从 help(etree.fromstring) 开始，如果没有将 parser 参数传递给函数，则使用默认解析器 .

fromstring(text, parser=None, *, base_url=None)
    fromstring(text, parser=None, base_url=None)

    Parses an XML document or fragment from a string.  Returns the
    root node (or the result returned by a parser target).

    To override the default parser with a different parser you can pass it to
    the ``parser`` keyword argument.

    The ``base_url`` keyword argument allows to set the original base URL of
    the document to support relative Paths when looking up external entities
    (DTD, XInclude, ...).

那么什么是默认解析器？传入的 XMLParser() 实例或解析器对应的数据类型（html，xml等）？

python lxml