Return absolute path if Python script exists (otherwise, return None)
    package=None -> module is in sys.path (standard library modules)

def python_script_exists(package=None, module=None):
    """
    Return absolute path if Python script exists (otherwise, return None)
    package=None -> module is in sys.path (standard library modules)
    """
    assert module is not None
    try:
        if package is None:
            path = imp.find_module(module)[1]
        else:
            path = osp.join(imp.find_module(package)[1], module)+'.py'
    except ImportError:
        return
    if not osp.isfile(path):
        path += 'w'
    if osp.isfile(path):
        return path