XML : how to query xml data with namespaces using xpath in python

I am trying to apply an XPath query to XML data which has namespaces using the following code:

  from lxml import etree  from io import StringIO    xml = '''  <gpx creator="udos" version="1.1" xmlns="http://www.topografix.com/GPX/1/1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd http://www.garmin.com/xmlschemas/GpxExtensions/v3 http://www.garmin.com/xmlschemas/GpxExtensionsv3.xsd http://www.garmin.com/xmlschemas/TrackPointExtension/v1 http://www.garmin.com/xmlschemas/TrackPointExtensionv1.xsd" xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1" xmlns:gpxx="http://www.garmin.com/xmlschemas/GpxExtensions/v3">   <metadata>    <time>2015-07-07T15:16:40Z</time>   </metadata>   <trk>    <name>some name</name>    <trkseg>     <trkpt lat="46.3884140" lon="10.0286290">      <ele>2261.8</ele>      <time>2015-07-07T15:30:42Z</time>     </trkpt>     <trkpt lat="46.3884050" lon="10.0286240">      <ele>2261.6</ele>      <time>2015-07-07T15:30:43Z</time>     </trkpt>     <trkpt lat="46.3884000" lon="10.0286210">      <ele>2262.0</ele>      <time>2015-07-07T15:30:46Z</time>     </trkpt>     <trkpt lat="46.3884000" lon="10.0286210">      <ele>2261.8</ele>      <time>2015-07-07T15:30:47Z</time>     </trkpt>    </trkseg>   </trk>  </gpx>  '''    # this is to simulate that above xml was read from a file  file = StringIO(unicode(xml))   # with python 3 use "file = StringIO(xml)"    # reading the xml from a file  tree = etree.parse(file)    ns = {'xmlns': 'http://www.topografix.com/GPX/1/1',        'xmlns:xsi': 'http://www.w3.org/2001/XMLSchema-instance',        'xmlns:gpxtpx': 'http://www.garmin.com/xmlschemas/TrackPointExtension/v1',        'xmlns:gpxx': 'http://www.garmin.com/xmlschemas/GpxExtensions/v3'}    expr = 'trk/trkseg/trkpt/ele'    for element in tree.xpath(expr, namespaces=ns):      print(element.text)    

I expect the following output from the code:

2261.8
2261.6
2262.0
2261.8

when you substitute the XML root element

  <gpx creator="udos" version="1.1" xmlns="http://www.topografix.com/GPX/1/1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd http://www.garmin.com/xmlschemas/GpxExtensions/v3 http://www.garmin.com/xmlschemas/GpxExtensionsv3.xsd http://www.garmin.com/xmlschemas/TrackPointExtension/v1 http://www.garmin.com/xmlschemas/TrackPointExtensionv1.xsd" xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1" xmlns:gpxx="http://www.garmin.com/xmlschemas/GpxExtensions/v3">    

with

  <gpx>    

the code is working...

any suggestions how to get it to work with namespaces as well?