使用python 3的XML Canonicalization

Question

我正在尝试编写xml规范化usign python 3.6的方法

现在我有此功能，可以接收xml作为以base64编码的参数，因此我必须对其进行解码才能使用它

def canonicalize(xml, c14n_exc=True):
    "Return the canonical (c14n) form of the xml document for hashing"
    # UTF8, normalization of line feeds/spaces, quoting, attribute ordering...
    xml_decoder = base64.b64decode(xml).decode("utf-8")
    output = StringIO()

    if xml_decoder is not None:
        # use faster libxml2 / lxml canonicalization function if available
        et = lxml.etree.parse(StringIO(xml_decoder))
        et.write_c14n(output, exclusive=c14n_exc)
    #else:
        # use pure-python implementation: c14n.py (avoid recursive import)
        #from .simplexml import SimpleXMLElement
        #SimpleXMLElement(xml).write_c14n(output, exclusive=c14n_exc)
    return output.getvalue()

问题是我收到以下错误 valueError ：：“字符串参数预期，得到了'字节'” 。

知道我在做什么错吗？

谢谢