foo.py

class Buffer(object):
    def __init__(self):
        self.s = ''
    def write(self, s):
        self.s += s

def test_write_partisan_input(...):
    buf = Buffer();
    ...
    write_partisan_input(self, buf, ...)
    # check buf.s

...
def write_partisan_input(self, f, geometry, mesh, etc.):
    #blablabla
    f.write(...)
    ...