\section{\module{xml.sax.handler} ---
        Base classes for SAX handlers}

\declaremodule{standard}{xml.sax.handler}
\modulesynopsis{Base classes for SAX event handlers.}
\sectionauthor{Martin v. L\"owis}{[email protected]}
\moduleauthor{Lars Marius Garshol}{[email protected]}

\versionadded{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 \module{xml.sax.handler}, so that all
methods get default implementations.

\begin{classdesc*}{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.
\end{classdesc*}

\begin{classdesc*}{DTDHandler}
 Handle DTD events.

 This interface specifies only those DTD events required for basic
 parsing (unparsed entities and attributes).
\end{classdesc*}

\begin{classdesc*}{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.
\end{classdesc*}

\begin{classdesc*}{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.
\end{classdesc*}

In addition to these classes, \module{xml.sax.handler} provides
symbolic constants for the feature and property names.

\begin{datadesc}{feature_namespaces}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{feature_namespace_prefixes}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{feature_string_interning}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{feature_validation}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{feature_external_ges}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{feature_external_pes}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{all_features}
 List of all features.
\end{datadesc}

\begin{datadesc}{property_lexical_handler}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{property_declaration_handler}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{property_dom_node}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{property_xml_string}
 Value: \code{"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
\end{datadesc}

\begin{datadesc}{all_properties}
 List of all known property names.
\end{datadesc}


\subsection{ContentHandler Objects \label{content-handler-objects}}

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

\begin{methoddesc}[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.
\end{methoddesc}

\begin{methoddesc}[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
 \method{setDocumentLocator()}).
\end{methoddesc}

\begin{methoddesc}[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.
\end{methoddesc}

\begin{methoddesc}[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
 \code{feature_namespaces} feature is enabled (the default).

%% XXX This is not really the default, is it? MvL

 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 \method{startPrefixMapping()} and
 \method{endPrefixMapping()} events supply the information to the
 application to expand prefixes in those contexts itself, if
 necessary.

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

\begin{methoddesc}[ContentHandler]{endPrefixMapping}{prefix}
 End the scope of a prefix-URI mapping.

 See \method{startPrefixMapping()} for details. This event will
 always occur after the corresponding \method{endElement()} event,
 but the order of \method{endPrefixMapping()} events is not otherwise
 guaranteed.
\end{methoddesc}

\begin{methoddesc}[ContentHandler]{startElement}{name, attrs}
 Signals the start of an element in non-namespace mode.

 The \var{name} parameter contains the raw XML 1.0 name of the
 element type as a string and the \var{attrs} parameter holds an
 object of the \ulink{\class{Attributes}
 interface}{attributes-objects.html} containing the attributes of the
 element.  The object passed as \var{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 \method{copy()} method of the \var{attrs} object.
\end{methoddesc}

\begin{methoddesc}[ContentHandler]{endElement}{name}
 Signals the end of an element in non-namespace mode.

 The \var{name} parameter contains the name of the element type, just
 as with the \method{startElement()} event.
\end{methoddesc}

\begin{methoddesc}[ContentHandler]{startElementNS}{name, qname, attrs}
 Signals the start of an element in namespace mode.

 The \var{name} parameter contains the name of the element type as a
 \code{(\var{uri}, \var{localname})} tuple, the \var{qname} parameter
 contains the raw XML 1.0 name used in the source document, and the
 \var{attrs} parameter holds an instance of the
 \ulink{\class{AttributesNS} interface}{attributes-ns-objects.html}
 containing the attributes of the element.  If no namespace is
 associated with the element, the \var{uri} component of \var{name}
 will be \code{None}.  The object passed as \var{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 \method{copy()} method of the \var{attrs}
 object.

 Parsers may set the \var{qname} parameter to \code{None}, unless the
 \code{feature_namespace_prefixes} feature is activated.
\end{methoddesc}

\begin{methoddesc}[ContentHandler]{endElementNS}{name, qname}
 Signals the end of an element in namespace mode.

 The \var{name} parameter contains the name of the element type, just
 as with the \method{startElementNS()} method, likewise the
 \var{qname} parameter.
\end{methoddesc}

\begin{methoddesc}[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.

 \var{content} may be a Unicode string or a byte string; the
 \code{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 \var{content}
 instead of slicing content with the old \var{offset} and
 \var{length} parameters.}
\end{methoddesc}

\begin{methoddesc}[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.
\end{methoddesc}

\begin{methoddesc}[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.
\end{methoddesc}

\begin{methoddesc}[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
 \code{feature_external_ges} and the
 \code{feature_external_pes} properties.
\end{methoddesc}


\subsection{DTDHandler Objects \label{dtd-handler-objects}}

\class{DTDHandler} instances provide the following methods:

\begin{methoddesc}[DTDHandler]{notationDecl}{name, publicId, systemId}
 Handle a notation declaration event.
\end{methoddesc}

\begin{methoddesc}[DTDHandler]{unparsedEntityDecl}{name, publicId,
                                                  systemId, ndata}
 Handle an unparsed entity declaration event.
\end{methoddesc}


\subsection{EntityResolver Objects \label{entity-resolver-objects}}

\begin{methoddesc}[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 \var{systemId}.
\end{methoddesc}


\subsection{ErrorHandler Objects \label{sax-error-handler}}

Objects with this interface are used to receive error and warning
information from the \class{XMLReader}.  If you create an object that
implements this interface, then register the object with your
\class{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 \exception{SAXParseException} as the only
parameter.  Errors and warnings may be converted to an exception by
raising the passed-in exception object.

\begin{methoddesc}[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.
\end{methoddesc}

\begin{methoddesc}[ErrorHandler]{fatalError}{exception}
 Called when the parser encounters an error it cannot recover from;
 parsing is expected to terminate when this method returns.
\end{methoddesc}

\begin{methoddesc}[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.
\end{methoddesc}

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