19.2. "sgmllib" — Simple SGML parser
************************************

2.6 版后已移除: The "sgmllib" module has been removed in Python 3.

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.  Another HTML parser which
supports XHTML and offers a somewhat different interface is available
in the "HTMLParser" module.

class sgmllib.SGMLParser

   The "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 sgmllib.SGMLParseError

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

   2.1 新版功能.

"SGMLParser" instances have the following methods:

SGMLParser.reset()

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

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.)

SGMLParser.setliteral()

   Enter literal mode (CDATA mode).

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 "close()" is called.

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 "close()".

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.).

SGMLParser.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. 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.

   2.5 新版功能: Handling of entity and character references within
   attribute values.

SGMLParser.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*.

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.

SGMLParser.handle_charref(ref)

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

   在 2.5 版更改: Use "convert_charref()" instead of hard-coding the
   conversion.

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 "convert_codepoint()" method. If *ref* is
   invalid or out of range, this method returns "None".  This method
   is called by the default "handle_charref()" implementation and by
   the attribute value parser.

   2.5 新版功能.

SGMLParser.convert_codepoint(codepoint)

   Convert a code point to a "str" value.  Encodings can be handled
   here if appropriate, though the rest of "sgmllib" is oblivious on
   this matter.

   2.5 新版功能.

SGMLParser.handle_entityref(ref)

   This method is called to process a general entity reference of the
   form "&ref;" where *ref* is a general entity reference.  It
   converts *ref* by passing it to "convert_entityref()".  If a
   translation is returned, 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;".

   在 2.5 版更改: Use "convert_entityref()" instead of hard-coding the
   conversion.

SGMLParser.convert_entityref(ref)

   Convert a named entity reference to a "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 "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 "handle_entityref()" implementation and by
   the attribute value parser.

   2.5 新版功能.

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.

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.

SGMLParser.report_unbalanced(tag)

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

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.

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.

SGMLParser.unknown_charref(ref)

   This method is called to process unresolvable numeric character
   references. Refer to "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.

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:

SGMLParser.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.

SGMLParser.do_tag(attributes)

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

SGMLParser.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.
