What is the best way to handle the lack of a namespace on some of the nodes in an xml document using lxml? Should I first modify all None named nodes to add the "gmd" name and then change the tree attributes to name http://www.isotc211.org/2005/gmd as "gmd"? If so, is there a clean way to do this with lxml or something else that would be relatively clean/safe?
from lxml import etree
nsmap = charts_tree.nsmap
nsmap.pop(None) # complains without this on the xpath with
# TypeError: empty namespace prefix is not supported in XPath
len (charts_tree.xpath('//*/gml:Polygon',namespaces=nsmap))
# 1180
len (charts_tree.xpath('//*/DS_DataSet',namespaces=nsmap))
# 0 ... Bummer!
len (charts_tree.xpath('//*/DS_DataSet'))
# 0 ... Also a bummer
e.g. http://www.charts.noaa.gov/ENCs/ENCProdCat_19115.xml
<DS_Series xmlns="http://www.isotc211.org/2005/gmd" xmlns:gco="http://www.isotc211.org/2005/gco" xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:gsr="http://www.isotc211.org/2005/gsr" xmlns:gss="http://www.isotc211.org/2005/gss" xmlns:gts="http://www.isotc211.org/2005/gts" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.isotc211.org/2005/gmd http://schemas.opengis.net/iso/19139/20070417/gmd/gmd.xsd">
<composedOf>
<DS_DataSet>
<has>
<MD_Metadata>
<parentIdentifier>
<gco:CharacterString>NOAA ENC Product Catalog</gco:CharacterString>
</parentIdentifier>
...
<EX_BoundingPolygon>
<polygon>
<gml:Polygon gml:id="US1AK90M_P1">
<gml:exterior>
<gml:LinearRing>
<gml:pos>67.61505 -178.99979</gml:pos>
<gml:pos>73.99999 -178.99979</gml:pos>
...
<gml:pos>64.99997 -178.99979</gml:pos>
<gml:pos>67.61505 -178.99979</gml:pos>
</gml:LinearRing>
I believe your DS_DataSet is by virtue of being within the DS_Series (implying a default namespace of "http://www.isotc211.org/2005/gmd") carrying a namespace.
Try and map that into your namespace dictionary (you can probably first test through a print to see if it's already in there, otherwise add it and refer to the namespace by your new key).
nsmap['some_ns'] = "http://www.isotc211.org/2005/gmd"
len (charts_tree.xpath('//*/some_ns:DS_DataSet',namespaces=nsmap))
Which becomes:
nsmap['gmd'] = nsmap[None]
nsmap.pop(None)
len(charts_tree.xpath('//*/gmd:DS_DataSet',namespaces=nsmap))
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