19.2. sgmllib — Simple SGML parser

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.

New in version 2.1.

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.

New in version 2.5: 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.

Changed in version 2.5: 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.

New in version 2.5.

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.

New in version 2.5.

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 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;.

Changed in version 2.5: 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.

New in version 2.5.

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.