19.12. "xml.sax.handler" — Base classes for SAX handlers
********************************************************

New in version 2.0.

The SAX API defines four kinds of handlers: content handlers, DTD
handlers, error handlers, and entity resolvers. Applications normally
only need to implement those interfaces whose events they are
interested in; they can implement the interfaces in a single object or
in multiple objects. Handler implementations should inherit from the
base classes provided in the module "xml.sax.handler", so that all
methods get default implementations.

class xml.sax.handler.ContentHandler

   This is the main callback interface in SAX, and the one most
   important to applications. The order of events in this interface
   mirrors the order of the information in the document.

class xml.sax.handler.DTDHandler

   Handle DTD events.

   This interface specifies only those DTD events required for basic
   parsing (unparsed entities and attributes).

class xml.sax.handler.EntityResolver

   Basic interface for resolving entities. If you create an object
   implementing this interface, then register the object with your
   Parser, the parser will call the method in your object to resolve
   all external entities.

class xml.sax.handler.ErrorHandler

   Interface used by the parser to present error and warning messages
   to the application.  The methods of this object control whether
   errors are immediately converted to exceptions or are handled in
   some other way.

In addition to these classes, "xml.sax.handler" provides symbolic
constants for the feature and property names.

xml.sax.handler.feature_namespaces

      value: ""http://xml.org/sax/features/namespaces""
      true: Perform Namespace processing.
      false: Optionally do not perform Namespace processing (implies
      namespace-prefixes; default).
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.feature_namespace_prefixes

      value: ""http://xml.org/sax/features/namespace-prefixes""
      true: Report the original prefixed names and attributes used for Namespace
      declarations.
      false: Do not report attributes used for Namespace declarations, and
      optionally do not report original prefixed names (default).
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.feature_string_interning

      value: ""http://xml.org/sax/features/string-interning""
      true: All element names, prefixes, attribute names, Namespace URIs, and
      local names are interned using the built-in intern function.
      false: Names are not necessarily interned, although they may be (default).
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.feature_validation

      value: ""http://xml.org/sax/features/validation""
      true: Report all validation errors (implies external-general-entities and
      external-parameter-entities).
      false: Do not report validation errors.
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.feature_external_ges

      value: ""http://xml.org/sax/features/external-general-entities""
      true: Include all external general (text) entities.
      false: Do not include external general entities.
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.feature_external_pes

      value: ""http://xml.org/sax/features/external-parameter-entities""
      true: Include all external parameter entities, including the external DTD
      subset.
      false: Do not include any external parameter entities, even the external
      DTD subset.
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.all_features

   List of all features.

xml.sax.handler.property_lexical_handler

      value: ""http://xml.org/sax/properties/lexical-handler""
      data type: xml.sax.sax2lib.LexicalHandler (not supported in Python 2)
      description: An optional extension handler for lexical events like
      comments.
      access: read/write

xml.sax.handler.property_declaration_handler

      value: ""http://xml.org/sax/properties/declaration-handler""
      data type: xml.sax.sax2lib.DeclHandler (not supported in Python 2)
      description: An optional extension handler for DTD-related events other
      than notations and unparsed entities.
      access: read/write

xml.sax.handler.property_dom_node

      value: ""http://xml.org/sax/properties/dom-node""
      data type: org.w3c.dom.Node (not supported in Python 2)
      description: When parsing, the current DOM node being visited if this is
      a DOM iterator; when not parsing, the root DOM node for iteration.
      access: (parsing) read-only; (not parsing) read/write

xml.sax.handler.property_xml_string

      value: ""http://xml.org/sax/properties/xml-string""
      data type: String
      description: The literal string of characters that was the source for the
      current event.
      access: read-only

xml.sax.handler.all_properties

   List of all known property names.


19.12.1. ContentHandler Objects
===============================

Users are expected to subclass "ContentHandler" to support their
application.  The following methods are called by the parser on the
appropriate events in the input document:

ContentHandler.setDocumentLocator(locator)

   Called by the parser to give the application a locator for locating
   the origin of document events.

   SAX parsers are strongly encouraged (though not absolutely
   required) to supply a locator: if it does so, it must supply the
   locator to the application by invoking this method before invoking
   any of the other methods in the DocumentHandler interface.

   The locator allows the application to determine the end position of
   any document-related event, even if the parser is not reporting an
   error. Typically, the application will use this information for
   reporting its own errors (such as character content that does not
   match an application’s business rules). The information returned by
   the locator is probably not sufficient for use with a search
   engine.

   Note that the locator will return correct information only during
   the invocation of the events in this interface. The application
   should not attempt to use it at any other time.

ContentHandler.startDocument()

   Receive notification of the beginning of a document.

   The SAX parser will invoke this method only once, before any other
   methods in this interface or in DTDHandler (except for
   "setDocumentLocator()").

ContentHandler.endDocument()

   Receive notification of the end of a document.

   The SAX parser will invoke this method only once, and it will be
   the last method invoked during the parse. The parser shall not
   invoke this method until it has either abandoned parsing (because
   of an unrecoverable error) or reached the end of input.

ContentHandler.startPrefixMapping(prefix, uri)

   Begin the scope of a prefix-URI Namespace mapping.

   The information from this event is not necessary for normal
   Namespace processing: the SAX XML reader will automatically replace
   prefixes for element and attribute names when the
   "feature_namespaces" feature is enabled (the default).

   There are cases, however, when applications need to use prefixes in
   character data or in attribute values, where they cannot safely be
   expanded automatically; the "startPrefixMapping()" and
   "endPrefixMapping()" events supply the information to the
   application to expand prefixes in those contexts itself, if
   necessary.

   Note that "startPrefixMapping()" and "endPrefixMapping()" events
   are not guaranteed to be properly nested relative to each-other:
   all "startPrefixMapping()" events will occur before the
   corresponding "startElement()" event, and all "endPrefixMapping()"
   events will occur after the corresponding "endElement()" event, but
   their order is not guaranteed.

ContentHandler.endPrefixMapping(prefix)

   End the scope of a prefix-URI mapping.

   See "startPrefixMapping()" for details. This event will always
   occur after the corresponding "endElement()" event, but the order
   of "endPrefixMapping()" events is not otherwise guaranteed.

ContentHandler.startElement(name, attrs)

   Signals the start of an element in non-namespace mode.

   The *name* parameter contains the raw XML 1.0 name of the element
   type as a string and the *attrs* parameter holds an object of the
   "Attributes" interface (see The Attributes Interface) containing
   the attributes of the element.  The object passed as *attrs* may be
   re-used by the parser; holding on to a reference to it is not a
   reliable way to keep a copy of the attributes. To keep a copy of
   the attributes, use the "copy()" method of the *attrs* object.

ContentHandler.endElement(name)

   Signals the end of an element in non-namespace mode.

   The *name* parameter contains the name of the element type, just as
   with the "startElement()" event.

ContentHandler.startElementNS(name, qname, attrs)

   Signals the start of an element in namespace mode.

   The *name* parameter contains the name of the element type as a
   "(uri, localname)" tuple, the *qname* parameter contains the raw
   XML 1.0 name used in the source document, and the *attrs* parameter
   holds an instance of the "AttributesNS" interface (see The
   AttributesNS Interface) containing the attributes of the element.
   If no namespace is associated with the element, the *uri* component
   of *name* will be "None".  The object passed as *attrs* may be re-
   used by the parser; holding on to a reference to it is not a
   reliable way to keep a copy of the attributes.  To keep a copy of
   the attributes, use the "copy()" method of the *attrs* object.

   Parsers may set the *qname* parameter to "None", unless the
   "feature_namespace_prefixes" feature is activated.

ContentHandler.endElementNS(name, qname)

   Signals the end of an element in namespace mode.

   The *name* parameter contains the name of the element type, just as
   with the "startElementNS()" method, likewise the *qname* parameter.

ContentHandler.characters(content)

   Receive notification of character data.

   The Parser will call this method to report each chunk of character
   data. SAX parsers may return all contiguous character data in a
   single chunk, or they may split it into several chunks; however,
   all of the characters in any single event must come from the same
   external entity so that the Locator provides useful information.

   *content* may be a Unicode string or a byte string; the "expat"
   reader module produces always Unicode strings.

   Note: The earlier SAX 1 interface provided by the Python XML
     Special Interest Group used a more Java-like interface for this
     method. Since most parsers used from Python did not take
     advantage of the older interface, the simpler signature was
     chosen to replace it. To convert old code to the new interface,
     use *content* instead of slicing content with the old *offset*
     and *length* parameters.

ContentHandler.ignorableWhitespace(whitespace)

   Receive notification of ignorable whitespace in element content.

   Validating Parsers must use this method to report each chunk of
   ignorable whitespace (see the W3C XML 1.0 recommendation, section
   2.10): non-validating parsers may also use this method if they are
   capable of parsing and using content models.

   SAX parsers may return all contiguous whitespace in a single chunk,
   or they may split it into several chunks; however, all of the
   characters in any single event must come from the same external
   entity, so that the Locator provides useful information.

ContentHandler.processingInstruction(target, data)

   Receive notification of a processing instruction.

   The Parser will invoke this method once for each processing
   instruction found: note that processing instructions may occur
   before or after the main document element.

   A SAX parser should never report an XML declaration (XML 1.0,
   section 2.8) or a text declaration (XML 1.0, section 4.3.1) using
   this method.

ContentHandler.skippedEntity(name)

   Receive notification of a skipped entity.

   The Parser will invoke this method once for each entity skipped.
   Non-validating processors may skip entities if they have not seen
   the declarations (because, for example, the entity was declared in
   an external DTD subset). All processors may skip external entities,
   depending on the values of the "feature_external_ges" and the
   "feature_external_pes" properties.


19.12.2. DTDHandler Objects
===========================

"DTDHandler" instances provide the following methods:

DTDHandler.notationDecl(name, publicId, systemId)

   Handle a notation declaration event.

DTDHandler.unparsedEntityDecl(name, publicId, systemId, ndata)

   Handle an unparsed entity declaration event.


19.12.3. EntityResolver Objects
===============================

EntityResolver.resolveEntity(publicId, systemId)

   Resolve the system identifier of an entity and return either the
   system identifier to read from as a string, or an InputSource to
   read from. The default implementation returns *systemId*.


19.12.4. ErrorHandler Objects
=============================

Objects with this interface are used to receive error and warning
information from the "XMLReader".  If you create an object that
implements this interface, then register the object with your
"XMLReader", the parser will call the methods in your object to report
all warnings and errors. There are three levels of errors available:
warnings, (possibly) recoverable errors, and unrecoverable errors.
All methods take a "SAXParseException" as the only parameter.  Errors
and warnings may be converted to an exception by raising the passed-in
exception object.

ErrorHandler.error(exception)

   Called when the parser encounters a recoverable error.  If this
   method does not raise an exception, parsing may continue, but
   further document information should not be expected by the
   application.  Allowing the parser to continue may allow additional
   errors to be discovered in the input document.

ErrorHandler.fatalError(exception)

   Called when the parser encounters an error it cannot recover from;
   parsing is expected to terminate when this method returns.

ErrorHandler.warning(exception)

   Called when the parser presents minor warning information to the
   application. Parsing is expected to continue when this method
   returns, and document information will continue to be passed to the
   application. Raising an exception in this method will cause parsing
   to end.
