20.9. "xml.sax" --- Support for SAX2 parsers
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**Source code:** Lib/xml/sax/__init__.py

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The "xml.sax" package provides a number of modules which implement the
Simple API for XML (SAX) interface for Python.  The package itself
provides the SAX exceptions and the convenience functions which will
be most used by users of the SAX API.

警告: The "xml.sax" module is not secure against maliciously
  constructed data.  If you need to parse untrusted or unauthenticated
  data see XML 漏洞.

3.6.7 版更變: The SAX parser no longer processes general external
entities by default to increase security. Before, the parser created
network connections to fetch remote files or loaded local files from
the file system for DTD and entities. The feature can be enabled again
with method "setFeature()" on the parser object and argument
"feature_external_ges".

The convenience functions are:

xml.sax.make_parser(parser_list=[])

   Create and return a SAX "XMLReader" object.  The first parser found
   will be used.  If *parser_list* is provided, it must be a list of
   strings which name modules that have a function named
   "create_parser()".  Modules listed in *parser_list* will be used
   before modules in the default list of parsers.

xml.sax.parse(filename_or_stream, handler, error_handler=handler.ErrorHandler())

   Create a SAX parser and use it to parse a document.  The document,
   passed in as *filename_or_stream*, can be a filename or a file
   object.  The *handler* parameter needs to be a SAX "ContentHandler"
   instance.  If *error_handler* is given, it must be a SAX
   "ErrorHandler" instance; if omitted,  "SAXParseException" will be
   raised on all errors.  There is no return value; all work must be
   done by the *handler* passed in.

xml.sax.parseString(string, handler, error_handler=handler.ErrorHandler())

   Similar to "parse()", but parses from a buffer *string* received as
   a parameter.  *string* must be a "str" instance or a *bytes-like
   object*.

   3.5 版更變: Added support of "str" instances.

A typical SAX application uses three kinds of objects: readers,
handlers and input sources.  "Reader" in this context is another term
for parser, i.e. some piece of code that reads the bytes or characters
from the input source, and produces a sequence of events. The events
then get distributed to the handler objects, i.e. the reader invokes a
method on the handler.  A SAX application must therefore obtain a
reader object, create or open the input sources, create the handlers,
and connect these objects all together.  As the final step of
preparation, the reader is called to parse the input. During parsing,
methods on the handler objects are called based on structural and
syntactic events from the input data.

For these objects, only the interfaces are relevant; they are normally
not instantiated by the application itself.  Since Python does not
have an explicit notion of interface, they are formally introduced as
classes, but applications may use implementations which do not inherit
from the provided classes.  The "InputSource", "Locator",
"Attributes", "AttributesNS", and "XMLReader" interfaces are defined
in the module "xml.sax.xmlreader".  The handler interfaces are defined
in "xml.sax.handler".  For convenience, "InputSource" (which is often
instantiated directly) and the handler classes are also available from
"xml.sax".  These interfaces are described below.

In addition to these classes, "xml.sax" provides the following
exception classes.

exception xml.sax.SAXException(msg, exception=None)

   Encapsulate an XML error or warning.  This class can contain basic
   error or warning information from either the XML parser or the
   application: it can be subclassed to provide additional
   functionality or to add localization.  Note that although the
   handlers defined in the "ErrorHandler" interface receive instances
   of this exception, it is not required to actually raise the
   exception --- it is also useful as a container for information.

   When instantiated, *msg* should be a human-readable description of
   the error. The optional *exception* parameter, if given, should be
   "None" or an exception that was caught by the parsing code and is
   being passed along as information.

   This is the base class for the other SAX exception classes.

exception xml.sax.SAXParseException(msg, exception, locator)

   Subclass of "SAXException" raised on parse errors. Instances of
   this class are passed to the methods of the SAX "ErrorHandler"
   interface to provide information about the parse error.  This class
   supports the SAX "Locator" interface as well as the "SAXException"
   interface.

exception xml.sax.SAXNotRecognizedException(msg, exception=None)

   Subclass of "SAXException" raised when a SAX "XMLReader" is
   confronted with an unrecognized feature or property.  SAX
   applications and extensions may use this class for similar
   purposes.

exception xml.sax.SAXNotSupportedException(msg, exception=None)

   Subclass of "SAXException" raised when a SAX "XMLReader" is asked
   to enable a feature that is not supported, or to set a property to
   a value that the implementation does not support.  SAX applications
   and extensions may use this class for similar purposes.

也參考:

  SAX: The Simple API for XML
     This site is the focal point for the definition of the SAX API.
     It provides a Java implementation and online documentation.
     Links to implementations and historical information are also
     available.

  Module "xml.sax.handler"
     Definitions of the interfaces for application-provided objects.

  Module "xml.sax.saxutils"
     Convenience functions for use in SAX applications.

  Module "xml.sax.xmlreader"
     Definitions of the interfaces for parser-provided objects.


20.9.1. SAXException Objects
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The "SAXException" exception class supports the following methods:

SAXException.getMessage()

   Return a human-readable message describing the error condition.

SAXException.getException()

   Return an encapsulated exception object, or "None".
