def import_sphinx():
    """
    sphinx is an optional extra, so only load it when necessary
    """
    try:
        import sphinx
    except ImportError:
        raise ImportError(
            'Sphinx is not installed. Please install ipypublish with the sphinx extras: >> pip install ipypublish[sphinx]'
            )
    if not sphinx.__version__ >= '1.8':
        raise ImportError('Sphinx version must be >= 1.8')
    return sphinx
