This is a 'for the record' email describing how to use boost.python
with sage on Mac OS X. Thanks to the gentlefolk who walked me through
it (especially Carl Witty).
I am assuming a working boost.python extension called 'my_ext.so'.
"Working" means that you should be able to start up python at the
command line, run "import my_ext" and call your functions /
instantiate classes etc.
Attempting to do the same with sage fails. Start sage, run "import
my_ext" and it will fail with a message like
ImportError: dlopen(./my_ext.so, 2): Symbol not found:
_PyUnicodeUCS2_AsWideChar
The problem is that the version of python distributed inside sage uses
UCS4 (for consistency with linux) rather than UCS2 (which is used by
Apple's system python). Because libboost_python is compiled against
Apple's Python, it assumes UCS2. There are two possible fixes: (1)
recompile boost against sage's python, and it will use UCS4. (2)
Recompile sage to use UCS2.
The second option is better for me, because I want my python
extensions binary compatible with both system python and sage python.
(The alternative is to have system & sage versions of boost, and
system & sage versions of every extension.) Changing sage's default
unicode setting might have consequences that I don't understand, but I
haven't been bitten yet. You will need to do this for each successive
version of sage, which could be tedious.
===== Rebuilding sage with UCS2 =====
Sage needs to be rebuilt from source, with python using the option --
enable-unicode=ucs2 instead of --enable-unicode=ucs4
To do this:
1. download the sage source,
2. extract it:
tar xf sage-3.0.3.tar
3. find the sage packages
cd sage-3.0.3/spkg/standard
4. open up the python spkg:
tar xjf python-2.5.2.p2.spkg
5. open the file python-2.5.2.p2/spkg-install
open -e python-2.5.2.p2/spkg-install
6. find two instances of "--enable-unicode=ucs4" and replace them with
"--enable-unicode=ucs2"
7. recreate the spkg:
tar cjf python-2.5.2.p2.spkg python-2.5.2.p2/
8. Then return to the main sage directory, and build sage
cd ../../
make
Once it's built, starting sage and importing my_ext should work.
==== other issues with boost.python ====
boost.python does not automagically convert sage values to ints or
floats---for example, assume you have made the following accessible to
python:
struct s_type {
int n;
void set_x(double x);
}
Running
s_inst = my_ext.s_type()
will work, but
s_inst.n = 1
will fail with a message like
ArgumentError: Python argument types in
None.None(s_type, sage.rings.integer.Integer)
did not match C++ signature:
None(s_type {lvalue}, int)
s_inst.set_x(3)
will similarly fail.
There are various fixes, including:
s_inst.n = int(1)
s_inst.set_x(float(3))
Another option is to switch off the sage preparser:
preparser(False)
s_inst.n = 1
preparser(True)
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