html.py 文件源码

def _parser_dispatch(flavor):
    """Choose the parser based on the input flavor.

    Parameters
    ----------
    flavor : str
        The type of parser to use. This must be a valid backend.

    Returns
    -------
    cls : _HtmlFrameParser subclass
        The parser class based on the requested input flavor.

    Raises
    ------
    ValueError
        * If `flavor` is not a valid backend.
    ImportError
        * If you do not have the requested `flavor`
    """
    valid_parsers = list(_valid_parsers.keys())
    if flavor not in valid_parsers:
        raise ValueError('%r is not a valid flavor, valid flavors are %s' %
                         (flavor, valid_parsers))

    if flavor in ('bs4', 'html5lib'):
        if not _HAS_HTML5LIB:
            raise ImportError("html5lib not found, please install it")
        if not _HAS_BS4:
            raise ImportError(
                "BeautifulSoup4 (bs4) not found, please install it")
        import bs4
        if bs4.__version__ == LooseVersion('4.2.0'):
            raise ValueError("You're using a version"
                             " of BeautifulSoup4 (4.2.0) that has been"
                             " known to cause problems on certain"
                             " operating systems such as Debian. "
                             "Please install a version of"
                             " BeautifulSoup4 != 4.2.0, both earlier"
                             " and later releases will work.")
    else:
        if not _HAS_LXML:
            raise ImportError("lxml not found, please install it")
    return _valid_parsers[flavor]