# 13.3 `htmllib` -- A parser for HTML documents

This module defines a class which can serve as a base for parsing text files formatted in the HyperText Mark-up Language (HTML). The class is not directly concerned with I/O -- it must be provided with input in string form via a method, and makes calls to methods of a ``formatter'' object in order to produce output. The `HTMLParser` class is designed to be used as a base class for other classes in order to add functionality, and allows most of its methods to be extended or overridden. In turn, this class is derived from and extends the `SGMLParser` class defined in module `sgmllib`. The `HTMLParser` implementation supports the HTML 2.0 language as described in RFC 1866. Two implementations of formatter objects are provided in the `formatter` module; refer to the documentation for that module for information on the formatter interface.

The following is a summary of the interface defined by `sgmllib.SGMLParser`:

- The interface to feed data to an instance is through the `feed()` method, which takes a string argument. This can be called with as little or as much text at a time as desired; "`p.feed(a); p.feed(b)`" has the same effect as "`p.feed(a+b)`". When the data contains complete HTML markup constructs, these are processed immediately; incomplete constructs are saved in a buffer. To force processing of all unprocessed data, call the `close()` method.

  For example, to parse the entire contents of a file, use:

      parser.feed(open('myfile.html').read())
      parser.close()

- The interface to define semantics for HTML tags is very simple: derive a class and define methods called `start_tag()`, `end_tag()`, or `do_tag()`. The parser will call these at appropriate moments: `start_tag` or `do_tag()` is called when an opening tag of the form `<tag ...>` is encountered; `end_tag()` is called when a closing tag of the form `<tag>` is encountered. If an opening tag requires a corresponding closing tag, like `<H1>` ... `</H1>`, the class should define the `start_tag()` method; if a tag requires no closing tag, like `<P>`, the class should define the `do_tag()` method.

The module defines a parser class and an exception:

class `HTMLParser`( | formatter)

  This is the basic HTML parser class. It supports all entity names required by the XHTML 1.0 Recommendation (http://www.w3.org/TR/xhtml1). It also defines handlers for all HTML 2.0 and many HTML 3.0 and 3.2 elements.

exception `HTMLParseError`
  Exception raised by the `HTMLParser` class when it encounters an error while parsing. New in version 2.4.

See Also:

Module `formatter`:
  Interface definition for transforming an abstract flow of formatting events into specific output events on writer objects.

Module `HTMLParser`:
  Alternate HTML parser that offers a slightly lower-level view of the input, but is designed to work with XHTML, and does not implement some of the SGML syntax not used in ``HTML as deployed'' and which isn't legal for XHTML.

Module `htmlentitydefs`:
  Definition of replacement text for XHTML 1.0 entities.

Module `sgmllib`:
  Base class for `HTMLParser`.