XML Tutorial – Learn to use for the Raspberry Pi and Arduino! #2

This series of articles discusses the use of XML in applications for the Raspberry Pi and Arduino. Part One introduced XML and the format of its data structures. In Part Two we cover building and parsing XML in Python, while in Part Three we will show how XML is used as a communications protocol for a client / server application, RasPiConnect. RasPiConnect is an iPad/iPhone app that connects and displays information for any number of Raspberry Pi’s via a defined XML interface.

XML Tutorial – Learn to use this important language for the Raspberry Pi!  Part 1

XML Tutorial – Learn to use this important language for the Raspberry Pi!  Part 2

XML Tutorial – Learn to use this important language for the Raspberry Pi!  Part 3

What do we mean by parsing?

Parsing refers to the syntactic analysis of the XML input into its component parts in order to facilitate executing code based on the result of the analysis. In other words, the program is “reading” the XML to find values that it is looking for, paying attention to proper syntax and form. XML syntax includes a nested hierarchy of elements. This means that each level of the hierarchy is included as a fully enclosed subset of the previous level. In our example below, each object <XMLCOMMAND> is fully enclosed (“nested”) in the <XMLObjectXMLRequest>. You can extend this nesting as far down as you like. When you write parsing code this nesting usually results in for loops in Python iterating through all the objects at a level in the hierarchy.

Options for Parsing XML in Python

There are many different packages for parsing XML in Python. We will be using xml.etree.ElementTree. ElementTree is a simple to use, fast XML tree library built into Python. It is somewhat limited in features, but for straightforward XML message parsing it is hard to beat.

What do you need to know about ElementTree? Very few commands are needed to parse simple XML. These few will be illustrated below.

Python Example Code

 import xml.etree.ElementTree as ET

 incomingXML = """
 <XMLObjectXMLRequests>

  <XMLCOMMAND>
   <OBJECTSERVERID>W-1</OBJECTSERVERID>
   <OBJECTNAME>StatusWebView</OBJECTNAME>
   <OBJECTTYPE>1</OBJECTTYPE>
   <OBJECTID>7</OBJECTID>

  </XMLCOMMAND>
  <XMLCOMMAND>

   <OBJECTSERVERID>M-2</OBJECTSERVERID>
   <OBJECTNAME>Processes</OBJECTNAME>
   <OBJECTTYPE>64</OBJECTTYPE>
   <OBJECTID>0</OBJECTID>

  </XMLCOMMAND>

 </XMLObjectXMLRequests>"""

 root = ET.fromstring(incomingXML)
 print incomingXML

 # iterate through all the values
 for element in
 root.findall('XMLCOMMAND'):

     print 'XMLCOMMAND'
     print 'OBJECTNAME:',\

         element.find('OBJECTNAME').text
     print 'OBJECTTYPE:',\

         element.find('OBJECTTYPE').text
     print 'OBJECTSERVERID:',\

 element.find('OBJECTSERVERID').text
     print 'OBJECTID:',\

         element.find('OBJECTID').text

Setup the ElementTree data

After the import of the ElementTree code and writing the XML to a string (note: You could be reading this from a file or a web request), we first set up the root of the XML hierarchy. The root of this XML code is <XMLObjectXMLRequests>.

Iterate through the list

We know from looking at the XML file, that <XMLObjectXMLRequests> consists of a number of <XMLCOMMAND> objects. We use a for loop to do this (each element inside the root is a <XMLCOMMAND> object) using the ElementTree command findall (finding all XMLCOMMAND objects in this case).

Parse the individual items

In the interior of the for loop, we now parse the individual elements of the <XMLCOMMAND> object. Here we use the ElementTree element command with the text attribute. Note that the <XMLCOMMAND> elements are not in the same order! XML does not care if elements on the same level are in any particular order. Furthermore, it is not guaranteed that the first <XMLCOMMAND> element will be the first one retrieved by ElementTree.

Expected elements can be missing from objects. In the case of missing elements in Python (using ElementTree) you absolutely must use an if statement to deal with the missing element. If you

 if (element.find('XXXX').text == None):
                 #do something