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#14020: improve HTMLParser documentation.

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Ezio Melotti 14 years ago
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      Doc/library/html.parser.rst

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Doc/library/html.parser.rst
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@ -19,14 +19,15 @@ parsing text files formatted in HTML (HyperText Mark-up Language) and XHTML.
.. class:: HTMLParser(strict=True)
Create a parser instance. If *strict* is ``True`` (the default), invalid
html results in :exc:`~html.parser.HTMLParseError` exceptions [#]_. If
HTML results in :exc:`~html.parser.HTMLParseError` exceptions [#]_. If
*strict* is ``False``, the parser uses heuristics to make a best guess at
the intention of any invalid html it encounters, similar to the way most
browsers do.
the intention of any invalid HTML it encounters, similar to the way most
browsers do. Using ``strict=False`` is advised.
An :class:`HTMLParser` instance is fed HTML data and calls handler functions when tags
begin and end. The :class:`HTMLParser` class is meant to be overridden by the
user to provide a desired behavior.
An :class:`.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 :class:`.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.
@ -39,25 +40,61 @@ An exception is defined as well:
.. exception:: HTMLParseError
Exception raised by the :class:`HTMLParser` class when it encounters an error
while parsing. This exception provides three attributes: :attr:`msg` is a brief
message explaining the error, :attr:`lineno` is the number of the line on which
the broken construct was detected, and :attr:`offset` is the number of
characters into the line at which the construct starts.
while parsing and *strict* is ``True``. This exception provides three
attributes: :attr:`msg` is a brief message explaining the error,
:attr:`lineno` is the number of the line on which the broken construct was
detected, and :attr:`offset` is the number of characters into the line at
which the construct starts.
:class:`HTMLParser` instances have the following methods:
Example HTML Parser Application
-------------------------------
.. method:: HTMLParser.reset()
As a basic example, below is a simple HTML parser that uses the
:class:`HTMLParser` class to print out start tags, end tags, and data
as they are encountered::
Reset the instance. Loses all unprocessed data. This is called implicitly at
instantiation time.
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(strict=False)
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
:class:`.HTMLParser` Methods
----------------------------
:class:`HTMLParser` instances have the following methods:
.. method:: 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
:meth:`close` is called.
:meth:`close` is called. *data* must be :class:`str`.
.. method:: HTMLParser.close()
@ -68,6 +105,12 @@ An exception is defined as well:
the :class:`HTMLParser` base class method :meth:`close`.
.. method:: HTMLParser.reset()
Reset the instance. Loses all unprocessed data. This is called implicitly at
instantiation time.
.. method:: HTMLParser.getpos()
Return current line number and offset.
@ -81,23 +124,35 @@ An exception is defined as well:
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 :meth:`~HTMLParser.handle_startendtag`):
.. method:: HTMLParser.handle_starttag(tag, attrs)
This method is called to handle the start of a tag. It is intended to be
overridden by a derived class; the base class implementation does nothing.
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="http://www.cwi.nl/">``, this method would be called as
``handle_starttag('a', [('href', 'http://www.cwi.nl/')])``.
have been replaced.
For instance, for the tag ``<A HREF="http://www.cwi.nl/">``, this method
would be called as ``handle_starttag('a', [('href', 'http://www.cwi.nl/')])``.
All entity references from :mod:`html.entities` are replaced in the attribute
values.
.. method:: 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.
.. method:: HTMLParser.handle_startendtag(tag, attrs)
Similar to :meth:`handle_starttag`, but called when the parser encounters an
@ -106,57 +161,46 @@ An exception is defined as well:
implementation simply calls :meth:`handle_starttag` and :meth:`handle_endtag`.
.. method:: HTMLParser.handle_endtag(tag)
This method is called to handle the end tag of an element. It is intended to be
overridden by a derived class; the base class implementation does nothing. The
*tag* argument is the name of the tag converted to lower case.
.. method:: HTMLParser.handle_data(data)
This method is called to process arbitrary data (e.g. the content of
``<script>...</script>`` and ``<style>...</style>``). It is intended to be
overridden by a derived class; the base class implementation does nothing.
This method is called to process arbitrary data (e.g. text nodes and the
content of ``<script>...</script>`` and ``<style>...</style>``).
.. method:: HTMLParser.handle_charref(name)
.. method:: HTMLParser.handle_entityref(name)
This method is called to process a character reference of the form ``&#ref;``.
It is intended to be overridden by a derived class; the base class
implementation does nothing.
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'``).
.. method:: HTMLParser.handle_entityref(name)
.. method:: HTMLParser.handle_charref(name)
This method is called to process a general entity reference of the form
``&name;`` where *name* is an general entity reference. It is intended to be
overridden by a derived class; the base class implementation does nothing.
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'``.
.. method:: HTMLParser.handle_comment(data)
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'``. It is intended to be
overridden by a derived class; the base class implementation does nothing.
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 '``.
.. method:: HTMLParser.handle_decl(decl)
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]>IE-specific content<![endif]'``.
Method called when an SGML ``doctype`` declaration is read by the parser.
The *decl* parameter will be the entire contents of the declaration inside
the ``<!...>`` markup. It is intended to be overridden by a derived class;
the base class implementation does nothing.
.. method:: HTMLParser.handle_decl(decl)
.. method:: HTMLParser.unknown_decl(data)
This method is called to handle an HTML doctype declaration (e.g.
``<!DOCTYPE html>``).
Method called when an unrecognized SGML 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 raises an :exc:`HTMLParseError`.
The *decl* parameter will be the entire contents of the declaration inside
the ``<!...>`` markup (e.g. ``'DOCTYPE html'``).
.. method:: HTMLParser.handle_pi(data)
@ -174,29 +218,123 @@ An exception is defined as well:
cause the ``'?'`` to be included in *data*.
.. _htmlparser-example:
.. method:: HTMLParser.unknown_decl(data)
Example HTML Parser Application
-------------------------------
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 raises an :exc:`HTMLParseError`
when *strict* is ``True``.
As a basic example, below is a simple HTML parser that uses the
:class:`HTMLParser` class to print out start tags, end tags, and data
as they are encountered::
.. _htmlparser-examples:
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("Encountered a start tag:", tag)
print("Start tag:", tag)
for attr in attrs:
print(" attr:", attr)
def handle_endtag(self, tag):
print("Encountered an end tag:", tag)
print("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>')
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(strict=False)
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 :meth:`~HTMLParser.feed` works, but
:meth:`~HTMLParser.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
.. rubric:: Footnotes

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