Our C++ lib works fine with Python2.4 using Swig, returning a C++ char* back to a python str. But this solution hit problem in Python3.0, error is: Exception=(, UnicodeDecodeError('utf8', b"\xb6\x9d\xa.....",0, 1, 'unexpected code byte') Our definition is like(working fine in Python 2.4): void cGetPubModulus( void* pSslRsa, char* cMod, int* nLen ); %include "cstring.i" %cstring_output_withsize( char* cMod, int* nLen ); Suspect swig is doing a Bytes-Str conversion automatically. In python2.4 it can be implicit but in Python3.0 it's no long allowed.. Anyone got a good idea? thanks