# 11.8 Standard Module `sgmllib`

This module defines a class `SGMLParser` which serves as the basis for parsing text files formatted in SGML (Standard Generalized Mark-up Language). In fact, it does not provide a full SGML parser -- it only parses SGML insofar as it is used by HTML, and the module only exists as a base for the `htmllib` module.

In particular, the parser is hardcoded to recognize the following constructs:

- Opening and closing tags of the form ```<tag attr="value" ...>`'' and ```</tag>`'', respectively.
- Numeric character references of the form ```&#name;`''.
- Entity references of the form ```&name;`''.
- SGML comments of the form ```<!-text->`''. Note that spaces, tabs, and newlines are allowed between the trailing ```>`'' and the immediately preceeding ```-`''.

The `SGMLParser` class must be instantiated without arguments. It has the following interface methods:

`reset` ()
  Reset the instance. Loses all unprocessed data. This is called implicitly at instantiation time.

`setnomoretags` ()
  Stop processing tags. Treat all following input as literal input (CDATA). (This is only provided so the HTML tag `<PLAINTEXT>` can be implemented.)

`setliteral` ()
  Enter literal mode (CDATA mode).

`feed` (data)
  Feed some text to the parser. It is processed insofar as it consists of complete elements; incomplete data is buffered until more data is fed or `close()` is called.

`close` ()
  Force processing of all buffered data as if it were followed by an end-of-file mark. This method may be redefined by a derived class to define additional processing at the end of the input, but the redefined version should always call `SGMLParser.close()`.

`handle_starttag` (tag, method, attributes)
  This method is called to handle start tags for which either a `start_tag()` or `do_tag()` method has been defined. The `tag` argument is the name of the tag converted to lower case, and the `method` argument is the bound method which should be used to support semantic interpretation of the start tag. The attributes argument is a list of (name, value) pairs containing the attributes found inside the tag's `<>` brackets. The name has been translated to lower case and double quotes and backslashes in the value have been interpreted. For instance, for the tag `<A HREF="http://www.cwi.nl/">`, this method would be called as `unknown_starttag('a', [('href', 'http://www.cwi.nl/')])`. The base implementation simply calls `method` with `attributes` as the only argument.

`handle_endtag` (tag, method)

  This method is called to handle endtags for which an `end_tag()` method has been defined. The `tag` argument is the name of the tag converted to lower case, and the `method` argument is the bound method which should be used to support semantic interpretation of the end tag. If no `end_tag()` method is defined for the closing element, this handler is not called. The base implementation simply calls `method`.

`handle_data` (data)
  This method is called to process arbitrary data. It is intended to be overridden by a derived class; the base class implementation does nothing.

`handle_charref` (ref)
  This method is called to process a character reference of the form ```&#ref;`''. In the base implementation, ref must be a decimal number in the range 0-255. It translates the character to ASCII and calls the method `handle_data()` with the character as argument. If ref is invalid or out of range, the method `unknown_charref(ref)` is called to handle the error. A subclass must override this method to provide support for named character entities.

`handle_entityref` (ref)
  This method is called to process a general entity reference of the form ```&ref;`'' where ref is an general entity reference. It looks for ref in the instance (or class) variable `entitydefs` which should be a mapping from entity names to corresponding translations. If a translation is found, it calls the method `handle_data()` with the translation; otherwise, it calls the method `unknown_entityref(ref)`. The default `entitydefs` defines translations for `&amp;`, `&apos`, `&gt;`, `&lt;`, and `&quot;`.

`handle_comment` (comment)
  This method is called when a comment is encountered. The `comment` argument is a string containing the text between the ```<!-`'' and ```->`'' delimiters, but not the delimiters themselves. For example, the comment ```<!-text->`'' will cause this method to be called with the argument `'text'`. The default method does nothing.

`report_unbalanced` (tag)
  This method is called when an end tag is found which does not correspond to any open element.

`unknown_starttag` (tag, attributes)
  This method is called to process an unknown start tag. It is intended to be overridden by a derived class; the base class implementation does nothing.

`unknown_endtag` (tag)
  This method is called to process an unknown end tag. It is intended to be overridden by a derived class; the base class implementation does nothing.

`unknown_charref` (ref)
  This method is called to process unresolvable numeric character references. It is intended to be overridden by a derived class; the base class implementation does nothing.

`unknown_entityref` (ref)
  This method is called to process an unknown entity reference. It is intended to be overridden by a derived class; the base class implementation does nothing.

Apart from overriding or extending the methods listed above, derived classes may also define methods of the following form to define processing of specific tags. Tag names in the input stream are case independent; the tag occurring in method names must be in lower case:

`start_tag` (attributes)
  This method is called to process an opening tag tag. It has preference over `do_tag()`. The attributes argument has the same meaning as described for `handle_starttag()` above.

`do_tag` (attributes)
  This method is called to process an opening tag tag that does not come with a matching closing tag. The attributes argument has the same meaning as described for `handle_starttag()` above.

`end_tag` ()
  This method is called to process a closing tag tag.

Note that the parser maintains a stack of open elements for which no end tag has been found yet. Only tags processed by `start_tag()` are pushed on this stack. Definition of an `end_tag()` method is optional for these tags. For tags processed by `do_tag()` or by `unknown_tag()`, no `end_tag()` method must be defined; if defined, it will not be used. If both `start_tag()` and `do_tag()` methods exist for a tag, the `start_tag()` method takes precedence.