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13.6.1 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:

`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.

`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()`).

`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.

`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 `http://xml.org/sax/features/namespaces` feature is true (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 start/endPrefixMapping event supplies the information to the application to expand prefixes in those contexts itself, if necessary.

  Note that start/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.

`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.

`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 instance of the `Attributes` class containing the attributes of the element.

`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.

`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 the raw XML 1.0 name used in the source document, and the attrs parameter holds an instance of the `AttributesNS` class containing the attributes of the element.

  Parsers may set the qname parameter to `None`, unless the `http://xml.org/sax/features/namespace-prefixes` feature is activated.

`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 event, likewise the qname parameter.

`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.

`ignorableWhitespace` ()
  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.

`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.

`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 `http://xml.org/sax/features/external-general-entities` and the `http://xml.org/sax/features/external-parameter-entities` properties.