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ReStructuredText
:mod:`xml.sax.handler` --- Base classes for SAX handlers
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========================================================
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.. module:: xml.sax.handler
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:synopsis: Base classes for SAX event handlers.
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.. moduleauthor:: Lars Marius Garshol <larsga@garshol.priv.no>
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.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>
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**Source code:** :source:`Lib/xml/sax/handler.py`
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--------------
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The SAX API defines four kinds of handlers: content handlers, DTD handlers,
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error handlers, and entity resolvers. Applications normally only need to
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implement those interfaces whose events they are interested in; they can
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implement the interfaces in a single object or in multiple objects. Handler
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implementations should inherit from the base classes provided in the module
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:mod:`xml.sax.handler`, so that all methods get default implementations.
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.. class:: ContentHandler
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This is the main callback interface in SAX, and the one most important to
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applications. The order of events in this interface mirrors the order of the
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information in the document.
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.. class:: DTDHandler
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Handle DTD events.
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This interface specifies only those DTD events required for basic parsing
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(unparsed entities and attributes).
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.. class:: EntityResolver
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Basic interface for resolving entities. If you create an object implementing
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this interface, then register the object with your Parser, the parser will call
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the method in your object to resolve all external entities.
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.. class:: ErrorHandler
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Interface used by the parser to present error and warning messages to the
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application. The methods of this object control whether errors are immediately
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converted to exceptions or are handled in some other way.
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In addition to these classes, :mod:`xml.sax.handler` provides symbolic constants
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for the feature and property names.
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.. data:: feature_namespaces
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| value: ``"http://xml.org/sax/features/namespaces"``
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| true: Perform Namespace processing.
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| false: Optionally do not perform Namespace processing (implies
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namespace-prefixes; default).
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: feature_namespace_prefixes
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| value: ``"http://xml.org/sax/features/namespace-prefixes"``
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| true: Report the original prefixed names and attributes used for Namespace
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declarations.
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| false: Do not report attributes used for Namespace declarations, and
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optionally do not report original prefixed names (default).
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: feature_string_interning
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| value: ``"http://xml.org/sax/features/string-interning"``
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| true: All element names, prefixes, attribute names, Namespace URIs, and
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local names are interned using the built-in intern function.
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| false: Names are not necessarily interned, although they may be (default).
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: feature_validation
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| value: ``"http://xml.org/sax/features/validation"``
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| true: Report all validation errors (implies external-general-entities and
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external-parameter-entities).
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| false: Do not report validation errors.
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: feature_external_ges
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| value: ``"http://xml.org/sax/features/external-general-entities"``
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| true: Include all external general (text) entities.
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| false: Do not include external general entities.
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: feature_external_pes
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| value: ``"http://xml.org/sax/features/external-parameter-entities"``
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| true: Include all external parameter entities, including the external DTD
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subset.
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| false: Do not include any external parameter entities, even the external
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DTD subset.
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: all_features
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List of all features.
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.. data:: property_lexical_handler
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| value: ``"http://xml.org/sax/properties/lexical-handler"``
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| data type: xml.sax.sax2lib.LexicalHandler (not supported in Python 2)
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| description: An optional extension handler for lexical events like
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comments.
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| access: read/write
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.. data:: property_declaration_handler
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| value: ``"http://xml.org/sax/properties/declaration-handler"``
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| data type: xml.sax.sax2lib.DeclHandler (not supported in Python 2)
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| description: An optional extension handler for DTD-related events other
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than notations and unparsed entities.
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| access: read/write
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.. data:: property_dom_node
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| value: ``"http://xml.org/sax/properties/dom-node"``
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| data type: org.w3c.dom.Node (not supported in Python 2)
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| description: When parsing, the current DOM node being visited if this is
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a DOM iterator; when not parsing, the root DOM node for iteration.
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| access: (parsing) read-only; (not parsing) read/write
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.. data:: property_xml_string
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| value: ``"http://xml.org/sax/properties/xml-string"``
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| data type: String
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| description: The literal string of characters that was the source for the
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current event.
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| access: read-only
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.. data:: all_properties
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List of all known property names.
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.. _content-handler-objects:
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ContentHandler Objects
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----------------------
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Users are expected to subclass :class:`ContentHandler` to support their
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application. The following methods are called by the parser on the appropriate
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events in the input document:
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.. method:: ContentHandler.setDocumentLocator(locator)
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Called by the parser to give the application a locator for locating the origin
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of document events.
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SAX parsers are strongly encouraged (though not absolutely required) to supply a
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locator: if it does so, it must supply the locator to the application by
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invoking this method before invoking any of the other methods in the
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DocumentHandler interface.
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The locator allows the application to determine the end position of any
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document-related event, even if the parser is not reporting an error. Typically,
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the application will use this information for reporting its own errors (such as
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character content that does not match an application's business rules). The
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information returned by the locator is probably not sufficient for use with a
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search engine.
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Note that the locator will return correct information only during the invocation
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of the events in this interface. The application should not attempt to use it at
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any other time.
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.. method:: ContentHandler.startDocument()
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Receive notification of the beginning of a document.
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The SAX parser will invoke this method only once, before any other methods in
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this interface or in DTDHandler (except for :meth:`setDocumentLocator`).
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.. method:: ContentHandler.endDocument()
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Receive notification of the end of a document.
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The SAX parser will invoke this method only once, and it will be the last method
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invoked during the parse. The parser shall not invoke this method until it has
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either abandoned parsing (because of an unrecoverable error) or reached the end
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of input.
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.. method:: ContentHandler.startPrefixMapping(prefix, uri)
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Begin the scope of a prefix-URI Namespace mapping.
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The information from this event is not necessary for normal Namespace
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processing: the SAX XML reader will automatically replace prefixes for element
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and attribute names when the ``feature_namespaces`` feature is enabled (the
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default).
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There are cases, however, when applications need to use prefixes in character
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data or in attribute values, where they cannot safely be expanded automatically;
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the :meth:`startPrefixMapping` and :meth:`endPrefixMapping` events supply the
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information to the application to expand prefixes in those contexts itself, if
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necessary.
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.. XXX This is not really the default, is it? MvL
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Note that :meth:`startPrefixMapping` and :meth:`endPrefixMapping` events are not
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guaranteed to be properly nested relative to each-other: all
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:meth:`startPrefixMapping` events will occur before the corresponding
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:meth:`startElement` event, and all :meth:`endPrefixMapping` events will occur
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after the corresponding :meth:`endElement` event, but their order is not
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guaranteed.
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.. method:: ContentHandler.endPrefixMapping(prefix)
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End the scope of a prefix-URI mapping.
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See :meth:`startPrefixMapping` for details. This event will always occur after
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the corresponding :meth:`endElement` event, but the order of
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:meth:`endPrefixMapping` events is not otherwise guaranteed.
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.. method:: ContentHandler.startElement(name, attrs)
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Signals the start of an element in non-namespace mode.
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The *name* parameter contains the raw XML 1.0 name of the element type as a
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string and the *attrs* parameter holds an object of the
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:class:`~xml.sax.xmlreader.Attributes`
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interface (see :ref:`attributes-objects`) containing the attributes of
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the element. The object passed as *attrs* may be re-used by the parser; holding
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on to a reference to it is not a reliable way to keep a copy of the attributes.
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To keep a copy of the attributes, use the :meth:`copy` method of the *attrs*
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object.
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.. method:: ContentHandler.endElement(name)
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Signals the end of an element in non-namespace mode.
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The *name* parameter contains the name of the element type, just as with the
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:meth:`startElement` event.
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.. method:: ContentHandler.startElementNS(name, qname, attrs)
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Signals the start of an element in namespace mode.
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The *name* parameter contains the name of the element type as a ``(uri,
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localname)`` tuple, the *qname* parameter contains the raw XML 1.0 name used in
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the source document, and the *attrs* parameter holds an instance of the
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:class:`~xml.sax.xmlreader.AttributesNS` interface (see
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:ref:`attributes-ns-objects`)
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containing the attributes of the element. If no namespace is associated with
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the element, the *uri* component of *name* will be ``None``. The object passed
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as *attrs* may be re-used by the parser; holding on to a reference to it is not
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a reliable way to keep a copy of the attributes. To keep a copy of the
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attributes, use the :meth:`copy` method of the *attrs* object.
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Parsers may set the *qname* parameter to ``None``, unless the
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``feature_namespace_prefixes`` feature is activated.
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.. method:: ContentHandler.endElementNS(name, qname)
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Signals the end of an element in namespace mode.
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The *name* parameter contains the name of the element type, just as with the
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:meth:`startElementNS` method, likewise the *qname* parameter.
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.. method:: ContentHandler.characters(content)
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Receive notification of character data.
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The Parser will call this method to report each chunk of character data. SAX
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parsers may return all contiguous character data in a single chunk, or they may
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split it into several chunks; however, all of the characters in any single event
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must come from the same external entity so that the Locator provides useful
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information.
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*content* may be a string or bytes instance; the ``expat`` reader module
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always produces strings.
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.. note::
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The earlier SAX 1 interface provided by the Python XML Special Interest Group
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used a more Java-like interface for this method. Since most parsers used from
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Python did not take advantage of the older interface, the simpler signature was
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chosen to replace it. To convert old code to the new interface, use *content*
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instead of slicing content with the old *offset* and *length* parameters.
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.. method:: ContentHandler.ignorableWhitespace(whitespace)
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Receive notification of ignorable whitespace in element content.
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Validating Parsers must use this method to report each chunk of ignorable
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whitespace (see the W3C XML 1.0 recommendation, section 2.10): non-validating
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parsers may also use this method if they are capable of parsing and using
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content models.
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SAX parsers may return all contiguous whitespace in a single chunk, or they may
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split it into several chunks; however, all of the characters in any single event
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must come from the same external entity, so that the Locator provides useful
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information.
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.. method:: ContentHandler.processingInstruction(target, data)
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Receive notification of a processing instruction.
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The Parser will invoke this method once for each processing instruction found:
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note that processing instructions may occur before or after the main document
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element.
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A SAX parser should never report an XML declaration (XML 1.0, section 2.8) or a
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text declaration (XML 1.0, section 4.3.1) using this method.
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.. method:: ContentHandler.skippedEntity(name)
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Receive notification of a skipped entity.
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The Parser will invoke this method once for each entity skipped. Non-validating
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processors may skip entities if they have not seen the declarations (because,
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for example, the entity was declared in an external DTD subset). All processors
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may skip external entities, depending on the values of the
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``feature_external_ges`` and the ``feature_external_pes`` properties.
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.. _dtd-handler-objects:
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DTDHandler Objects
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------------------
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:class:`DTDHandler` instances provide the following methods:
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.. method:: DTDHandler.notationDecl(name, publicId, systemId)
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Handle a notation declaration event.
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.. method:: DTDHandler.unparsedEntityDecl(name, publicId, systemId, ndata)
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Handle an unparsed entity declaration event.
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.. _entity-resolver-objects:
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EntityResolver Objects
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----------------------
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.. method:: EntityResolver.resolveEntity(publicId, systemId)
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Resolve the system identifier of an entity and return either the system
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identifier to read from as a string, or an InputSource to read from. The default
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implementation returns *systemId*.
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.. _sax-error-handler:
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ErrorHandler Objects
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--------------------
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Objects with this interface are used to receive error and warning information
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from the :class:`~xml.sax.xmlreader.XMLReader`. If you create an object that
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implements this interface, then register the object with your
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:class:`~xml.sax.xmlreader.XMLReader`, the parser
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will call the methods in your object to report all warnings and errors. There
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are three levels of errors available: warnings, (possibly) recoverable errors,
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and unrecoverable errors. All methods take a :exc:`SAXParseException` as the
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only parameter. Errors and warnings may be converted to an exception by raising
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the passed-in exception object.
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.. method:: ErrorHandler.error(exception)
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Called when the parser encounters a recoverable error. If this method does not
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raise an exception, parsing may continue, but further document information
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should not be expected by the application. Allowing the parser to continue may
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allow additional errors to be discovered in the input document.
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.. method:: ErrorHandler.fatalError(exception)
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Called when the parser encounters an error it cannot recover from; parsing is
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expected to terminate when this method returns.
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.. method:: ErrorHandler.warning(exception)
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Called when the parser presents minor warning information to the application.
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Parsing is expected to continue when this method returns, and document
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information will continue to be passed to the application. Raising an exception
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in this method will cause parsing to end.
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