This chapter describes a number of modules that are used to parse different file formats.
Python comes with extensive support for the Extensible Markup Language XML and Hypertext Markup Language (HTML) file formats. Python also provides basic support for Standard Generalized Markup Language (SGML).
All these formats share the same basic structure (this isn’t so strange, since both HTML and XML are derived from SGML). Each document contains a mix of start tags, end tags, plain text (also called character data), and entity references.
<document name="sample.xml"> <header>This is a header</header> <body>This is the body text. The text can contain plain text ("character data"), tags, and entities. </body> </document>
In the above example, <document>, <header>, and <body> are start tags. For each start tag, there’s a corresponding end tag which looks similar, but has a slash before the tag name. The start tag can also contain one or more attributes, like the name attribute in this example.
Everything between a start tag and its matching end tag is called an element. In the above example, the document element contains two other elements, header and body.
Finally, " is a character entity. It is used to represent reserved characters in the text sections (in this case, it’s an ampersand (&) which is used to start the entity itself. Other common entities include < for “less than” (<), and > for “greater than” (>).
While XML, HTML, and SGML all share the same building blocks, there are important differences between them. In XML, all elements must have both start tags and end tags, and the tags must be properly nested (if they are, the document is said to be well-formed). In addition, XML is case-sensitive, so <document> and <Document> are two different element types.
HTML, in contrast, is much more flexible. The HTML parser can often fill in missing tags; for example, if you open a new paragraph in HTML using the <P> tag without closing the previous paragraph, the parser automatically adds a </P> end tag. HTML is also case-insensitive. On the other hand, XML allows you to define your own elements, while HTML uses a fixed element set, as defined by the HTML specifications.
SGML is even more flexible. In its full incarnation, you can use a custom declaration to define how to translate the source text into an element structure, and a document type description (DTD) to validate the structure, and fill in missing tags. Technically, both HTML and XML are SGML applications; they both have their own SGML declaration, and HTML also has a standard DTD.
Python comes with parsers for all markup flavors. While SGML is the most flexible of the formats, Python’s sgmllib parser is actually pretty simple. It avoids most of the problems by only understanding enough of the SGML standard to be able to deal with HTML. It doesn’t handle document type descriptions either; instead, you can customize the parser via subclassing.
The XML support is most complex. In Python 1.5.2, the built-in support was limited to the xmllib parser, which is pretty similar to the sgmllib module (with one important difference; xmllib actually tries to support the entire XML standard).
Python 2.0 comes with more advanced XML tools, based on the optional expat parser.
The ConfigParser module reads and writes a simple configuration file format, similar to Windows INI files.
Python’s standard library also provides support for the popular GZIP and ZIP (2.0 only) formats. The gzip module can read and write GZIP files, and the zipfile reads and writes ZIP files. Both modules depend on the zlib data compression module.