# 13.10 `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`, so that all methods get default implementations.

class `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 `DTDHandler`
  Handle DTD events.

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

class `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 `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.

`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

`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

`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

`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

`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

`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

`all_features`
  List of all features.

`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

`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

`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

`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

`all_properties`
  List of all known property names.