Calling C++ function from python script

[Pankaj]

The module which i am creating is like

Part A:
1. It does some processing by using python code.
2. The result of this python code execution is written to a text file.
[This part is already compelete]]

Part B:
1. I read a text file which is outputted by above python script in a
C++ program
2. and again output of this c++ code is text file
[This part is as well complete with graph data structure construction
part involved, which i feel can be done in better way in C++ than in
python]

Now i want to integrate this flow.

The communication between part A and part B is by call to a function
present in C++
How should i do that?
Kindly suggest some ways.

Regards
Pankaj

 

[Ravi Teja]

The module which i am creating is like

Part A:
1. It does some processing by using python code.
2. The result of this python code execution is written to a text file.
[This part is already compelete]]

Part B:
1. I read a text file which is outputted by above python script in a
C++ program
2. and again output of this c++ code is text file
[This part is as well complete with graph data structure construction
part involved, which i feel can be done in better way in C++ than in
python]

Now i want to integrate this flow.

The communication between part A and part B is by call to a function
present in C++
How should i do that?
Kindly suggest some ways.

Regards
Pankaj

If the bulk of your code is in Python, write an extension module in
C++. You can use Swig or Boost here.
If the bulk of your code is in C++, embed Python.
The general rule is "extend, not embed"

Python documentation
http://docs.python.org/ext/ext.html
Unless you know C/C++ well, these are not easy topics.

Boost
http://www.boost.org/libs/python/doc/tutorial/doc/html/index.html
Swig, SIP are other good alternatives.
All the above are relatively easy.

If your C++ can be simplified to C, Pyrex is remarkably easy to write
extensions in. In this case you will import/include a C header
containing your function and write a Pyrex wrapper around it rather
than directly using Pyrex as a language.

If you are on MS Windows, you can always write a COM server in either
language (COM in C++ is not easy unless if you are using something like
C++ Builder).

Or you can simply use popen from Python or use pipes or just read
stdout from either side.

As you can see, there are a number of options.

 

[Pankaj]

In my case, i want to use python script as parent script and call C++
function from within the python script.

If this is the case, then what am i doing?
1. extending
2. embedding.

?

The module which i am creating is like

Part A:
1. It does some processing by using python code.
2. The result of this python code execution is written to a text file.
[This part is already compelete]]

Part B:
1. I read a text file which is outputted by above python script in a
C++ program
2. and again output of this c++ code is text file
[This part is as well complete with graph data structure construction
part involved, which i feel can be done in better way in C++ than in
python]

Now i want to integrate this flow.

The communication between part A and part B is by call to a function
present in C++
How should i do that?
Kindly suggest some ways.

Regards
Pankaj

If the bulk of your code is in Python, write an extension module in
C++. You can use Swig or Boost here.
If the bulk of your code is in C++, embed Python.
The general rule is "extend, not embed"

Python documentation
http://docs.python.org/ext/ext.html
Unless you know C/C++ well, these are not easy topics.

Boost
http://www.boost.org/libs/python/doc/tutorial/doc/html/index.html
Swig, SIP are other good alternatives.
All the above are relatively easy.

If your C++ can be simplified to C, Pyrex is remarkably easy to write
extensions in. In this case you will import/include a C header
containing your function and write a Pyrex wrapper around it rather
than directly using Pyrex as a language.

If you are on MS Windows, you can always write a COM server in either
language (COM in C++ is not easy unless if you are using something like
C++ Builder).

Or you can simply use popen from Python or use pipes or just read
stdout from either side.

As you can see, there are a number of options.

 

[Ravi Teja]

Extending

 

[Pankaj]

The examples given are too complicated.

So, if it can be explained using my sample example. Would be thankful
for that.

/***** 1.c File ******/
func( char a[10] )
{
int i;
for( i =0; i < 10; i++)
printf("\n array element is: %c", a);
}

/***** 1.py File ******/

f = open( "x.txt", "r")
while 1:
l = f.readline( )

=>> Here i want to call func() as
func( l)

 

[Ravi Teja]

Right. They are complicated(Assuming that you are reading Python API
docs), especially if you have not done a substantial amount of C. You
spent 8 minutes trying to understand them. What did you expect? People
spend days sorting out the API issues.

If you are in a hurry, Pyrex is easiest approch to write Python
extensions. Try that instead.
http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/

Another approach... create a dll and call it from ctypes. Nothing to
learn (assuming you understand C calling conventions)

 

[Pankaj]

See, i tell u, nothing is difficult in this world.

I achieved the thing which i was asking for.

Thanks for the advice.

I used this paper:
http://www.swig.org/tutorial.html

for creating python modules from C code.

Which is what i needed. In order to interface both things, we need
convert atleast one thing to other so i choosed to convert C code to
python module.

The steps which i followed were:
These steps were to create a python module from C code. So a mandatory
condition for .c file is : it should not have main function, and any
variable in any function called from main function should be declared
global.

1. Creating wrapper from .i :
swig -python TestCase.i (where TestCase.i is interface file containing
declarations of functions and variables)
2. Creating .o's :
gcc -c TestCase.c TestCase_wrap.c (TestCase_wrap.c is file genereted
by swig and is a wrapper for creating a python module)
3. Shared Library:
ld -shared TestCase.o TestCase_wrap.o TestCase.so

Module was inserted as:
1. export LD_LIBRARY_PATH=$PWD:$LD_LIBRARY_PATH
2. python Tc.py
( In this python file i did: "import TestCase" and then used it as
"TestCase.main_module()"

**********************************

So, we don't need to write any code to do this.

 

[Pankaj]

One small thing was incorrect there.

gcc -c TestCase.c TestCase_wrap.c -I /usr/include/python2.2/

This include path is required for Python.h file