迭代 XPath 元素以获取单个元素而不是列表

Question

I'm parsing a XML document and reading values of different elements using XPath. Currently this works well to get all elements in lists. However, children elements are not always present for all parents (but are present in some!) and I need to know which as I'm parsing the xml to create a dataframe to insert in a database. So I want to iterate over elements and grab the values I need one at a time. I'm not sure how to do this as currently I'm getting the full list on each iteration. I'm extracting elements that are nested at different levels.

The xml I'm parsing is a TCX file by Garmin. Short example:

 <?xml version="1.0" encoding="UTF-8"?>
<TrainingCenterDatabase
  xsi:schemaLocation="http://www.garmin.com/xmlschemas/TrainingCenterDatabase/v2 http://www.garmin.com/xmlschemas/TrainingCenterDatabasev2.xsd"
  xmlns:ns5="http://www.garmin.com/xmlschemas/ActivityGoals/v1"
  xmlns:ns3="http://www.garmin.com/xmlschemas/ActivityExtension/v2"
  xmlns:ns2="http://www.garmin.com/xmlschemas/UserProfile/v2"
  xmlns="http://www.garmin.com/xmlschemas/TrainingCenterDatabase/v2"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ns4="http://www.garmin.com/xmlschemas/ProfileExtension/v1">
  <Activities>
    <Activity Sport="Running">
      <Id>2018-10-10T14:10:10.000Z</Id>
      <Lap StartTime="2018-10-10T14:10:10.000Z">
        <TotalTimeSeconds>343.0</TotalTimeSeconds>
        <DistanceMeters>1000.0</DistanceMeters>
        <MaximumSpeed>3.694999933242798</MaximumSpeed>
        <Calories>51</Calories>
        <AverageHeartRateBpm>
          <Value>136</Value>
        </AverageHeartRateBpm>
        <MaximumHeartRateBpm>
          <Value>162</Value>
        </MaximumHeartRateBpm>
        <Intensity>Active</Intensity>
        <TriggerMethod>Manual</TriggerMethod>
        <Track>
          <Trackpoint>
            <Time>2018-10-10T14:10:10.000Z</Time>
            <Position>
              <LatitudeDegrees>52.17917550355196</LatitudeDegrees>
              <LongitudeDegrees>6.532441098242998</LongitudeDegrees>
            </Position>
            <AltitudeMeters>-0.20000000298023224</AltitudeMeters>
            <DistanceMeters>0.0</DistanceMeters>
            <HeartRateBpm>
              <Value>94</Value>
            </HeartRateBpm>
            <Extensions>
              <ns3:TPX>
                <ns3:Speed>0.04699999839067459</ns3:Speed>
                <ns3:RunCadence>7</ns3:RunCadence>
              </ns3:TPX>
            </Extensions>
          </Trackpoint>
          <Trackpoint>
            <Time>2018-10-10T14:10:11.000Z</Time>
            <Position>
              <LatitudeDegrees>52.17917634174228</LatitudeDegrees>
              <LongitudeDegrees>6.532444199547172</LongitudeDegrees>
            </Position>
            <AltitudeMeters>0.0</AltitudeMeters>
            <DistanceMeters>0.23000000417232513</DistanceMeters>
            <HeartRateBpm>
              <Value>95</Value>
            </HeartRateBpm>
            <Extensions>
              <ns3:TPX>
                <ns3:Speed>0.0</ns3:Speed>
                <ns3:RunCadence>7</ns3:RunCadence>
              </ns3:TPX>
            </Extensions>
          </Trackpoint>
          <Trackpoint>
            <Time>2018-10-10T14:10:12.000Z</Time>
            <Position>
              <LatitudeDegrees>52.17917206697166</LatitudeDegrees>
              <LongitudeDegrees>6.532468926161528</LongitudeDegrees>
            </Position>
            <AltitudeMeters>0.0</AltitudeMeters>
            <DistanceMeters>1.9700000286102295</DistanceMeters>
            <Extensions>
              <ns3:TPX>
                <ns3:Speed>0.0</ns3:Speed>
                <ns3:RunCadence>7</ns3:RunCadence>
              </ns3:TPX>
            </Extensions>
          </Trackpoint>
          <Trackpoint>
            <Time>2018-10-10T14:10:13.000Z</Time>
            <Position>
              <LatitudeDegrees>52.17916024848819</LatitudeDegrees>
              <LongitudeDegrees>6.5325202234089375</LongitudeDegrees>
            </Position>
            <AltitudeMeters>0.0</AltitudeMeters>
            <DistanceMeters>5.679999828338623</DistanceMeters>
            <HeartRateBpm>
              <Value>96</Value>
            </HeartRateBpm>
            <Extensions>
              <ns3:TPX>
                <ns3:Speed>0.08399999886751175</ns3:Speed>
                <ns3:RunCadence>7</ns3:RunCadence>
              </ns3:TPX>
            </Extensions>
          </Trackpoint>
          <Trackpoint>
            <Time>2018-10-10T14:10:14.000Z</Time>
            <Position>
              <LatitudeDegrees>52.17914817854762</LatitudeDegrees>
              <LongitudeDegrees>6.532532041892409</LongitudeDegrees>
            </Position>
            <AltitudeMeters>0.0</AltitudeMeters>
            <DistanceMeters>7.150000095367432</DistanceMeters>
            <HeartRateBpm>
              <Value>98</Value>
            </HeartRateBpm>
            <Extensions>
              <ns3:TPX>
                <ns3:Speed>0.10300000011920929</ns3:Speed>
                <ns3:RunCadence>10</ns3:RunCadence>
              </ns3:TPX>
            </Extensions>
          </Trackpoint>

Code that is working that gives me all values in the file as a list:

from lxml import etree, objectify
from os import listdir
from os.path import isfile, join

def tcxParse(tcxFile):
    parser = etree.XMLParser(remove_blank_text=True)
    tree = etree.parse(tcxFile, parser)
    root = tree.getroot()

    ####
    #strip namespaces
    for elem in root.getiterator():
        if not hasattr(elem.tag, 'find'): continue  # (1)
        i = elem.tag.find('}')
        if i >= 0:
            elem.tag = elem.tag[i + 1:]
    objectify.deannotate(root, cleanup_namespaces=True)
    ####
#check if we are dealing with .tcx or other format
    if tcxFile.lower().endswith('.tcx'):
        tcxParse.activity = tree.xpath('//*[@Sport]/@Sport')
        tcxParse.HR = list(map(int, tree.xpath('//Track/Trackpoint/HeartRateBpm/Value/text()')))
        tcxParse.Time = tree.xpath('//Time/text()')
        tcxParse.Speed = list(map(float, tree.xpath('//Track/Trackpoint/Extensions/TPX/Speed/text()')))
        tcxParse.Cadence = list(map(int, tree.xpath('//Track/Trackpoint/Extensions/TPX/RunCadence/text()')))
        tcxParse.Lat = list(map(float, tree.xpath('//Track/Trackpoint/Position/LatitudeDegrees/text()')))
        tcxParse.Lon = list(map(float, tree.xpath('//Track/Trackpoint/Position/LongitudeDegrees/text()')))
        tcxParse.Alt = list(map(float, tree.xpath('//Track/Trackpoint/AltitudeMeters/text()')))
        tcxParse.Distance = list(map(float, tree.xpath('//Track/Trackpoint/DistanceMeters/text()')))

I know I can use tree.iter() to iterate over the elements, but not sure how to grab the values one at a time instead of the full list.

To be clear: Current output for tcxParse.HR for instance would be:

94,95,96,98

But I need it to be

94,95,nan,96,98 

as the HeartRateBpm is missing in the 3rd Trackpoint element

Answer 1

As I understand you need to iterate <Trackpoint> 's in <Track> .
I propose you to do it like this:

trackpoints = [{
    'HR': tp.findtext('HeartRateBpm/Value'),
    'Time': tp.findtext('Time'),
    'Speed': tp.findtext('Extensions/TPX/Speed'),
    'Cadence': tp.findtext('Extensions/TPX/RunCadence'),
    'Lat': tp.findtext('Position/LatitudeDegrees'),
    'Lon': tp.findtext('Position/LongitudeDegrees'),
    'Alt': tp.findtext('AltitudeMeters'),
    'Distance': tp.findtext('DistanceMeters')
    }
for tp in tree.xpath('//Track/Trackpoint')]

For xml chunk in question (with deleted <HeartRateBpm> in second <Trackpoint> ) - trackpoints will contain such list:

[{'HR': '94', 'Time': '2018-10-10T14:10:10.000Z', 'Speed': '0.04699999839067459', 'Cadence': '7', 'Lat': '52.17917550355196', 'Lon': '6.532441098242998', 'Alt': '-0.20000000298023224', 'Distance': '0.0'}, 
 {'HR': None, 'Time': '2018-10-10T14:10:11.000Z', 'Speed': '0.0', 'Cadence': '7', 'Lat': '52.17917634174228', 'Lon': '6.532444199547172', 'Alt': '0.0', 'Distance': '0.23000000417232513'}]