20.2. "html.parser" --- Simple HTML and XHTML parser
****************************************************

**Source code:** Lib/html/parser.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.

class html.parser.HTMLParser(*, convert_charrefs=True)

   Create a parser instance able to parse invalid markup.

   If *convert_charrefs* is "True" (the default), all character
   references (except the ones in "script"/"style" elements) are
   automatically converted to the corresponding Unicode characters.

   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.

   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.

   3.4 版更變: *convert_charrefs* keyword argument added.

   3.5 版更變: The default value for argument *convert_charrefs* is
   now "True".


20.2.1. 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 html.parser import HTMLParser

   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)

   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


20.2.2. "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* must be "str".

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/')])".

   All entity references from "html.entities" are 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'").  This method is never called if
   *convert_charrefs* is "True".

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'".  This method is never called if
   *convert_charrefs* is "True".

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)

   Method 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'")".  It
   is intended to be overridden by a derived class; the base class
   implementation does nothing.

   備註: 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.  The base class implementation does
   nothing.


20.2.3. Examples
================

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

   from html.parser import HTMLParser
   from html.entities 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 = chr(name2codepoint[name])
           print("Named ent:", c)

       def handle_charref(self, name):
           if name.startswith('x'):
               c = chr(int(name[1:], 16))
           else:
               c = chr(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 (unless *convert_charrefs* is set to "True"):

   >>> 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
