xpath 错误，XPath 位置 >= 1 预期

问：

我正在尝试从字符串中解析 xml。

下面是字符串中的 xml。

<xc:Application class="bril::lumistore::Application" id="111" instance="0" logpolicy="inherit" network="local" service="lumistore">
  <ns4:properties xsi:type="soapenc:Struct">
    <ns4:datasources soapenc:arrayType="xsd:ur-type[1]" xsi:type="soapenc:Array">
      <ns4:item soapenc:position="[0]" xsi:type="soapenc:Struct">
        <ns4:properties xsi:type="soapenc:Struct">
          <ns4:bus xsi:type="xsd:string">brildata</ns4:bus>
          <ns4:topics xsi:type="xsd:string">tcds,beam,bestlumi,bcm1fagghist,bcm1flumi,bcm1fbkg,pltaggzero,pltlumizero,hfoclumi,hfOcc1Agg,bunchmask,ScopeData,atlasbeam,hfetlumi,hfEtSumAgg,hfafterglowfrac,hfEtPedestal,dtlumi,bunchlength,radmonraw,radmonflux,radmonlumi,pltslinklumi,bcm1futca_bkg12,bcm1futca_background,bcm1futcalumi,remuslumi,remuslumi_5514,remuslumi_5515,remuslumi_5516,remuslumi_5517,bcm1futca_agg_hist</ns4:topics>
        </ns4:properties>
      </ns4:item>
    </ns4:datasources>
    <ns4:maxstalesec xsi:type="xsd:unsignedInt">30</ns4:maxstalesec>
    <ns4:checkagesec xsi:type="xsd:unsignedInt">10</ns4:checkagesec>
    <ns4:maxsizeMB xsi:type="xsd:unsignedInt">120</ns4:maxsizeMB>
    <ns4:fileformat xsi:type="xsd:string">hd5</ns4:fileformat>
    <ns4:filepath xsi:type="xsd:string">/scratch/central_current</ns4:filepath>
    <ns4:nrowperwbuf xsi:type="xsd:unsignedInt">102</ns4:nrowperwbuf>
    <ns4:workinterval xsi:type="xsd:unsignedInt">50000</ns4:workinterval>
  </ns4:properties>
</xc:Application>

这是我用来解析字符串的代码

import xml.etree.ElementTree as ET

root = ET.fromstring(xml)
node = root.find(field['xpath'], ns)

哪里

field['xpath'] = ".//xc:Application[@class='bril::lumistore::Application']/lst:properties/lst:datasources/lst:item[0]/lst:properties/lst:topics"

和

ns = {'xc': 'http://path/XMLConfiguration-30', 'lst': 'urn:application-urn:bril::lumistore::Application'}

我收到以下错误

XPath position >= 1 expected
Traceback (most recent call last):
File "/usr/lib64/python3.6/xml/etree/ElementPath.py", line 263, in iterfind
    selector = _cache[cache_key]
KeyError: (".//xc:Application[@class='bril::lumistore::Application']/lst:properties/lst:datasources/lst:item[0]/lst:properties/lst:topics", (('lst', 'urn:application-urn:bril::lumistore::Application'), ('xc', 'http://path/XMLConfiguration-30')))
During handling of the above exception, another exception occurred:
SyntaxError: XPath position >= 1 expected

任何帮助都非常感谢

注意：Usin Python 2.7 似乎工作正常，但不适用于 Python 3.6。