20.13. "xml.parsers.expat" — Fast XML parsing using Expat
*********************************************************

======================================================================

Warning: The "pyexpat" module is not secure against maliciously
  constructed data.  If you need to parse untrusted or unauthenticated
  data see XML vulnerabilities.

The "xml.parsers.expat" module is a Python interface to the Expat non-
validating XML parser. The module provides a single extension type,
"xmlparser", that represents the current state of an XML parser.
After an "xmlparser" object has been created, various attributes of
the object can be set to handler functions.  When an XML document is
then fed to the parser, the handler functions are called for the
character data and markup in the XML document.

This module uses the "pyexpat" module to provide access to the Expat
parser.  Direct use of the "pyexpat" module is deprecated.

This module provides one exception and one type object:

exception xml.parsers.expat.ExpatError

   The exception raised when Expat reports an error.  See section
   ExpatError Exceptions for more information on interpreting Expat
   errors.

exception xml.parsers.expat.error

   Alias for "ExpatError".

xml.parsers.expat.XMLParserType

   The type of the return values from the "ParserCreate()" function.

The "xml.parsers.expat" module contains two functions:

xml.parsers.expat.ErrorString(errno)

   Returns an explanatory string for a given error number *errno*.

xml.parsers.expat.ParserCreate(encoding=None, namespace_separator=None)

   Creates and returns a new "xmlparser" object.   *encoding*, if
   specified, must be a string naming the encoding  used by the XML
   data.  Expat doesn’t support as many encodings as Python does, and
   its repertoire of encodings can’t be extended; it supports UTF-8,
   UTF-16, ISO-8859-1 (Latin1), and ASCII.  If *encoding* [1] is given
   it will override the implicit or explicit encoding of the document.

   Expat can optionally do XML namespace processing for you, enabled
   by providing a value for *namespace_separator*.  The value must be
   a one-character string; a "ValueError" will be raised if the string
   has an illegal length ("None" is considered the same as omission).
   When namespace processing is enabled, element type names and
   attribute names that belong to a namespace will be expanded.  The
   element name passed to the element handlers "StartElementHandler"
   and "EndElementHandler" will be the concatenation of the namespace
   URI, the namespace separator character, and the local part of the
   name.  If the namespace separator is a zero byte ("chr(0)") then
   the namespace URI and the local part will be concatenated without
   any separator.

   For example, if *namespace_separator* is set to a space character
   ("' '") and the following document is parsed:

      <?xml version="1.0"?>
      <root xmlns    = "http://default-namespace.org/"
            xmlns:py = "http://www.python.org/ns/">
        <py:elem1 />
        <elem2 xmlns="" />
      </root>

   "StartElementHandler" will receive the following strings for each
   element:

      http://default-namespace.org/ root
      http://www.python.org/ns/ elem1
      elem2

   Due to limitations in the "Expat" library used by "pyexpat", the
   "xmlparser" instance returned can only be used to parse a single
   XML document.  Call "ParserCreate" for each document to provide
   unique parser instances.

See also:

  The Expat XML Parser
     Home page of the Expat project.


20.13.1. XMLParser Objects
==========================

"xmlparser" objects have the following methods:

xmlparser.Parse(data[, isfinal])

   Parses the contents of the string *data*, calling the appropriate
   handler functions to process the parsed data.  *isfinal* must be
   true on the final call to this method; it allows the parsing of a
   single file in fragments, not the submission of multiple files.
   *data* can be the empty string at any time.

xmlparser.ParseFile(file)

   Parse XML data reading from the object *file*.  *file* only needs
   to provide the "read(nbytes)" method, returning the empty string
   when there’s no more data.

xmlparser.SetBase(base)

   Sets the base to be used for resolving relative URIs in system
   identifiers in declarations.  Resolving relative identifiers is
   left to the application: this value will be passed through as the
   *base* argument to the "ExternalEntityRefHandler()",
   "NotationDeclHandler()", and "UnparsedEntityDeclHandler()"
   functions.

xmlparser.GetBase()

   Returns a string containing the base set by a previous call to
   "SetBase()", or "None" if  "SetBase()" hasn’t been called.

xmlparser.GetInputContext()

   Returns the input data that generated the current event as a
   string. The data is in the encoding of the entity which contains
   the text. When called while an event handler is not active, the
   return value is "None".

xmlparser.ExternalEntityParserCreate(context[, encoding])

   Create a “child” parser which can be used to parse an external
   parsed entity referred to by content parsed by the parent parser.
   The *context* parameter should be the string passed to the
   "ExternalEntityRefHandler()" handler function, described below. The
   child parser is created with the "ordered_attributes" and
   "specified_attributes" set to the values of this parser.

xmlparser.SetParamEntityParsing(flag)

   Control parsing of parameter entities (including the external DTD
   subset). Possible *flag* values are
   "XML_PARAM_ENTITY_PARSING_NEVER",
   "XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE" and
   "XML_PARAM_ENTITY_PARSING_ALWAYS".  Return true if setting the flag
   was successful.

xmlparser.UseForeignDTD([flag])

   Calling this with a true value for *flag* (the default) will cause
   Expat to call the "ExternalEntityRefHandler" with "None" for all
   arguments to allow an alternate DTD to be loaded.  If the document
   does not contain a document type declaration, the
   "ExternalEntityRefHandler" will still be called, but the
   "StartDoctypeDeclHandler" and "EndDoctypeDeclHandler" will not be
   called.

   Passing a false value for *flag* will cancel a previous call that
   passed a true value, but otherwise has no effect.

   This method can only be called before the "Parse()" or
   "ParseFile()" methods are called; calling it after either of those
   have been called causes "ExpatError" to be raised with the "code"
   attribute set to
   "errors.codes[errors.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING]".

"xmlparser" objects have the following attributes:

xmlparser.buffer_size

   The size of the buffer used when "buffer_text" is true. A new
   buffer size can be set by assigning a new integer value to this
   attribute. When the size is changed, the buffer will be flushed.

xmlparser.buffer_text

   Setting this to true causes the "xmlparser" object to buffer
   textual content returned by Expat to avoid multiple calls to the
   "CharacterDataHandler()" callback whenever possible.  This can
   improve performance substantially since Expat normally breaks
   character data into chunks at every line ending.  This attribute is
   false by default, and may be changed at any time.

xmlparser.buffer_used

   If "buffer_text" is enabled, the number of bytes stored in the
   buffer. These bytes represent UTF-8 encoded text.  This attribute
   has no meaningful interpretation when "buffer_text" is false.

xmlparser.ordered_attributes

   Setting this attribute to a non-zero integer causes the attributes
   to be reported as a list rather than a dictionary.  The attributes
   are presented in the order found in the document text.  For each
   attribute, two list entries are presented: the attribute name and
   the attribute value.  (Older versions of this module also used this
   format.)  By default, this attribute is false; it may be changed at
   any time.

xmlparser.specified_attributes

   If set to a non-zero integer, the parser will report only those
   attributes which were specified in the document instance and not
   those which were derived from attribute declarations.  Applications
   which set this need to be especially careful to use what additional
   information is available from the declarations as needed to comply
   with the standards for the behavior of XML processors.  By default,
   this attribute is false; it may be changed at any time.

The following attributes contain values relating to the most recent
error encountered by an "xmlparser" object, and will only have correct
values once a call to "Parse()" or "ParseFile()" has raised an
"xml.parsers.expat.ExpatError" exception.

xmlparser.ErrorByteIndex

   Byte index at which an error occurred.

xmlparser.ErrorCode

   Numeric code specifying the problem.  This value can be passed to
   the "ErrorString()" function, or compared to one of the constants
   defined in the "errors" object.

xmlparser.ErrorColumnNumber

   Column number at which an error occurred.

xmlparser.ErrorLineNumber

   Line number at which an error occurred.

The following attributes contain values relating to the current parse
location in an "xmlparser" object.  During a callback reporting a
parse event they indicate the location of the first of the sequence of
characters that generated the event.  When called outside of a
callback, the position indicated will be just past the last parse
event (regardless of whether there was an associated callback).

xmlparser.CurrentByteIndex

   Current byte index in the parser input.

xmlparser.CurrentColumnNumber

   Current column number in the parser input.

xmlparser.CurrentLineNumber

   Current line number in the parser input.

Here is the list of handlers that can be set.  To set a handler on an
"xmlparser" object *o*, use "o.handlername = func".  *handlername*
must be taken from the following list, and *func* must be a callable
object accepting the correct number of arguments.  The arguments are
all strings, unless otherwise stated.

xmlparser.XmlDeclHandler(version, encoding, standalone)

   Called when the XML declaration is parsed.  The XML declaration is
   the (optional) declaration of the applicable version of the XML
   recommendation, the encoding of the document text, and an optional
   “standalone” declaration. *version* and *encoding* will be strings,
   and *standalone* will be "1" if the document is declared
   standalone, "0" if it is declared not to be standalone, or "-1" if
   the standalone clause was omitted. This is only available with
   Expat version 1.95.0 or newer.

xmlparser.StartDoctypeDeclHandler(doctypeName, systemId, publicId, has_internal_subset)

   Called when Expat begins parsing the document type declaration
   ("<!DOCTYPE ...").  The *doctypeName* is provided exactly as
   presented.  The *systemId* and *publicId* parameters give the
   system and public identifiers if specified, or "None" if omitted.
   *has_internal_subset* will be true if the document contains and
   internal document declaration subset. This requires Expat version
   1.2 or newer.

xmlparser.EndDoctypeDeclHandler()

   Called when Expat is done parsing the document type declaration.
   This requires Expat version 1.2 or newer.

xmlparser.ElementDeclHandler(name, model)

   Called once for each element type declaration.  *name* is the name
   of the element type, and *model* is a representation of the content
   model.

xmlparser.AttlistDeclHandler(elname, attname, type, default, required)

   Called for each declared attribute for an element type.  If an
   attribute list declaration declares three attributes, this handler
   is called three times, once for each attribute.  *elname* is the
   name of the element to which the declaration applies and *attname*
   is the name of the attribute declared.  The attribute type is a
   string passed as *type*; the possible values are "'CDATA'", "'ID'",
   "'IDREF'", … *default* gives the default value for the attribute
   used when the attribute is not specified by the document instance,
   or "None" if there is no default value ("#IMPLIED" values).  If the
   attribute is required to be given in the document instance,
   *required* will be true. This requires Expat version 1.95.0 or
   newer.

xmlparser.StartElementHandler(name, attributes)

   Called for the start of every element.  *name* is a string
   containing the element name, and *attributes* is the element
   attributes. If "ordered_attributes" is true, this is a list (see
   "ordered_attributes" for a full description). Otherwise it’s a
   dictionary mapping names to values.

xmlparser.EndElementHandler(name)

   Called for the end of every element.

xmlparser.ProcessingInstructionHandler(target, data)

   Called for every processing instruction.

xmlparser.CharacterDataHandler(data)

   Called for character data.  This will be called for normal
   character data, CDATA marked content, and ignorable whitespace.
   Applications which must distinguish these cases can use the
   "StartCdataSectionHandler", "EndCdataSectionHandler", and
   "ElementDeclHandler" callbacks to collect the required information.

xmlparser.UnparsedEntityDeclHandler(entityName, base, systemId, publicId, notationName)

   Called for unparsed (NDATA) entity declarations.  This is only
   present for version 1.2 of the Expat library; for more recent
   versions, use "EntityDeclHandler" instead.  (The underlying
   function in the Expat library has been declared obsolete.)

xmlparser.EntityDeclHandler(entityName, is_parameter_entity, value, base, systemId, publicId, notationName)

   Called for all entity declarations.  For parameter and internal
   entities, *value* will be a string giving the declared contents of
   the entity; this will be "None" for external entities.  The
   *notationName* parameter will be "None" for parsed entities, and
   the name of the notation for unparsed entities.
   *is_parameter_entity* will be true if the entity is a parameter
   entity or false for general entities (most applications only need
   to be concerned with general entities). This is only available
   starting with version 1.95.0 of the Expat library.

xmlparser.NotationDeclHandler(notationName, base, systemId, publicId)

   Called for notation declarations.  *notationName*, *base*, and
   *systemId*, and *publicId* are strings if given.  If the public
   identifier is omitted, *publicId* will be "None".

xmlparser.StartNamespaceDeclHandler(prefix, uri)

   Called when an element contains a namespace declaration.  Namespace
   declarations are processed before the "StartElementHandler" is
   called for the element on which declarations are placed.

xmlparser.EndNamespaceDeclHandler(prefix)

   Called when the closing tag is reached for an element  that
   contained a namespace declaration.  This is called once for each
   namespace declaration on the element in the reverse of the order
   for which the "StartNamespaceDeclHandler" was called to indicate
   the start of each namespace declaration’s scope.  Calls to this
   handler are made after the corresponding "EndElementHandler" for
   the end of the element.

xmlparser.CommentHandler(data)

   Called for comments.  *data* is the text of the comment, excluding
   the leading "'<!-""-'" and trailing "'-""->'".

xmlparser.StartCdataSectionHandler()

   Called at the start of a CDATA section.  This and
   "EndCdataSectionHandler" are needed to be able to identify the
   syntactical start and end for CDATA sections.

xmlparser.EndCdataSectionHandler()

   Called at the end of a CDATA section.

xmlparser.DefaultHandler(data)

   Called for any characters in the XML document for which no
   applicable handler has been specified.  This means characters that
   are part of a construct which could be reported, but for which no
   handler has been supplied.

xmlparser.DefaultHandlerExpand(data)

   This is the same as the "DefaultHandler()",  but doesn’t inhibit
   expansion of internal entities. The entity reference will not be
   passed to the default handler.

xmlparser.NotStandaloneHandler()

   Called if the XML document hasn’t been declared as being a
   standalone document. This happens when there is an external subset
   or a reference to a parameter entity, but the XML declaration does
   not set standalone to "yes" in an XML declaration.  If this handler
   returns "0", then the parser will raise an
   "XML_ERROR_NOT_STANDALONE" error.  If this handler is not set, no
   exception is raised by the parser for this condition.

xmlparser.ExternalEntityRefHandler(context, base, systemId, publicId)

   Called for references to external entities.  *base* is the current
   base, as set by a previous call to "SetBase()".  The public and
   system identifiers, *systemId* and *publicId*, are strings if
   given; if the public identifier is not given, *publicId* will be
   "None".  The *context* value is opaque and should only be used as
   described below.

   For external entities to be parsed, this handler must be
   implemented. It is responsible for creating the sub-parser using
   "ExternalEntityParserCreate(context)", initializing it with the
   appropriate callbacks, and parsing the entity.  This handler should
   return an integer; if it returns "0", the parser will raise an
   "XML_ERROR_EXTERNAL_ENTITY_HANDLING" error, otherwise parsing will
   continue.

   If this handler is not provided, external entities are reported by
   the "DefaultHandler" callback, if provided.


20.13.2. ExpatError Exceptions
==============================

"ExpatError" exceptions have a number of interesting attributes:

ExpatError.code

   Expat’s internal error number for the specific error.  The
   "errors.messages" dictionary maps these error numbers to Expat’s
   error messages.  For example:

      from xml.parsers.expat import ParserCreate, ExpatError, errors

      p = ParserCreate()
      try:
          p.Parse(some_xml_document)
      except ExpatError as err:
          print("Error:", errors.messages[err.code])

   The "errors" module also provides error message constants and a
   dictionary "codes" mapping these messages back to the error codes,
   see below.

ExpatError.lineno

   Line number on which the error was detected.  The first line is
   numbered "1".

ExpatError.offset

   Character offset into the line where the error occurred.  The first
   column is numbered "0".


20.13.3. Example
================

The following program defines three handlers that just print out their
arguments.

   import xml.parsers.expat

   # 3 handler functions
   def start_element(name, attrs):
       print('Start element:', name, attrs)
   def end_element(name):
       print('End element:', name)
   def char_data(data):
       print('Character data:', repr(data))

   p = xml.parsers.expat.ParserCreate()

   p.StartElementHandler = start_element
   p.EndElementHandler = end_element
   p.CharacterDataHandler = char_data

   p.Parse("""<?xml version="1.0"?>
   <parent id="top"><child1 name="paul">Text goes here</child1>
   <child2 name="fred">More text</child2>
   </parent>""", 1)

The output from this program is:

   Start element: parent {'id': 'top'}
   Start element: child1 {'name': 'paul'}
   Character data: 'Text goes here'
   End element: child1
   Character data: '\n'
   Start element: child2 {'name': 'fred'}
   Character data: 'More text'
   End element: child2
   Character data: '\n'
   End element: parent


20.13.4. Content Model Descriptions
===================================

Content models are described using nested tuples.  Each tuple contains
four values: the type, the quantifier, the name, and a tuple of
children.  Children are simply additional content model descriptions.

The values of the first two fields are constants defined in the
"xml.parsers.expat.model" module.  These constants can be collected in
two groups: the model type group and the quantifier group.

The constants in the model type group are:

xml.parsers.expat.model.XML_CTYPE_ANY

   The element named by the model name was declared to have a content
   model of "ANY".

xml.parsers.expat.model.XML_CTYPE_CHOICE

   The named element allows a choice from a number of options; this is
   used for content models such as "(A | B | C)".

xml.parsers.expat.model.XML_CTYPE_EMPTY

   Elements which are declared to be "EMPTY" have this model type.

xml.parsers.expat.model.XML_CTYPE_MIXED

xml.parsers.expat.model.XML_CTYPE_NAME

xml.parsers.expat.model.XML_CTYPE_SEQ

   Models which represent a series of models which follow one after
   the other are indicated with this model type.  This is used for
   models such as "(A, B, C)".

The constants in the quantifier group are:

xml.parsers.expat.model.XML_CQUANT_NONE

   No modifier is given, so it can appear exactly once, as for "A".

xml.parsers.expat.model.XML_CQUANT_OPT

   The model is optional: it can appear once or not at all, as for
   "A?".

xml.parsers.expat.model.XML_CQUANT_PLUS

   The model must occur one or more times (like "A+").

xml.parsers.expat.model.XML_CQUANT_REP

   The model must occur zero or more times, as for "A*".


20.13.5. Expat error constants
==============================

The following constants are provided in the "xml.parsers.expat.errors"
module.  These constants are useful in interpreting some of the
attributes of the "ExpatError" exception objects raised when an error
has occurred. Since for backwards compatibility reasons, the
constants’ value is the error *message* and not the numeric error
*code*, you do this by comparing its "code" attribute with
"errors.codes[errors.XML_ERROR_*CONSTANT_NAME*]".

The "errors" module has the following attributes:

xml.parsers.expat.errors.codes

   A dictionary mapping numeric error codes to their string
   descriptions.

   New in version 3.2.

xml.parsers.expat.errors.messages

   A dictionary mapping string descriptions to their error codes.

   New in version 3.2.

xml.parsers.expat.errors.XML_ERROR_ASYNC_ENTITY

xml.parsers.expat.errors.XML_ERROR_ATTRIBUTE_EXTERNAL_ENTITY_REF

   An entity reference in an attribute value referred to an external
   entity instead of an internal entity.

xml.parsers.expat.errors.XML_ERROR_BAD_CHAR_REF

   A character reference referred to a character which is illegal in
   XML (for example, character "0", or ‘"&#0;"’).

xml.parsers.expat.errors.XML_ERROR_BINARY_ENTITY_REF

   An entity reference referred to an entity which was declared with a
   notation, so cannot be parsed.

xml.parsers.expat.errors.XML_ERROR_DUPLICATE_ATTRIBUTE

   An attribute was used more than once in a start tag.

xml.parsers.expat.errors.XML_ERROR_INCORRECT_ENCODING

xml.parsers.expat.errors.XML_ERROR_INVALID_TOKEN

   Raised when an input byte could not properly be assigned to a
   character; for example, a NUL byte (value "0") in a UTF-8 input
   stream.

xml.parsers.expat.errors.XML_ERROR_JUNK_AFTER_DOC_ELEMENT

   Something other than whitespace occurred after the document
   element.

xml.parsers.expat.errors.XML_ERROR_MISPLACED_XML_PI

   An XML declaration was found somewhere other than the start of the
   input data.

xml.parsers.expat.errors.XML_ERROR_NO_ELEMENTS

   The document contains no elements (XML requires all documents to
   contain exactly one top-level element)..

xml.parsers.expat.errors.XML_ERROR_NO_MEMORY

   Expat was not able to allocate memory internally.

xml.parsers.expat.errors.XML_ERROR_PARAM_ENTITY_REF

   A parameter entity reference was found where it was not allowed.

xml.parsers.expat.errors.XML_ERROR_PARTIAL_CHAR

   An incomplete character was found in the input.

xml.parsers.expat.errors.XML_ERROR_RECURSIVE_ENTITY_REF

   An entity reference contained another reference to the same entity;
   possibly via a different name, and possibly indirectly.

xml.parsers.expat.errors.XML_ERROR_SYNTAX

   Some unspecified syntax error was encountered.

xml.parsers.expat.errors.XML_ERROR_TAG_MISMATCH

   An end tag did not match the innermost open start tag.

xml.parsers.expat.errors.XML_ERROR_UNCLOSED_TOKEN

   Some token (such as a start tag) was not closed before the end of
   the stream or the next token was encountered.

xml.parsers.expat.errors.XML_ERROR_UNDEFINED_ENTITY

   A reference was made to an entity which was not defined.

xml.parsers.expat.errors.XML_ERROR_UNKNOWN_ENCODING

   The document encoding is not supported by Expat.

xml.parsers.expat.errors.XML_ERROR_UNCLOSED_CDATA_SECTION

   A CDATA marked section was not closed.

xml.parsers.expat.errors.XML_ERROR_EXTERNAL_ENTITY_HANDLING

xml.parsers.expat.errors.XML_ERROR_NOT_STANDALONE

   The parser determined that the document was not “standalone” though
   it declared itself to be in the XML declaration, and the
   "NotStandaloneHandler" was set and returned "0".

xml.parsers.expat.errors.XML_ERROR_UNEXPECTED_STATE

xml.parsers.expat.errors.XML_ERROR_ENTITY_DECLARED_IN_PE

xml.parsers.expat.errors.XML_ERROR_FEATURE_REQUIRES_XML_DTD

   An operation was requested that requires DTD support to be compiled
   in, but Expat was configured without DTD support.  This should
   never be reported by a standard build of the "xml.parsers.expat"
   module.

xml.parsers.expat.errors.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING

   A behavioral change was requested after parsing started that can
   only be changed before parsing has started.  This is (currently)
   only raised by "UseForeignDTD()".

xml.parsers.expat.errors.XML_ERROR_UNBOUND_PREFIX

   An undeclared prefix was found when namespace processing was
   enabled.

xml.parsers.expat.errors.XML_ERROR_UNDECLARING_PREFIX

   The document attempted to remove the namespace declaration
   associated with a prefix.

xml.parsers.expat.errors.XML_ERROR_INCOMPLETE_PE

   A parameter entity contained incomplete markup.

xml.parsers.expat.errors.XML_ERROR_XML_DECL

   The document contained no document element at all.

xml.parsers.expat.errors.XML_ERROR_TEXT_DECL

   There was an error parsing a text declaration in an external
   entity.

xml.parsers.expat.errors.XML_ERROR_PUBLICID

   Characters were found in the public id that are not allowed.

xml.parsers.expat.errors.XML_ERROR_SUSPENDED

   The requested operation was made on a suspended parser, but isn’t
   allowed.  This includes attempts to provide additional input or to
   stop the parser.

xml.parsers.expat.errors.XML_ERROR_NOT_SUSPENDED

   An attempt to resume the parser was made when the parser had not
   been suspended.

xml.parsers.expat.errors.XML_ERROR_ABORTED

   This should not be reported to Python applications.

xml.parsers.expat.errors.XML_ERROR_FINISHED

   The requested operation was made on a parser which was finished
   parsing input, but isn’t allowed.  This includes attempts to
   provide additional input or to stop the parser.

xml.parsers.expat.errors.XML_ERROR_SUSPEND_PE

-[ Footnotes ]-

[1] The encoding string included in XML output should conform to
    the appropriate standards. For example, “UTF-8” is valid, but
    “UTF8” is not. See https://www.w3.org/TR/2006/REC-
    xml11-20060816/#NT- EncodingDecl and
    https://www.iana.org/assignments/character-sets /character-
    sets.xhtml.
