path: root/Doc/library/sgmllib.rst

:mod:`sgmllib` --- Simple SGML parser
=====================================

.. module:: sgmllib
   :synopsis: Only as much of an SGML parser as needed to parse HTML.


.. index:: single: SGML

This module defines a class :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
:mod:`htmllib` module.  Another HTML parser which supports XHTML and offers a
somewhat different interface is available in the :mod:`HTMLParser` module.


.. class:: SGMLParser()

   The :class:`SGMLParser` class is instantiated without arguments. 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 preceding
     ``--``.

A single exception is defined as well:


.. exception:: SGMLParseError

   Exception raised by the :class:`SGMLParser` class when it encounters an error
   while parsing.

   .. versionadded:: 2.1

:class:`SGMLParser` instances have the following methods:


.. method:: SGMLParser.reset()

   Reset the instance.  Loses all unprocessed data.  This is called implicitly at
   instantiation time.


.. method:: SGMLParser.setnomoretags()

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


.. method:: SGMLParser.setliteral()

   Enter literal mode (CDATA mode).


.. method:: SGMLParser.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
   :meth:`close` is called.


.. method:: SGMLParser.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
   :meth:`close`.


.. method:: SGMLParser.get_starttag_text()

   Return the text of the most recently opened start tag.  This should not normally
   be needed for structured processing, but may be useful in dealing with HTML "as
   deployed" or for re-generating input with minimal changes (whitespace between
   attributes can be preserved, etc.).


.. method:: SGMLParser.handle_starttag(tag, method, attributes)

   This method is called to handle start tags for which either a :meth:`start_tag`
   or :meth:`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. Double quotes and backslashes in
   the *value* have been interpreted, as well as known character references and
   known entity references terminated by a semicolon (normally, entity references
   can be terminated by any non-alphanumerical character, but this would break the
   very common case of ``<A HREF="url?spam=1&eggs=2">`` when ``eggs`` is a valid
   entity name).

   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.

   .. versionadded:: 2.5
      Handling of entity and character references within attribute values.


.. method:: SGMLParser.handle_endtag(tag, method)

   This method is called to handle endtags for which an :meth:`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 :meth:`end_tag` method is
   defined for the closing element, this handler is not called.  The base
   implementation simply calls *method*.


.. method:: SGMLParser.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.


.. method:: SGMLParser.handle_charref(ref)

   This method is called to process a character reference of the form ``&#ref;``.
   The base implementation uses :meth:`convert_charref` to convert the reference to
   a string.  If that method returns a string, it is passed to :meth:`handle_data`,
   otherwise ``unknown_charref(ref)`` is called to handle the error.

   .. versionchanged:: 2.5
      Use :meth:`convert_charref` instead of hard-coding the conversion.


.. method:: SGMLParser.convert_charref(ref)

   Convert a character reference to a string, or ``None``.  *ref* is the reference
   passed in as a string.  In the base implementation, *ref* must be a decimal
   number in the range 0-255.  It converts the code point found using the
   :meth:`convert_codepoint` method. If *ref* is invalid or out of range, this
   method returns ``None``.  This method is called by the default
   :meth:`handle_charref` implementation and by the attribute value parser.

   .. versionadded:: 2.5


.. method:: SGMLParser.convert_codepoint(codepoint)

   Convert a codepoint to a :class:`str` value.  Encodings can be handled here if
   appropriate, though the rest of :mod:`sgmllib` is oblivious on this matter.

   .. versionadded:: 2.5


.. method:: SGMLParser.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 converts *ref* by
   passing it to :meth:`convert_entityref`.  If a translation is returned, it calls
   the method :meth:`handle_data` with the translation; otherwise, it calls the
   method ``unknown_entityref(ref)``. The default :attr:`entitydefs` defines
   translations for ``&amp;``, ``&apos``, ``&gt;``, ``&lt;``, and ``&quot;``.

   .. versionchanged:: 2.5
      Use :meth:`convert_entityref` instead of hard-coding the conversion.


.. method:: SGMLParser.convert_entityref(ref)

   Convert a named entity reference to a :class:`str` value, or ``None``.  The
   resulting value will not be parsed.  *ref* will be only the name of the entity.
   The default implementation looks for *ref* in the instance (or class) variable
   :attr:`entitydefs` which should be a mapping from entity names to corresponding
   translations.  If no translation is available for *ref*, this method returns
   ``None``.  This method is called by the default :meth:`handle_entityref`
   implementation and by the attribute value parser.

   .. versionadded:: 2.5


.. method:: SGMLParser.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.


.. method:: SGMLParser.handle_decl(data)

   Method called when an SGML declaration is read by the parser.  In practice, the
   ``DOCTYPE`` declaration is the only thing observed in HTML, but the parser does
   not discriminate among different (or broken) declarations.  Internal subsets in
   a ``DOCTYPE`` declaration are not supported.  The *data* parameter will be the
   entire contents of the declaration inside the ``<!``...\ ``>`` markup.  The
   default implementation does nothing.


.. method:: SGMLParser.report_unbalanced(tag)

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


.. method:: SGMLParser.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.


.. method:: SGMLParser.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.


.. method:: SGMLParser.unknown_charref(ref)

   This method is called to process unresolvable numeric character references.
   Refer to :meth:`handle_charref` to determine what is handled by default.  It is
   intended to be overridden by a derived class; the base class implementation does
   nothing.


.. method:: SGMLParser.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:


.. method:: SGMLParser.start_tag(attributes)
   :noindex:

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


.. method:: SGMLParser.do_tag(attributes)
   :noindex:

   This method is called to process an opening tag *tag*  for which no
   :meth:`start_tag` method is defined.   The *attributes* argument has the same
   meaning as described for :meth:`handle_starttag` above.


.. method:: SGMLParser.end_tag()
   :noindex:

   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 :meth:`start_tag` are pushed on this
stack.  Definition of an :meth:`end_tag` method is optional for these tags.  For
tags processed by :meth:`do_tag` or by :meth:`unknown_tag`, no :meth:`end_tag`
method must be defined; if defined, it will not be used.  If both
:meth:`start_tag` and :meth:`do_tag` methods exist for a tag, the
:meth:`start_tag` method takes precedence.