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&Units=0&stat=TNKCN</aws:WebURL>
<aws:InputLocationURL>http://weather.weatherbug.com/PA/Tunkhannock-weather.html?ZCode=Z5546&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="&deg;F">-100</aws:aux-temp>
<aws:aux-temp-rate units="&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="&deg;F">40</aws:dew-point>
<aws:elevation units="ft">886</aws:elevation>
<aws:feels-like units="&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="&deg;F">77</aws:indoor-temp>
<aws:indoor-temp-rate units="&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=""">30.09</aws:pressure>
<aws:pressure-high units=""">30.5</aws:pressure-high>
<aws:pressure-low units=""">30.08</aws:pressure-low>
<aws:pressure-rate units=""/h">-0.01</aws:pressure-rate>
<aws:rain-month units=""">0.11</aws:rain-month>
<aws:rain-rate units=""/h">0</aws:rain-rate>
<aws:rain-rate-max units=""/h">0.12</aws:rain-rate-max>
<aws:rain-today units=""">0.09</aws:rain-today>
<aws:rain-year units=""">0.11</aws:rain-year>
<aws:temp units="&deg;F">41</aws:temp>
<aws:temp-high units="&deg;F">42</aws:temp-high>
<aws:temp-low units="&deg;F">29</aws:temp-low>
<aws:temp-rate units="&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="&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!
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:
The need for this mapping between the namespace prefix to a value is documented at http://lxml.de/xpathxslt.html.