Scott Huey[/color]

John Machin has answered most of your questions in a separate post. All you
need is to take a look at ctypes module for Python. You can call virtually
any C based API that is stored in a .DLL using ctypes. I've called .DLL
APIs for Castelle's Faxpress, Expervision's OCR toolkit and Softrak's ADS.DLL
with no problems. The trick is creating some functions/classes that help
with the C structures that need to be passed back and forth. For that you
will probably also need to take a look at the Python struct module, but ctypes
has some built-in helper functions also.

ctypes can be located here: http://starship.python.net/crew/theller/ctypes/

-Larry Bates

[Cameron Laird]

Re: First question on extending Python...

In article <448A048B.70000 07@websafe.com> ,
Larry Bates <larry.bates@we bsafe.com> wrote:[color=blue]
>Redefined Horizons wrote:[/color]