I am transforming word documents to xml to compare them using the following code:

word = win32com.client.Dispatch('Word.Application')
wd = word.Documents.Open(inFile)
# Converts the word infile to xml outfile
wd.SaveAs(outFile,11)
wd.Close()
dom=parse(outFile)

The xml file I get looks like:

<?xml version="1.0" encoding="utf-8"?>
<?mso-application progid="Word.Document"?>
<w:wordDocument w:embeddedObjPresent="no" w:macrosPresent="no" w:ocxPresent="no" xml:space="preserve" xmlns:aml="http://schemas.microsoft.com/aml/2001/core" xmlns:dt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:sl="http://schemas.microsoft.com/schemaLibrary/2003/core" xmlns:v="urn:schemas-microsoft-com:vml" xmlns:w="http://schemas.microsoft.com/office/word/2003/wordml" xmlns:w10="urn:schemas-microsoft-com:office:word" xmlns:wne="http://schemas.microsoft.com/office/word/2006/wordml" xmlns:wpc="http://schemas.microsoft.com/office/word/2010/wordprocessingCanvas" xmlns:wsp="http://schemas.microsoft.com/office/word/2003/wordml/sp2" xmlns:wx="http://schemas.microsoft.com/office/word/2003/auxHint"><w:ignoreSubtree w:val="http://schemas.microsoft.com/office/word/2003/wordml/sp2"/><w:shapeDefaults><o:shapedefaults spidmax="1027" v:ext="edit"/><o:shapelayout v:ext="edit"><o:idmap data="1" v:ext="edit"/></o:shapelayout></w:shapeDefaults><w:body><wx:sect><w:tbl><w:tblGrid><w:gridCol w:w="200"/>...</w:tblGrid><w:pict><v:shapetype coordsize="21600,21600" filled="f" id="_x0000_t75" o:preferrelative="t" o:spt="75" path="m@4@5l@4@11@9@11@9@5xe" stroked="f"><v:stroke joinstyle="miter"/><v:formulas><v:f eqn="if lineDrawn pixelLineWidth 0"/>...</v:formulas><v:path gradientshapeok="t" o:connecttype="rect" o:extrusionok="f"/><o:lock aspectratio="t" v:ext="edit"/></v:shapetype><v:shape id="Picture" o:spid="_x0000_s1026" style="position:absolute;left:0;text-align:left;margin-left:0;margin-top:0;width:400pt;height:40pt;z-index:1;visibility:visible;mso-wrap-style:square;mso-wrap-distance-left:0;mso-wrap-distance-top:0;mso-wrap-distance-right:0;mso-wrap-distance-bottom:0;mso-position-horizontal:left;mso-position-horizontal-relative:text;mso-position-vertical:absolute;mso-position-vertical-relative:line" type="#_x0000_t75"><v:imagedata o:title="" src="wordml://03000001.png"/><w10:wrap anchory="line"/><w10:anchorlock/></v:shape></w:pict> ...

I can't use xpath function (lxml library) when I try for example :

import lxml.etree as et
tree = et.parse(xmlFile)
for elt in tree.xpath("//w:gridCol"):elt.getparent().remove(elt)

I get the following error:

 for elt in tree.xpath("//w:gridCol"):File "lxml.etree.pyx", line 2029, in lxml.etree._ElementTree.xpath (src/lxml/lxml.etree.c:45934)File "xpath.pxi", line 379, in lxml.etree.XPathDocumentEvaluator.__call__ (src/lxml/lxml.etree.c:114389)File "xpath.pxi", line 242, in lxml.etree._XPathEvaluatorBase._handle_result (src/lxml/lxml.etree.c:113063)File "xpath.pxi", line 227, in lxml.etree._XPathEvaluatorBase._raise_eval_error (src/lxml/lxml.etree.c:112894)
XPathEvalError: Undefined namespace prefix

I did some research and I guess it's a namespace matter, but I don't know how to fix it?

In this code:

for elt in tree.xpath("//w:gridCol"):

w: isn't a namespace; it's a namespace prefix which is effectively shorthand for the actual namespace, http://schemas.microsoft.com/office/word/2003/wordml. If you want to search for elements in this namespace using the xpath method, you need to provide it with a mapping of namespace prefixes to namespaces:

tree.xpath("//w:gridCol", namespaces={'w': 'http://schemas.microsoft.com/office/word/2003/wordml',})

Also, note that there is no requirement that you use the same namespace prefix. The following would find the same elements:

tree.xpath("//bob:gridCol", namespaces={'bob': 'http://schemas.microsoft.com/office/word/2003/wordml'})