def should_build_with_cython(previous_cython_version, is_release):
    """
    Returns the previously used Cython version (or 'unknown' if not
    previously built) if Cython should be used to build extension modules from
    pyx files.
    """
    have_cython = False
    try:
        from Cython import __version__ as cython_version
        have_cython = True
    except ImportError:
        pass
    if have_cython and (not is_release or previous_cython_version == 'unknown'
        ):
        return cython_version
    else:
        return False
