19.1. "HTMLParser" — Simple HTML and XHTML parser
*************************************************

Note: The "HTMLParser" module has been renamed to "html.parser" in
  Python 3.  The *2to3* tool will automatically adapt imports when
  converting your sources to Python 3.

New in version 2.2.

**Source code:** Lib/HTMLParser.py

======================================================================

This module defines a class "HTMLParser" which serves as the basis for
parsing text files formatted in HTML (HyperText Mark-up Language) and
XHTML. Unlike the parser in "htmllib", this parser is not based on the
SGML parser in "sgmllib".

class HTMLParser.HTMLParser

   An "HTMLParser" instance is fed HTML data and calls handler methods
   when start tags, end tags, text, comments, and other markup
   elements are encountered.  The user should subclass "HTMLParser"
   and override its methods to implement the desired behavior.

   The "HTMLParser" class is instantiated without arguments.

   Unlike the parser in "htmllib", this parser does not check that end
   tags match start tags or call the end-tag handler for elements
   which are closed implicitly by closing an outer element.

An exception is defined as well:

exception HTMLParser.HTMLParseError

   "HTMLParser" is able to handle broken markup, but in some cases it
   might raise this exception when it encounters an error while
   parsing. This exception provides three attributes: "msg" is a brief
   message explaining the error, "lineno" is the number of the line on
   which the broken construct was detected, and "offset" is the number
   of characters into the line at which the construct starts.


19.1.1. Example HTML Parser Application
=======================================

As a basic example, below is a simple HTML parser that uses the
"HTMLParser" class to print out start tags, end tags and data as they
are encountered:

   from HTMLParser import HTMLParser

   # create a subclass and override the handler methods
   class MyHTMLParser(HTMLParser):
       def handle_starttag(self, tag, attrs):
           print "Encountered a start tag:", tag

       def handle_endtag(self, tag):
           print "Encountered an end tag :", tag

       def handle_data(self, data):
           print "Encountered some data  :", data

   # instantiate the parser and fed it some HTML
   parser = MyHTMLParser()
   parser.feed('<html><head><title>Test</title></head>'
               '<body><h1>Parse me!</h1></body></html>')

The output will then be:

   Encountered a start tag: html
   Encountered a start tag: head
   Encountered a start tag: title
   Encountered some data  : Test
   Encountered an end tag : title
   Encountered an end tag : head
   Encountered a start tag: body
   Encountered a start tag: h1
   Encountered some data  : Parse me!
   Encountered an end tag : h1
   Encountered an end tag : body
   Encountered an end tag : html


19.1.2. "HTMLParser" Methods
============================

"HTMLParser" instances have the following methods:

HTMLParser.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.  *data* can be either
   "unicode" or "str", but passing "unicode" is advised.

HTMLParser.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 the "HTMLParser" base class
   method "close()".

HTMLParser.reset()

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

HTMLParser.getpos()

   Return current line number and offset.

HTMLParser.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.).

The following methods are called when data or markup elements are
encountered and they are meant to be overridden in a subclass.  The
base class implementations do nothing (except for
"handle_startendtag()"):

HTMLParser.handle_starttag(tag, attrs)

   This method is called to handle the start of a tag (e.g. "<div
   id="main">").

   The *tag* argument is the name of the tag converted to lower case.
   The *attrs* argument is a list of "(name, value)" pairs containing
   the attributes found inside the tag’s "<>" brackets.  The *name*
   will be translated to lower case, and quotes in the *value* have
   been removed, and character and entity references have been
   replaced.

   For instance, for the tag "<A HREF="https://www.cwi.nl/">", this
   method would be called as "handle_starttag('a', [('href',
   'https://www.cwi.nl/')])".

   Changed in version 2.6: All entity references from "htmlentitydefs"
   are now replaced in the attribute values.

HTMLParser.handle_endtag(tag)

   This method is called to handle the end tag of an element (e.g.
   "</div>").

   The *tag* argument is the name of the tag converted to lower case.

HTMLParser.handle_startendtag(tag, attrs)

   Similar to "handle_starttag()", but called when the parser
   encounters an XHTML-style empty tag ("<img ... />").  This method
   may be overridden by subclasses which require this particular
   lexical information; the default implementation simply calls
   "handle_starttag()" and "handle_endtag()".

HTMLParser.handle_data(data)

   This method is called to process arbitrary data (e.g. text nodes
   and the content of "<script>...</script>" and
   "<style>...</style>").

HTMLParser.handle_entityref(name)

   This method is called to process a named character reference of the
   form "&name;" (e.g. "&gt;"), where *name* is a general entity
   reference (e.g. "'gt'").

HTMLParser.handle_charref(name)

   This method is called to process decimal and hexadecimal numeric
   character references of the form "&#NNN;" and "&#xNNN;".  For
   example, the decimal equivalent for "&gt;" is "&#62;", whereas the
   hexadecimal is "&#x3E;"; in this case the method will receive
   "'62'" or "'x3E'".

HTMLParser.handle_comment(data)

   This method is called when a comment is encountered (e.g. "<!--
   comment-->").

   For example, the comment "<!-- comment -->" will cause this method
   to be called with the argument "' comment '".

   The content of Internet Explorer conditional comments (condcoms)
   will also be sent to this method, so, for "<!--[if IE
   9]>IE9-specific content<![endif]-->", this method will receive
   "'[if IE 9]>IE9-specific content<![endif]'".

HTMLParser.handle_decl(decl)

   This method is called to handle an HTML doctype declaration (e.g.
   "<!DOCTYPE html>").

   The *decl* parameter will be the entire contents of the declaration
   inside the "<!...>" markup (e.g. "'DOCTYPE html'").

HTMLParser.handle_pi(data)

   This method is called when a processing instruction is encountered.
   The *data* parameter will contain the entire processing
   instruction.  For example, for the processing instruction "<?proc
   color='red'>", this method would be called as "handle_pi("proc
   color='red'")".

   Note: The "HTMLParser" class uses the SGML syntactic rules for
     processing instructions.  An XHTML processing instruction using
     the trailing "'?'" will cause the "'?'" to be included in *data*.

HTMLParser.unknown_decl(data)

   This method is called when an unrecognized declaration is read by
   the parser.

   The *data* parameter will be the entire contents of the declaration
   inside the "<![...]>" markup.  It is sometimes useful to be
   overridden by a derived class.


19.1.3. Examples
================

The following class implements a parser that will be used to
illustrate more examples:

   from HTMLParser import HTMLParser
   from htmlentitydefs import name2codepoint

   class MyHTMLParser(HTMLParser):
       def handle_starttag(self, tag, attrs):
           print "Start tag:", tag
           for attr in attrs:
               print "     attr:", attr

       def handle_endtag(self, tag):
           print "End tag  :", tag

       def handle_data(self, data):
           print "Data     :", data

       def handle_comment(self, data):
           print "Comment  :", data

       def handle_entityref(self, name):
           c = unichr(name2codepoint[name])
           print "Named ent:", c

       def handle_charref(self, name):
           if name.startswith('x'):
               c = unichr(int(name[1:], 16))
           else:
               c = unichr(int(name))
           print "Num ent  :", c

       def handle_decl(self, data):
           print "Decl     :", data

   parser = MyHTMLParser()

Parsing a doctype:

   >>> parser.feed('<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" '
   ...             '"http://www.w3.org/TR/html4/strict.dtd">')
   Decl     : DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"

Parsing an element with a few attributes and a title:

   >>> parser.feed('<img src="python-logo.png" alt="The Python logo">')
   Start tag: img
        attr: ('src', 'python-logo.png')
        attr: ('alt', 'The Python logo')
   >>>
   >>> parser.feed('<h1>Python</h1>')
   Start tag: h1
   Data     : Python
   End tag  : h1

The content of "script" and "style" elements is returned as is,
without further parsing:

   >>> parser.feed('<style type="text/css">#python { color: green }</style>')
   Start tag: style
        attr: ('type', 'text/css')
   Data     : #python { color: green }
   End tag  : style

   >>> parser.feed('<script type="text/javascript">'
   ...             'alert("<strong>hello!</strong>");</script>')
   Start tag: script
        attr: ('type', 'text/javascript')
   Data     : alert("<strong>hello!</strong>");
   End tag  : script

Parsing comments:

   >>> parser.feed('<!-- a comment -->'
   ...             '<!--[if IE 9]>IE-specific content<![endif]-->')
   Comment  :  a comment
   Comment  : [if IE 9]>IE-specific content<![endif]

Parsing named and numeric character references and converting them to
the correct char (note: these 3 references are all equivalent to
"'>'"):

   >>> parser.feed('&gt;&#62;&#x3E;')
   Named ent: >
   Num ent  : >
   Num ent  : >

Feeding incomplete chunks to "feed()" works, but "handle_data()" might
be called more than once:

   >>> for chunk in ['<sp', 'an>buff', 'ered ', 'text</s', 'pan>']:
   ...     parser.feed(chunk)
   ...
   Start tag: span
   Data     : buff
   Data     : ered
   Data     : text
   End tag  : span

Parsing invalid HTML (e.g. unquoted attributes) also works:

   >>> parser.feed('<p><a class=link href=#main>tag soup</p ></a>')
   Start tag: p
   Start tag: a
        attr: ('class', 'link')
        attr: ('href', '#main')
   Data     : tag soup
   End tag  : p
   End tag  : a
