'Invoke subprocess.Popen.
This calls subprocess.Popen making sure the current working
directory is the PYTHONPATH.
You probably want to use :py:meth:`run` instead.'

def popen(self, cmdargs, stdout, stderr, **kw):

    env = os.environ.copy()
    env['PYTHONPATH'] = os.pathsep.join(filter(None, [str(os.getcwd()), env.get('PYTHONPATH', '')]))
    kw['env'] = env
    return subprocess.Popen(cmdargs, stdout=stdout, stderr=stderr, **kw)
