:mod:`!xml.parsers.expat` --- Fast XML parsing using Expat
==========================================================

.. module:: xml.parsers.expat
   :synopsis: An interface to the Expat non-validating XML parser.

--------------

.. Markup notes:

   Many of the attributes of the XMLParser objects are callbacks.  Since
   signature information must be presented, these are described using the method
   directive.  Since they are attributes which are set by client code, in-text
   references to these attributes should be marked using the :member: role.


.. note::

   If you need to parse untrusted or unauthenticated data, see
   :ref:`xml-security`.


.. index:: single: Expat

The :mod:`!xml.parsers.expat` module is a Python interface to the Expat
non-validating XML parser. The module provides a single extension type,
:class:`xmlparser`, that represents the current state of an XML parser.  After
an :class:`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.

.. index:: pair: module; pyexpat

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

This module provides one exception and one type object:


.. exception:: ExpatError

   The exception raised when Expat reports an error.  See section
   :ref:`expaterror-objects` for more information on interpreting Expat errors.


.. exception:: error

   Alias for :exc:`ExpatError`.


.. data:: XMLParserType

   The type of the return values from the :func:`ParserCreate` function.

The :mod:`!xml.parsers.expat` module contains two functions:


.. function:: ErrorString(errno)

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


.. function:: ParserCreate(encoding=None, namespace_separator=None)

   Creates and returns a new :class:`xmlparser` object.
   *encoding* [1]_, if specified, must be a string naming the encoding
   used by the XML data.
   If it is given it will override the implicit or explicit encoding
   of the document.

   .. impl-detail::

      Expat natively understands and processes UTF-8, UTF-16, UTF-16BE,
      UTF-16LE, ISO-8859-1, and US-ASCII.
      For other encodings (including aliases like Latin1 and ASCII) it
      falls back to Python.
      It supports most of 8-bit encodings and many multi-byte encodings
      like Shift_JIS, although only BMP characters (``U+0000-U+FFFF``)
      are supported with non-native encodings (this restriction is also
      applied to aliases like UTF8).
      These restrictions only apply if *encoding* is not given.

      .. versionchanged:: next
         Added support for multi-byte encodings.

   .. _xmlparser-non-root:

   Parsers created through :func:`!ParserCreate` are called "root" parsers,
   in the sense that they do not have any parent parser attached. Non-root
   parsers are created by :meth:`parser.ExternalEntityParserCreate
   <xmlparser.ExternalEntityParserCreate>`.

   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
   :exc:`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
   :attr:`StartElementHandler` and :attr:`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:

   .. code-block:: xml

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

   :attr:`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 :mod:`pyexpat`,
   the :class:`xmlparser` instance returned can only be used to parse a single
   XML document.  Call ``ParserCreate`` for each document to provide unique
   parser instances.

.. seealso::

   `The Expat XML Parser <http://www.libexpat.org/>`_
      Home page of the Expat project.


.. _xmlparser-objects:

XMLParser Objects
-----------------

:class:`xmlparser` objects have the following methods:


.. method:: 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.


.. method:: 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.


.. method:: 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
   :func:`ExternalEntityRefHandler`, :func:`NotationDeclHandler`, and
   :func:`UnparsedEntityDeclHandler` functions.


.. method:: xmlparser.GetBase()

   Returns a string containing the base set by a previous call to :meth:`SetBase`,
   or ``None`` if  :meth:`SetBase` hasn't been called.


.. method:: 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``.


.. method:: 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 :meth:`ExternalEntityRefHandler` handler
   function, described below. The child parser is created with the
   :attr:`ordered_attributes` and :attr:`specified_attributes` set to the values of
   this parser.

.. method:: xmlparser.SetParamEntityParsing(flag)

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

.. method:: xmlparser.UseForeignDTD([flag])

   Calling this with a true value for *flag* (the default) will cause Expat to call
   the :attr:`ExternalEntityRefHandler` with :const:`None` for all arguments to
   allow an alternate DTD to be loaded.  If the document does not contain a
   document type declaration, the :attr:`ExternalEntityRefHandler` will still be
   called, but the :attr:`StartDoctypeDeclHandler` and
   :attr:`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 :meth:`Parse` or :meth:`ParseFile`
   methods are called; calling it after either of those have been called causes
   :exc:`ExpatError` to be raised with the :attr:`code` attribute set to
   ``errors.codes[errors.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING]``.

.. method:: xmlparser.SetReparseDeferralEnabled(enabled)

   .. warning::

      Calling ``SetReparseDeferralEnabled(False)`` has security implications,
      as detailed below; please make sure to understand these consequences
      prior to using the ``SetReparseDeferralEnabled`` method.

   Expat 2.6.0 introduced a security mechanism called "reparse deferral"
   where instead of causing denial of service through quadratic runtime
   from reparsing large tokens, reparsing of unfinished tokens is now delayed
   by default until a sufficient amount of input is reached.
   Due to this delay, registered handlers may — depending of the sizing of
   input chunks pushed to Expat — no longer be called right after pushing new
   input to the parser.  Where immediate feedback and taking over responsibility
   of protecting against denial of service from large tokens are both wanted,
   calling ``SetReparseDeferralEnabled(False)`` disables reparse deferral
   for the current Expat parser instance, temporarily or altogether.
   Calling ``SetReparseDeferralEnabled(True)`` allows re-enabling reparse
   deferral.

   :meth:`!SetReparseDeferralEnabled`
   has been backported to some prior releases of CPython as a security fix.
   Check for availability using :func:`hasattr` if used in code running
   across a variety of Python versions.

   .. versionadded:: 3.13

.. method:: xmlparser.GetReparseDeferralEnabled()

   Returns whether reparse deferral is currently enabled for the given
   Expat parser instance.

   .. versionadded:: 3.13


:class:`!xmlparser` objects have the following methods to tune protections
against some common XML vulnerabilities.

.. method:: xmlparser.SetBillionLaughsAttackProtectionActivationThreshold(threshold, /)

   Sets the number of output bytes needed to activate protection against
   `billion laughs`_ attacks.

   The number of output bytes includes amplification from entity expansion
   and reading DTD files.

   Parser objects usually have a protection activation threshold of 8 MiB,
   but the actual default value depends on the underlying Expat library.

   An :exc:`ExpatError` is raised if this method is called on a
   |xml-non-root-parser| parser.
   The corresponding :attr:`~ExpatError.lineno` and :attr:`~ExpatError.offset`
   should not be used as they may have no special meaning.

   :meth:`!SetBillionLaughsAttackProtectionActivationThreshold`
   has been backported to some prior releases of CPython as a security fix.
   Check for availability using :func:`hasattr` if used in code running
   across a variety of Python versions.

   .. note::

      Activation thresholds below 4 MiB are known to break support for DITA 1.3
      payload and are hence not recommended.

   .. versionadded:: 3.15

.. method:: xmlparser.SetBillionLaughsAttackProtectionMaximumAmplification(max_factor, /)

   Sets the maximum tolerated amplification factor for protection against
   `billion laughs`_ attacks.

   The amplification factor is calculated as ``(direct + indirect) / direct``
   while parsing, where ``direct`` is the number of bytes read from
   the primary document in parsing and ``indirect`` is the number of
   bytes added by expanding entities and reading of external DTD files.

   The *max_factor* value must be a non-NaN :class:`float` value greater than
   or equal to 1.0. Peak amplifications of factor 15,000 for the entire payload
   and of factor 30,000 in the middle of parsing have been observed with small
   benign files in practice. In particular, the activation threshold should be
   carefully chosen to avoid false positives.

   Parser objects usually have a maximum amplification factor of 100,
   but the actual default value depends on the underlying Expat library.

   An :exc:`ExpatError` is raised if this method is called on a
   |xml-non-root-parser| parser or if *max_factor* is outside the valid range.
   The corresponding :attr:`~ExpatError.lineno` and :attr:`~ExpatError.offset`
   should not be used as they may have no special meaning.

   :meth:`!SetBillionLaughsAttackProtectionMaximumAmplification`
   has been backported to some prior releases of CPython as a security fix.
   Check for availability using :func:`hasattr` if used in code running
   across a variety of Python versions.

   .. note::

      The maximum amplification factor is only considered if the threshold
      that can be adjusted by :meth:`.SetBillionLaughsAttackProtectionActivationThreshold`
      is exceeded.

   .. versionadded:: 3.15

.. method:: xmlparser.SetAllocTrackerActivationThreshold(threshold, /)

   Sets the number of allocated bytes of dynamic memory needed to activate
   protection against disproportionate use of RAM.

   Parser objects usually have an allocation activation threshold of 64 MiB,
   but the actual default value depends on the underlying Expat library.

   An :exc:`ExpatError` is raised if this method is called on a
   |xml-non-root-parser| parser.
   The corresponding :attr:`~ExpatError.lineno` and :attr:`~ExpatError.offset`
   should not be used as they may have no special meaning.

   :meth:`!SetAllocTrackerActivationThreshold`
   has been backported to some prior releases of CPython as a security fix.
   Check for availability using :func:`hasattr` if used in code running
   across a variety of Python versions.

   .. versionadded:: 3.15

.. method:: xmlparser.SetAllocTrackerMaximumAmplification(max_factor, /)

   Sets the maximum amplification factor between direct input and bytes
   of dynamic memory allocated.

   The amplification factor is calculated as ``allocated / direct``
   while parsing, where ``direct`` is the number of bytes read from
   the primary document in parsing and ``allocated`` is the number
   of bytes of dynamic memory allocated in the parser hierarchy.

   The *max_factor* value must be a non-NaN :class:`float` value greater than
   or equal to 1.0. Amplification factors greater than 100.0 can be observed
   near the start of parsing even with benign files in practice. In particular,
   the activation threshold should be carefully chosen to avoid false positives.

   Parser objects usually have a maximum amplification factor of 100,
   but the actual default value depends on the underlying Expat library.

   An :exc:`ExpatError` is raised if this method is called on a
   |xml-non-root-parser| parser or if *max_factor* is outside the valid range.
   The corresponding :attr:`~ExpatError.lineno` and :attr:`~ExpatError.offset`
   should not be used as they may have no special meaning.

   :meth:`!SetAllocTrackerMaximumAmplification`
   has been backported to some prior releases of CPython as a security fix.
   Check for availability using :func:`hasattr` if used in code running
   across a variety of Python versions.

   .. note::

      The maximum amplification factor is only considered if the threshold
      that can be adjusted by :meth:`.SetAllocTrackerActivationThreshold`
      is exceeded.

   .. versionadded:: 3.15


:class:`xmlparser` objects have the following attributes:


.. attribute:: xmlparser.buffer_size

   The size of the buffer used when :attr:`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.


.. attribute:: xmlparser.buffer_text

   Setting this to true causes the :class:`xmlparser` object to buffer textual
   content returned by Expat to avoid multiple calls to the
   :meth:`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. Note that when it is false, data that does not contain newlines
   may be chunked too.


.. attribute:: xmlparser.buffer_used

   If :attr:`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 :attr:`buffer_text` is false.


.. attribute:: 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.


.. attribute:: 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 :class:`xmlparser` object, and will only have correct values
once a call to :meth:`Parse` or :meth:`ParseFile` has raised an
:exc:`xml.parsers.expat.ExpatError` exception.


.. attribute:: xmlparser.ErrorByteIndex

   Byte index at which an error occurred.


.. attribute:: xmlparser.ErrorCode

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


.. attribute:: xmlparser.ErrorColumnNumber

   Column number at which an error occurred.


.. attribute:: xmlparser.ErrorLineNumber

   Line number at which an error occurred.

The following attributes contain values relating to the current parse location
in an :class:`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).


.. attribute:: xmlparser.CurrentByteIndex

   Current byte index in the parser input.


.. attribute:: xmlparser.CurrentColumnNumber

   Current column number in the parser input.


.. attribute:: 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
:class:`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.


.. method:: 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.


.. method:: 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 an internal document declaration subset. This requires Expat version
   1.2 or newer.


.. method:: xmlparser.EndDoctypeDeclHandler()

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


.. method:: 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.


.. method:: 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.


.. method:: 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
   :attr:`ordered_attributes` is true, this is a list (see
   :attr:`ordered_attributes` for a full description). Otherwise it's a
   dictionary mapping names to values.


.. method:: xmlparser.EndElementHandler(name)

   Called for the end of every element.


.. method:: xmlparser.ProcessingInstructionHandler(target, data)

   Called for every processing instruction.


.. method:: 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 :attr:`StartCdataSectionHandler`,
   :attr:`EndCdataSectionHandler`, and :attr:`ElementDeclHandler` callbacks to
   collect the required information. Note that the character data may be
   chunked even if it is short and so you may receive more than one call to
   :meth:`CharacterDataHandler`. Set the :attr:`buffer_text` instance attribute
   to ``True`` to avoid that.


.. method:: 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
   :attr:`EntityDeclHandler` instead.  (The underlying function in the Expat
   library has been declared obsolete.)


.. method:: 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.


.. method:: 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``.


.. method:: xmlparser.StartNamespaceDeclHandler(prefix, uri)

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


.. method:: 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
   :attr:`StartNamespaceDeclHandler` was called to indicate the start of each
   namespace declaration's scope.  Calls to this handler are made after the
   corresponding :attr:`EndElementHandler` for the end of the element.


.. method:: xmlparser.CommentHandler(data)

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


.. method:: xmlparser.StartCdataSectionHandler()

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


.. method:: xmlparser.EndCdataSectionHandler()

   Called at the end of a CDATA section.


.. method:: 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.


.. method:: xmlparser.DefaultHandlerExpand(data)

   This is the same as the :func:`DefaultHandler`,  but doesn't inhibit expansion
   of internal entities. The entity reference will not be passed to the default
   handler.


.. method:: 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
   :const:`XML_ERROR_NOT_STANDALONE` error.  If this handler is not set, no
   exception is raised by the parser for this condition.


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

   .. warning::

      Implementing a handler that accesses local files and/or the network
      may create a vulnerability to
      `external entity attacks <https://en.wikipedia.org/wiki/XML_external_entity_attack>`_
      if :class:`xmlparser` is used with user-provided XML content.
      Please reflect on your `threat model <https://en.wikipedia.org/wiki/Threat_model>`_
      before implementing this handler.

   Called for references to external entities.  *base* is the current base, as set
   by a previous call to :meth:`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
   :const:`XML_ERROR_EXTERNAL_ENTITY_HANDLING` error, otherwise parsing will
   continue.

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


.. _expaterror-objects:

ExpatError Exceptions
---------------------

:exc:`ExpatError` exceptions have a number of interesting attributes:


.. attribute:: ExpatError.code

   Expat's internal error number for the specific error.  The
   :data:`errors.messages <xml.parsers.expat.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 :mod:`~xml.parsers.expat.errors` module also provides error message
   constants and a dictionary :data:`~xml.parsers.expat.errors.codes` mapping
   these messages back to the error codes, see below.


.. attribute:: ExpatError.lineno

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


.. attribute:: ExpatError.offset

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


.. _expat-example:

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


.. _expat-content-models:

Content Model Descriptions
--------------------------

.. module:: xml.parsers.expat.model

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
:mod:`!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:


.. data:: XML_CTYPE_ANY
   :noindex:

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


.. data:: XML_CTYPE_CHOICE
   :noindex:

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


.. data:: XML_CTYPE_EMPTY
   :noindex:

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


.. data:: XML_CTYPE_MIXED
   :noindex:


.. data:: XML_CTYPE_NAME
   :noindex:


.. data:: XML_CTYPE_SEQ
   :noindex:

   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:


.. data:: XML_CQUANT_NONE
   :noindex:

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


.. data:: XML_CQUANT_OPT
   :noindex:

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


.. data:: XML_CQUANT_PLUS
   :noindex:

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


.. data:: XML_CQUANT_REP
   :noindex:

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


.. _expat-errors:

Expat error constants
---------------------

.. module:: xml.parsers.expat.errors

The following constants are provided in the :mod:`!xml.parsers.expat.errors`
module.  These constants are useful in interpreting some of the attributes of
the :exc:`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
:attr:`code` attribute with
:samp:`errors.codes[errors.XML_ERROR_{CONSTANT_NAME}]`.

The ``errors`` module has the following attributes:

.. data:: codes

   A dictionary mapping string descriptions to their error codes.

   .. versionadded:: 3.2


.. data:: messages

   A dictionary mapping numeric error codes to their string descriptions.

   .. versionadded:: 3.2


.. data:: XML_ERROR_ASYNC_ENTITY


.. data:: XML_ERROR_ATTRIBUTE_EXTERNAL_ENTITY_REF

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


.. data:: XML_ERROR_BAD_CHAR_REF

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


.. data:: XML_ERROR_BINARY_ENTITY_REF

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


.. data:: XML_ERROR_DUPLICATE_ATTRIBUTE

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


.. data:: XML_ERROR_INCORRECT_ENCODING


.. data:: 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.


.. data:: XML_ERROR_JUNK_AFTER_DOC_ELEMENT

   Something other than whitespace occurred after the document element.


.. data:: XML_ERROR_MISPLACED_XML_PI

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


.. data:: XML_ERROR_NO_ELEMENTS

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


.. data:: XML_ERROR_NO_MEMORY

   Expat was not able to allocate memory internally.


.. data:: XML_ERROR_PARAM_ENTITY_REF

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


.. data:: XML_ERROR_PARTIAL_CHAR

   An incomplete character was found in the input.


.. data:: XML_ERROR_RECURSIVE_ENTITY_REF

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


.. data:: XML_ERROR_SYNTAX

   Some unspecified syntax error was encountered.


.. data:: XML_ERROR_TAG_MISMATCH

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


.. data:: 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.


.. data:: XML_ERROR_UNDEFINED_ENTITY

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


.. data:: XML_ERROR_UNKNOWN_ENCODING

   The document encoding is not supported by Expat.


.. data:: XML_ERROR_UNCLOSED_CDATA_SECTION

   A CDATA marked section was not closed.


.. data:: XML_ERROR_EXTERNAL_ENTITY_HANDLING


.. data:: 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 :attr:`NotStandaloneHandler` was
   set and returned ``0``.


.. data:: XML_ERROR_UNEXPECTED_STATE


.. data:: XML_ERROR_ENTITY_DECLARED_IN_PE


.. data:: 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 :mod:`!xml.parsers.expat` module.


.. data:: 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
   :meth:`UseForeignDTD`.


.. data:: XML_ERROR_UNBOUND_PREFIX

   An undeclared prefix was found when namespace processing was enabled.


.. data:: XML_ERROR_UNDECLARING_PREFIX

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


.. data:: XML_ERROR_INCOMPLETE_PE

   A parameter entity contained incomplete markup.


.. data:: XML_ERROR_XML_DECL

   The document contained no document element at all.


.. data:: XML_ERROR_TEXT_DECL

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


.. data:: XML_ERROR_PUBLICID

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


.. data:: 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.


.. data:: XML_ERROR_NOT_SUSPENDED

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


.. data:: XML_ERROR_ABORTED

   This should not be reported to Python applications.


.. data:: 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.


.. data:: XML_ERROR_SUSPEND_PE


.. data:: XML_ERROR_RESERVED_PREFIX_XML

   An attempt was made to
   undeclare reserved namespace prefix ``xml``
   or to bind it to another namespace URI.


.. data:: XML_ERROR_RESERVED_PREFIX_XMLNS

   An attempt was made to declare or undeclare reserved namespace prefix ``xmlns``.


.. data:: XML_ERROR_RESERVED_NAMESPACE_URI

   An attempt was made to bind the URI of one the reserved namespace
   prefixes ``xml`` and ``xmlns`` to another namespace prefix.


.. data:: XML_ERROR_INVALID_ARGUMENT

   This should not be reported to Python applications.


.. data:: XML_ERROR_NO_BUFFER

   This should not be reported to Python applications.


.. data:: XML_ERROR_AMPLIFICATION_LIMIT_BREACH

   The limit on input amplification factor (from DTD and entities)
   has been breached.


.. data:: XML_ERROR_NOT_STARTED

   The parser was tried to be stopped or suspended before it started.

   .. versionadded:: 3.14


.. rubric:: 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 valid in an XML document's declaration, even though
   Python accepts it as an encoding name.
   See https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-EncodingDecl
   and https://www.iana.org/assignments/character-sets/character-sets.xhtml.


.. _billion laughs: https://en.wikipedia.org/wiki/Billion_laughs_attack
.. |xml-non-root-parser| replace:: :ref:`non-root <xmlparser-non-root>`
