Reading XML: Introducing XPath. Andrew Walker

Transcription

1 Reading XML: Introducing XPath Andrew Walker

2 Why? Makes writing analysis scripts for your XML encoded results easy. Is used by many other XML technologies. XSLT, XPointer etc. A Fortran XPath API is in development. It helps you design your XML documents.

3 #! /usr/bin/perl -w $num = [+-]?\d+\.?\d* ; while (<>) { if (/temperature = ($num)/) { $temp = $1; } elsif (/pressure = ($num)/) { $pres = $1; } }...

4 #! /usr/bin/perl -w use XML::XPath; my $xp = XML::XPath ->new(filename => $ARGV[0]); $temp = $xp -> find( //temperature ); $pres = $xp -> find( //pressure );...

5 /XPath/queries/are/like/paths

6 kml Document Style Style Folder Placemark Placemark Folder Placemark Placemark Elements are nodes <kml> <Document> <Style/> <Style/> <Folder> <Placemark/> <Placemark/> </Folder> <Folder> <Placemark/> <Placemark/> </Folder> </Document> </kml>

7 /path//node path node <> <node/> </>

8 Everything is a node path node Some text < = foo > <node>some text</node> </>

9 comments elements attributes the root node Types of node text nodes namespace nodes processing instructions

10 / -- the root node elementname -- an -- an attribute text() -- a text node /text//node/text()

11 boolean: true or false string: some text in quotes Data s - What can be used or returned? number: nodeset: one or more nodes

12 /path//node/text() gives string: Some text path node Some text < = foo > <node>some text</node> </>

13 gives string: foo path node Some text < = foo > <node>some text</node> </>

14 /path/ gives: a nodeset path node Some text < = foo > <node>some text</node> </>

15 string(any) normalize-space(string) (Some) functions count(nodeset) namespace-uri(nodeset)

16 count(/path/) gives: number 2 path node Some text the end < = foo > <node>some text</node> </> <> = bar >the end</>

17 Predicates Path matches query if predicate is true foo ] Included in square brackets in query

18 bar ]/text() gives: string the end path node Some text the end < = foo > <node>some text</node> </> <> = bar >the end</>

19 XPath in python import lxml.etree docroot = lxml.etree.parse(source="monty.xml") answer = docroot.xpath("/film/@name") print answer

20 Namespaces Create a dictionary relating namespace URIs local namespace prefix. Pass dictionary XPath function. Include prefix before all elements separated by a colon. No inheritance in query!

21 count(/ns:path/ns:) gives: number 2 path node Some text the end <path xmlns= name1 > < = foo > <node>some text</node> </> <> = bar >the end</>

22 count(/ns:path/ns2:) gives: number 1 path node Some text the end <path xmlns= name1 > < = foo xmlns= name2 > <node>some text</node> </> <> = bar >the end</>

23 Namespaces in python import lxml.etree namespaces = {'q':' 'f':' docroot = lxml.etree.parse(source="monty.xml") answer = docroot.xpath ("/q:film/@name", namespaces) print answer

24 Delimiters / //... [ -- separates location steps -- descendent or self -- parent -- self -- predicate -- attribute

25 count(//ns:) gives: number 2 path node Some text the end <path xmlns= name1 > < = foo > <node>some text</node> </> <> = bar >the end</>

26 //ns:/ns:node/text() gives: string Some text path node Some text the end <path xmlns= name1 > < = foo > <node>some text</node> </> <> = bar >the end</>

27 Relative paths import lxml.etree docroot = lxml.etree.parse(source="films.xml") nodes = docroot.xpath("//film[@= comedy ]") for node in nodes: answer = node.xpath("name/text()") print answer

28 numeric valued predicates and the document order more functions What have I not covered unusual nodes the axis

29 XPath APIs Java: standard API provided by javax.xml.xpath as of Java 5 Perl: CPAN module XML::XPath Python: lxml.xpath C and bindings other languages: libxml at