Porting Python2 C-API/Swig based modules to Python 3

Adam Pridgen adam.pridgen at thecoverofnight.com
Thu Feb 24 01:18:53 CET 2011

Thanks for the help.  That information got me started in the right
direction.  I put my notes up on paste bin for others to use in the

For the future reference of others, I have included the pertinent
details below.  My process for identifying the various issues was
compiling and running the project to see what was still broken after
each fix.  I know this is not the most efficient way, but it worked
for this pylibcap since the code base was pretty small.  I don't think
everything is completely fixed, but its a step in the right direction.

The initial hurdle I encountered was swig.  I had to modify swig's
code output, because it was not using the PyCapsule_* API yet.  I
found someones solution here: http://paste.pocoo.org/show/253494/.
After applying that fix, I found that the pcap_interface.c and pcap.i
files in the project were using some deprecated API calls.  So I
converted the following APIs:

PyLong_AS_INT ==> PyLong_AsLong
PyInt_* ==> PyLong_*
PyString_* ==>  PyBytes_* (initialily changed to PyUnicode_* but that
was wrong, because I needed the Raw Bytes)

I also had to change s# to y#.  Before, s# stood for string and #, but
it now means unicode and number.

Hopefully these details will be helpful to others in the future.
Thanks again for the nudge,

-- Adam.

On Wed, Feb 23, 2011 at 11:58 AM, casevh <casevh at gmail.com> wrote:
> On Feb 23, 8:54 am, Adam Pridgen <adam.prid... at thecoverofnight.com>
> wrote:
>> Hello,
>> I am trying to get a compiled module to work with Python3.  The code I
>> am compiling was originally intended to be used in a Python 2.*
>> environment, but I updated all the Python 2.* elements and the swig
>> commands used by the setup.py script.  I got the library to
>> successfully compile, but I am getting the following errors on import
>> (below).  I am not sure how to trouble shoot this problem, and the
>> fact that only one symbol (_PyCObject_FromVoidPtr) is missing is
>> disconcerting.  I Googled some, but the symbol mentioned only showed
>> up in a few posts, where linking was an issue.  I have also gone
>> through the setup.py script and explicitly defined all the library
>> paths.
>> My questions:
>> - Has anyone ever ported Python 2* modules/libs to Python 3 that rely
>> on swig, and are there some changes in the C-API/swig I need to be
>> looking for to make this port successful?
>> - Does anyone have any advice/insght about how I can troubleshoot,
>> diagnose, and resolve this issue?
>> Thanks in advance,
>> -- Adam
>> <error message>
>> 0:pylibpcap-0.6.2$ python3
>> Python 3.2 (r32:88452, Feb 20 2011, 11:12:31)
>> [GCC 4.2.1 (Apple Inc. build 5664)] on darwin
>> Type "help", "copyright", "credits" or "license" for more information.>>> import pcap,py
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>>   File "/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packa­ges/pcap.py",
>> line 25, in <module>
>>     _pcap = swig_import_helper()
>>   File "/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packa­ges/pcap.py",
>> line 21, in swig_import_helper
>>     _mod = imp.load_module('_pcap', fp, pathname, description)
>> ImportError: dlopen(/Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site­-packages/_pcapmodule.so,
>> 2): Symbol not found: _PyCObject_FromVoidPtr
>>   Referenced from:
>> /Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packag­es/_pcapmodule.so
>>   Expected in: flat namespace
>>  in /Library/Frameworks/Python.framework/Versions/3.2/lib/python3.2/site-packag­es/_pcapmodule.so
>> >>> ^D- Hide quoted text -
>> - Show quoted text -
> This is a change in the C-API in 3.2. See http://bugs.python.org/issue5630
> casevh
> --
> http://mail.python.org/mailman/listinfo/python-list

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