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Python & XML by Christopher A. Jones, Fred L. Drake

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Key Advantages of XML

XML has a few key advantages that make it the data language of choice on the Internet. These advantages were designed into XML from the beginning, and, in fact, are what make it so appealing to Internet developers.

Application Neutrality

First, XML is both human- and machine-readable. This is not a subtle point. Have you ever tried to read a Microsoft Word document with a text editor? You can’t if it was saved as a .doc file, because the information in a .doc document is in a binary (computer readable only) format, even though most Word documents primarily consist of text. A Word document cannot be shared with any other application besides Word—unless that application has been taught the intricacies of Word’s binary format. In this case, the application must also be taught to expect changes in Word’s format each time there is a new release from Microsoft.

This sounds annoying for the developer, but how bad is it, really? After all, Word is incredibly popular, so it must not be too hard to figure out. Let’s look at the top of the Word file that contains this chapter:

Ï_ࡱ_á                > _ ÿ    _           _   B_       _  D_  _  
ÿÿÿ    ?_  @_  A_ ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ
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           ÿÿ_         ÿÿ_         ÿÿ_                 l     Ê_      
Ê_  Ê_      Ê_      Ê_      Ê_      Ê_  ¶           _

This certainly looks familiar to anyone who has ever opened a Word file with a text editor. We don’t see our recognizable text (the content we intended) so we must assume it is buried deep in the file. Determining what the true content is and where it is can be difficult, but it shouldn’t be. It is our data, after all. Let’s try another supported format: “Rich Text Format,” or RTF. Unlike the .doc file, this format is text-based, and should therefore be a bit easier to decipher. We search down in the file to find the start of our text:

\par }\pard \s34\qr
\li0\ri0\sb80\sa480\sl240\slmult0\widctlpar\aspalpha\aspnum\faauto\out
linelevel0\widctlpar\aspalpha\aspnum\faauto\outlinelevel0\pnrauth1\pnr
date-967302179\pnrnot1\adjustright\rin0\lin0\itap0 {\b0\fs48 Combining
Python and XML}{
\b0\deleted\fs48\revauthdel1\revdttmdel-2041034726 Fundamentals}{\b0\f
s48\revised\revauth1\revdttm-2041034726 ?}{\b0\fs48 
\par }\pard\plain \qj

This is better. The chapter title is visible, so we can try to decipher the structure from that point forward. The markup appears to be complex, and there’s a hint of an old version of the chapter title. To extract the text we actually want, we need to understand the Word model for revision tracking, which still presents many challenges.

XML, on the other hand, is application-neutral. In other words, an XML document is usually processed by an XML parser or processor, but if one is not available, an XML document can be easily read and parsed. Data kept in XML is not trapped within the constraints of one particular software application. The ability to read rich data files can become very valuable when, for example, 20 years from now, you dig up a CD-ROM of old business forms that you suddenly find you need again. Will QuickBooks still allow you to extract this same data in 2021? With XML, you can read the data with any text editor.

Let’s look at this chapter in XML. Using markup from a common document type for software manuals and documentation (DocBook), it appears somewhat verbose, and doesn’t include change-tracking information, but we can identify the text quite easily now:

<chapter>
  <title>Python and XML</title>
  <para>Python and XML are two very different animals, each with a
    rich history.  Python is a full-scale programming language that has grown
    from scripting world roots, and has done so in a very organic way

Note that additional characters appear in the document (other than the document content); these are called markup (or tags). We saw this in the RTF version of the document as well, but there were many more bits of text that were difficult to decipher, and we can reasonably surmise that the strange data in the MS Word document would correspond to this in some way. Were this a book on RTF, you would quickly surmise two things: RTF is much more like a printer control language than the example of XML we just looked at, and writing a program that understands RTF would be quite difficult. In this book, we’re going to show you that XML can be used to define languages that fit your application, and that creating programs that can decipher XML is not a difficult task, especially with the help of Python.

Hierarchical Structure

XML is hierarchical, and allows you to choose your own tag names. This is quite different from HTML. In XML, you are free to create elements of any type, and stack other elements within those elements. For example, consider an address entry:

<?xml version="1.0"?>
<address>
  <name>Bubba McBubba</name>
  <street>123 Happy Go Lucky Ln.</street>
  <city>Seattle</city><state>WA</state><zip>98056</zip>
</address>

In the above well-formed XML code, I came up with a few record names and then lumped them together with data. XML processing software, such as a parser (which you use to interpret the syntactic constructs in an XML document), would be able to represent this data in many ways, because its structure has been communicated. For example, if we were to look at what an application programmer might write in source code, we could turn this record into an object initialized this way:

addr = Address(  )
addr.name = "Bubba McBubba"
addr.street = "123 Happy Go Lucky Ln."
addr.city = "Seattle"
addr.state = "WA"
addr.zip = "98056"

This approach makes XML well-suited as a format for many serialized objects. (There are some constructs for which XML is not so well suited, including many formats for large numerical datasets used in scientific computing.) XML’s hierarchical structure makes it easy to apply the concept of object interfaces to documents—it’s quite simple to build application-specific objects directly from the information stream, given mappings from element names to object types. We later see that we can model more than simple hierarchical structures with XML.

Platform Neutrality

Remember that XML is cross-platform. While this is mainly a feature of its text-based format, it’s still very much true. The use of certain text encodings ensures that there are no misconceptions among platforms as to the arrangement of an XML document. Therefore, it’s easy to pass an XML purchase order from a Unix machine to a wireless personal digital assistant. XML is designed for use in conjunction with existing Internet infrastructure using HTTP, SSL, and other messaging protocols as they evolve. These qualities make XML lend itself to distributed applications; it has been successfully used as a foundation for message queuing systems, instant messaging applications, and remote procedure call frameworks. We examine these applications further in Chapter 9 and Chapter 10. It also means that the document example given earlier is more than simply application-neutral, and can be readily moved from one type of machine to another without loss of information. A chapter of a technical book can be written by a programmer on his or her favorite flavor of Unix, and then sent to a publisher using book composition software on a Macintosh. The many difficult format conversions can be avoided.

International Language Support

As the Internet becomes increasingly pervasive in our daily lives, we become more aware of the world around us — it is a culture-rich and diversified place. As technologists, however, we are still learning the significance of making our software work in ways that supports more than one language at a time; making our text-processing routines “8-bit safe” is not only no longer sufficient, it’s no longer even close.

Standards bodies all over the world have come up with ways that computers can interchange text written in their national languages, and sometimes they’ve come up with several, each having varying degrees of acceptance. Unfortunately, most applications do not include information about which language or interchange standard their data is written in, so it is difficult to share information across the cultural and linguistic boundaries the different standards represent. Sometimes it is difficult to share information within such boundaries if multiple standards are prominent.

The difficulties are compounded by very substantial cultural differences that present themselves about how text is handled. There are many different writing systems in addition to the western European left-to-right, top-to-bottom style in which this book is written; right-to-left is not uncommon, and top-to-bottom “lines” of text arranged right-to-left on the page is used in China. Hebrew uses a right-to-left writing system, but numbers are written using Arabic numerals from left to right. Other systems support textual annotations written in parallel with the text. Consider what happens when a document includes text from different writing systems!

Standards bodies are aware of this problem, and have been working on solutions for years. The editors of the XML specification have wisely avoided proposing new solutions to most of these issues, and are instead choosing to build on the work of experts on the topic and existing standards.

The International Organization for Standardization (ISO) and the Unicode Consortium (http://www.unicode.org/ ) have arrived at a single standard that, while not perfect, is perhaps the most capable standard attempting to unify the world’s text representations, with the intent that all languages and alphabets (including ideographic and hieroglyphic character sets) are representable. The standard is known as ISO/IEC 10646, or more commonly, Unicode. Not all national standards bodies have agreed that Unicode is the standard for all future text interchange applications, especially in Asia, but there is widespread belief that Unicode is the best thing available to serve everyone. The standard deals with issues including multidirectional text, capitalization rules, and encoding algorithms that can be used to ensure various properties of data streams. The standard does not deal specifically with language issues that are not tied intimately to character issues. Software sensitive to natural language may still need to do a lot beyond using Unicode to ensure proper collation of names in a particular language (or multiple languages!). Some languages will require substantial additional support for proper text rendering (Arabic, for instance, which requires different letterforms for characters based on their position within a word and based on neighboring letterforms).

The World Wide Web Consortium (W3C) made a simple and masterful stroke to make it easier to use both the older interchange standards and Unicode. It required that all XML documents be Unicode, and specified that they must describe their own encoding in such a way that all XML processors were able to determine what encoding the document was written in. A few specific encodings must be recognized by all processors, so that it is always possible to generate XML that can be read anywhere and represent all of the world’s characters. There is also a feature that allows the content of XML documents to be labeled with the actual language it is written in, but that’s not used as much as it could be at this time.

Since XML documents are Unicode documents, the languages of the world are supported. The use of Unicode and encodings in XML are discussed in some detail in Chapter 2. Unicode strings have been a part of Python since Version 2.0, and the Python standard library includes support for a large number of encodings.

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