How to parse XML in Python and LXML?
Question: Here's my project: I'm graphing weather data from WeatherBug using RRDTool. I
need a simple, efficient way to download the weather data from WeatherBug. I
was using a terribly inefficient bash-script-scraper but moved on to
BeautifulSoup. The performance is just too slow (it's running on a Raspberry
Pi) so I need to use LXML.

What I have so far:

    
    
    from lxml import etree
    doc=etree.parse('weather.xml')
    print doc.xpath("//aws:weather/aws:ob/aws:temp")
    

But I get an error message. Weather.xml is this:

    
    
    <?xml version="1.0" encoding="UTF-8"?>
    
    <aws:weather xmlns:aws="http://www.aws.com/aws">
      <aws:api version="2.0"/>
      <aws:WebURL>http://weather.weatherbug.com/PA/Tunkhannock-weather.html?ZCode=Z5546&amp;Units=0&amp;stat=TNKCN</aws:WebURL>
      <aws:InputLocationURL>http://weather.weatherbug.com/PA/Tunkhannock-weather.html?ZCode=Z5546&amp;Units=0</aws:InputLocationURL>
      <aws:ob>
        <aws:ob-date>
          <aws:year number="2013"/>
          <aws:month number="1" text="January" abbrv="Jan"/>
          <aws:day number="11" text="Friday" abbrv="Fri"/>
          <aws:hour number="10" hour-24="22"/>
          <aws:minute number="26"/>
          <aws:second number="00"/>
          <aws:am-pm abbrv="PM"/>
          <aws:time-zone offset="-5" text="Eastern Standard Time (USA)" abbrv="EST"/>
        </aws:ob-date>
        <aws:requested-station-id/>
        <aws:station-id>TNKCN</aws:station-id>
        <aws:station>Tunkhannock HS</aws:station>
        <aws:city-state zipcode="18657">Tunkhannock, PA</aws:city-state>
        <aws:country>USA</aws:country>
        <aws:latitude>41.5663871765137</aws:latitude>
        <aws:longitude>-75.9794464111328</aws:longitude>
        <aws:site-url>http://www.tasd.net/highschool/index.cfm</aws:site-url>
        <aws:aux-temp units="&amp;deg;F">-100</aws:aux-temp>
        <aws:aux-temp-rate units="&amp;deg;F">0</aws:aux-temp-rate>
        <aws:current-condition icon="http://deskwx.weatherbug.com/images/Forecast/icons/cond013.gif">Cloudy</aws:current-condition>
        <aws:dew-point units="&amp;deg;F">40</aws:dew-point>
        <aws:elevation units="ft">886</aws:elevation>
        <aws:feels-like units="&amp;deg;F">41</aws:feels-like>
        <aws:gust-time>
          <aws:year number="2013"/>
          <aws:month number="1" text="January" abbrv="Jan"/>
          <aws:day number="11" text="Friday" abbrv="Fri"/>
          <aws:hour number="12" hour-24="12"/>
          <aws:minute number="18"/>
          <aws:second number="00"/>
          <aws:am-pm abbrv="PM"/>
          <aws:time-zone offset="-5" text="Eastern Standard Time (USA)" abbrv="EST"/>
        </aws:gust-time>
        <aws:gust-direction>NNW</aws:gust-direction>
        <aws:gust-direction-degrees>323</aws:gust-direction-degrees>
        <aws:gust-speed units="mph">17</aws:gust-speed>
        <aws:humidity units="%">98</aws:humidity>
        <aws:humidity-high units="%">100</aws:humidity-high>
        <aws:humidity-low units="%">61</aws:humidity-low>
        <aws:humidity-rate>3</aws:humidity-rate>
        <aws:indoor-temp units="&amp;deg;F">77</aws:indoor-temp>
        <aws:indoor-temp-rate units="&amp;deg;F">-1.1</aws:indoor-temp-rate>
        <aws:light>0</aws:light>
        <aws:light-rate>0</aws:light-rate>
        <aws:moon-phase moon-phase-img="http://api.wxbug.net/images/moonphase/mphase01.gif">0</aws:moon-phase>
        <aws:pressure units="&quot;">30.09</aws:pressure>
        <aws:pressure-high units="&quot;">30.5</aws:pressure-high>
        <aws:pressure-low units="&quot;">30.08</aws:pressure-low>
        <aws:pressure-rate units="&quot;/h">-0.01</aws:pressure-rate>
        <aws:rain-month units="&quot;">0.11</aws:rain-month>
        <aws:rain-rate units="&quot;/h">0</aws:rain-rate>
        <aws:rain-rate-max units="&quot;/h">0.12</aws:rain-rate-max>
        <aws:rain-today units="&quot;">0.09</aws:rain-today>
        <aws:rain-year units="&quot;">0.11</aws:rain-year>
        <aws:temp units="&amp;deg;F">41</aws:temp>
        <aws:temp-high units="&amp;deg;F">42</aws:temp-high>
        <aws:temp-low units="&amp;deg;F">29</aws:temp-low>
        <aws:temp-rate units="&amp;deg;F/h">-0.9</aws:temp-rate>
        <aws:sunrise>
          <aws:year number="2013"/>
          <aws:month number="1" text="January" abbrv="Jan"/>
          <aws:day number="11" text="Friday" abbrv="Fri"/>
          <aws:hour number="7" hour-24="07"/>
          <aws:minute number="29"/>
          <aws:second number="53"/>
          <aws:am-pm abbrv="AM"/>
          <aws:time-zone offset="-5" text="Eastern Standard Time (USA)" abbrv="EST"/>
        </aws:sunrise>
        <aws:sunset>
          <aws:year number="2013"/>
          <aws:month number="1" text="January" abbrv="Jan"/>
          <aws:day number="11" text="Friday" abbrv="Fri"/>
          <aws:hour number="4" hour-24="16"/>
          <aws:minute number="54"/>
          <aws:second number="19"/>
          <aws:am-pm abbrv="PM"/>
          <aws:time-zone offset="-5" text="Eastern Standard Time (USA)" abbrv="EST"/>
        </aws:sunset>
        <aws:wet-bulb units="&amp;deg;F">40.802</aws:wet-bulb>
        <aws:wind-speed units="mph">3</aws:wind-speed>
        <aws:wind-speed-avg units="mph">1</aws:wind-speed-avg>
        <aws:wind-direction>S</aws:wind-direction>
        <aws:wind-direction-degrees>163</aws:wind-direction-degrees>
        <aws:wind-direction-avg>SE</aws:wind-direction-avg>
      </aws:ob>
    </aws:weather>
    

I used <http://www.xpathtester.com/test> to test my xpath and it worked there.
But I get the error message:

    
    
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "lxml.etree.pyx", line 2043, in lxml.etree._ElementTree.xpath (src/lxml/lxml.etree.c:47570)
      File "xpath.pxi", line 376, in lxml.etree.XPathDocumentEvaluator.__call__ (src/lxml/lxml.etree.c:118247)
      File "xpath.pxi", line 239, in lxml.etree._XPathEvaluatorBase._handle_result (src/lxml/lxml.etree.c:116911)
      File "xpath.pxi", line 224, in lxml.etree._XPathEvaluatorBase._raise_eval_error (src/lxml/lxml.etree.c:116728)
    lxml.etree.XPathEvalError: Undefined namespace prefix
    

This is all _very_ new to me -- Python, XML, and LXML. All I want is the
observed time and the temperature.

Do my problems have anything to do with that aws: prefix in front of
everything? What does that even mean?

Any help you can offer is greatly appreciated!


Answer: The problem has all "to do with that aws: prefix in front of everything"; it
is a namespace prefix which you have to define. This is easily achievable, as
in:

    
    
    print doc.xpath('//aws:weather/aws:ob/aws:temp', 
                    namespaces={'aws': 'http://www.aws.com/aws'})[0].text
    

The need for this mapping between the namespace prefix to a value is
documented at <http://lxml.de/xpathxslt.html>.

