I would like to call PARI/GP from Python only to calculate the function nextprime(n) for different ns that I define. Unfortunately I can't get pari-python to install so I thought I would just call it using a command line via os.system in Python. I can't see in the man page how to do get PARI/GP to run in non-interactive mode, however. Is there a way to achieve this?

You can pipe input into gp's stdin like so, using the -q flag to quash verbosity:

senderle:~ $ echo "print(isprime(5))" | gp -q
1

However, it's not much harder to create a simple python extension that allows you to pass strings to pari's internal parser and get results back (as strings). Here's a bare-bones version that I wrote some time ago so that I could call pari's implementation of the APRT test from python. You could extend this further to do appropriate conversions and so on.

//pariparse.c#include<Python.h>
#include<pari/pari.h>static PyObject * pariparse_run(PyObject *self, PyObject *args) {pari_init(40000000, 2);const char *pari_code;char *outstr;if (!PyArg_ParseTuple(args, "s", &pari_code)) { return NULL; }outstr = GENtostr(gp_read_str(pari_code));pari_close();return Py_BuildValue("s", outstr);
}static PyMethodDef PariparseMethods[] = {{"run", pariparse_run, METH_VARARGS, "Run a pari command."},{NULL, NULL, 0, NULL}
};PyMODINIT_FUNC initpariparse(void) {(void) Py_InitModule("pariparse", PariparseMethods);
}

And the setup file:

#setup.pyfrom distutils.core import setup, Extensionmodule1 = Extension('pariparse',include_dirs = ['/usr/include', '/usr/local/include'],libraries = ['pari'],library_dirs = ['/usr/lib', '/usr/local/lib'],sources = ['pariparse.c'])setup (name = 'pariparse',version = '0.01a',description = 'A super tiny python-pari interface',ext_modules = [module1])

Then just type python setup.py build to build the extension. You can then call it like this:

>>> pariparse.run('nextprime(5280)')
'5281'

I tested this just now and it compiled for me with the latest version of pari available via homebrew (on OS X). YMMV!