path: root/Doc/library/xml.dom.minidom.rst

:mod:`xml.dom.minidom` --- Lightweight DOM implementation
=========================================================

.. module:: xml.dom.minidom
   :synopsis: Lightweight Document Object Model (DOM) implementation.
.. moduleauthor:: Paul Prescod <paul@prescod.net>
.. sectionauthor:: Paul Prescod <paul@prescod.net>
.. sectionauthor:: Martin v. Löwis <martin@v.loewis.de>


:mod:`xml.dom.minidom` is a light-weight implementation of the Document Object
Model interface.  It is intended to be simpler than the full DOM and also
significantly smaller.

DOM applications typically start by parsing some XML into a DOM.  With
:mod:`xml.dom.minidom`, this is done through the parse functions::

   from xml.dom.minidom import parse, parseString

   dom1 = parse('c:\\temp\\mydata.xml') # parse an XML file by name

   datasource = open('c:\\temp\\mydata.xml')
   dom2 = parse(datasource)   # parse an open file

   dom3 = parseString('<myxml>Some data<empty/> some more data</myxml>')

The :func:`parse` function can take either a filename or an open file object.


.. function:: parse(filename_or_file, parser)

   Return a :class:`Document` from the given input. *filename_or_file* may be
   either a file name, or a file-like object. *parser*, if given, must be a SAX2
   parser object. This function will change the document handler of the parser and
   activate namespace support; other parser configuration (like setting an entity
   resolver) must have been done in advance.

If you have XML in a string, you can use the :func:`parseString` function
instead:


.. function:: parseString(string[, parser])

   Return a :class:`Document` that represents the *string*. This method creates a
   :class:`StringIO` object for the string and passes that on to :func:`parse`.

Both functions return a :class:`Document` object representing the content of the
document.

What the :func:`parse` and :func:`parseString` functions do is connect an XML
parser with a "DOM builder" that can accept parse events from any SAX parser and
convert them into a DOM tree.  The name of the functions are perhaps misleading,
but are easy to grasp when learning the interfaces.  The parsing of the document
will be completed before these functions return; it's simply that these
functions do not provide a parser implementation themselves.

You can also create a :class:`Document` by calling a method on a "DOM
Implementation" object.  You can get this object either by calling the
:func:`getDOMImplementation` function in the :mod:`xml.dom` package or the
:mod:`xml.dom.minidom` module. Using the implementation from the
:mod:`xml.dom.minidom` module will always return a :class:`Document` instance
from the minidom implementation, while the version from :mod:`xml.dom` may
provide an alternate implementation (this is likely if you have the `PyXML
package <http://pyxml.sourceforge.net/>`_ installed).  Once you have a
:class:`Document`, you can add child nodes to it to populate the DOM::

   from xml.dom.minidom import getDOMImplementation

   impl = getDOMImplementation()

   newdoc = impl.createDocument(None, "some_tag", None)
   top_element = newdoc.documentElement
   text = newdoc.createTextNode('Some textual content.')
   top_element.appendChild(text)

Once you have a DOM document object, you can access the parts of your XML
document through its properties and methods.  These properties are defined in
the DOM specification.  The main property of the document object is the
:attr:`documentElement` property.  It gives you the main element in the XML
document: the one that holds all others.  Here is an example program::

   dom3 = parseString("<myxml>Some data</myxml>")
   assert dom3.documentElement.tagName == "myxml"

When you are finished with a DOM, you should clean it up.  This is necessary
because some versions of Python do not support garbage collection of objects
that refer to each other in a cycle.  Until this restriction is removed from all
versions of Python, it is safest to write your code as if cycles would not be
cleaned up.

The way to clean up a DOM is to call its :meth:`unlink` method::

   dom1.unlink()
   dom2.unlink()
   dom3.unlink()

:meth:`unlink` is a :mod:`xml.dom.minidom`\ -specific extension to the DOM API.
After calling :meth:`unlink` on a node, the node and its descendants are
essentially useless.


.. seealso::

   `Document Object Model (DOM) Level 1 Specification <http://www.w3.org/TR/REC-DOM-Level-1/>`_
      The W3C recommendation for the DOM supported by :mod:`xml.dom.minidom`.


.. _minidom-objects:

DOM Objects
-----------

The definition of the DOM API for Python is given as part of the :mod:`xml.dom`
module documentation.  This section lists the differences between the API and
:mod:`xml.dom.minidom`.


.. method:: Node.unlink()

   Break internal references within the DOM so that it will be garbage collected on
   versions of Python without cyclic GC.  Even when cyclic GC is available, using
   this can make large amounts of memory available sooner, so calling this on DOM
   objects as soon as they are no longer needed is good practice.  This only needs
   to be called on the :class:`Document` object, but may be called on child nodes
   to discard children of that node.


.. method:: Node.writexml(writer[,indent=""[,addindent=""[,newl=""]]])

   Write XML to the writer object.  The writer should have a :meth:`write` method
   which matches that of the file object interface.  The *indent* parameter is the
   indentation of the current node.  The *addindent* parameter is the incremental
   indentation to use for subnodes of the current one.  The *newl* parameter
   specifies the string to use to terminate newlines.

   For the :class:`Document` node, an additional keyword argument *encoding* can be
   used to specify the encoding field of the XML header.


.. method:: Node.toxml([encoding])

   Return the XML that the DOM represents as a string.

   With no argument, the XML header does not specify an encoding, and the result is
   Unicode string if the default encoding cannot represent all characters in the
   document. Encoding this string in an encoding other than UTF-8 is likely
   incorrect, since UTF-8 is the default encoding of XML.

   With an explicit *encoding* [1]_ argument, the result is a byte string in the
   specified encoding. It is recommended that this argument is always specified. To
   avoid :exc:`UnicodeError` exceptions in case of unrepresentable text data, the
   encoding argument should be specified as "utf-8".


.. method:: Node.toprettyxml([indent[, newl[, encoding]]])

   Return a pretty-printed version of the document. *indent* specifies the
   indentation string and defaults to a tabulator; *newl* specifies the string
   emitted at the end of each line and defaults to ``\n``.

   There's also an *encoding* argument; see :meth:`toxml`.


The following standard DOM methods have special considerations with
:mod:`xml.dom.minidom`:

.. method:: Node.cloneNode(deep)

   Although this method was present in the version of :mod:`xml.dom.minidom`
   packaged with Python 2.0, it was seriously broken.  This has been corrected for
   subsequent releases.


.. _dom-example:

DOM Example
-----------

This example program is a fairly realistic example of a simple program. In this
particular case, we do not take much advantage of the flexibility of the DOM.

.. literalinclude:: ../includes/minidom-example.py


.. _minidom-and-dom:

minidom and the DOM standard
----------------------------

The :mod:`xml.dom.minidom` module is essentially a DOM 1.0-compatible DOM with
some DOM 2 features (primarily namespace features).

Usage of the DOM interface in Python is straight-forward.  The following mapping
rules apply:

* Interfaces are accessed through instance objects. Applications should not
  instantiate the classes themselves; they should use the creator functions
  available on the :class:`Document` object. Derived interfaces support all
  operations (and attributes) from the base interfaces, plus any new operations.

* Operations are used as methods. Since the DOM uses only :keyword:`in`
  parameters, the arguments are passed in normal order (from left to right).
  There are no optional arguments. ``void`` operations return ``None``.

* IDL attributes map to instance attributes. For compatibility with the OMG IDL
  language mapping for Python, an attribute ``foo`` can also be accessed through
  accessor methods :meth:`_get_foo` and :meth:`_set_foo`.  ``readonly``
  attributes must not be changed; this is not enforced at runtime.

* The types ``short int``, ``unsigned int``, ``unsigned long long``, and
  ``boolean`` all map to Python integer objects.

* The type ``DOMString`` maps to Python strings. :mod:`xml.dom.minidom` supports
  either bytes or strings, but will normally produce strings.
  Values of type ``DOMString`` may also be ``None`` where allowed to have the IDL
  ``null`` value by the DOM specification from the W3C.

* ``const`` declarations map to variables in their respective scope (e.g.
  ``xml.dom.minidom.Node.PROCESSING_INSTRUCTION_NODE``); they must not be changed.

* ``DOMException`` is currently not supported in :mod:`xml.dom.minidom`.
  Instead, :mod:`xml.dom.minidom` uses standard Python exceptions such as
  :exc:`TypeError` and :exc:`AttributeError`.

* :class:`NodeList` objects are implemented using Python's built-in list type.
  Starting with Python 2.2, these objects provide the interface defined in the DOM
  specification, but with earlier versions of Python they do not support the
  official API.  They are, however, much more "Pythonic" than the interface
  defined in the W3C recommendations.

The following interfaces have no implementation in :mod:`xml.dom.minidom`:

* :class:`DOMTimeStamp`

* :class:`DocumentType` (added in Python 2.1)

* :class:`DOMImplementation` (added in Python 2.1)

* :class:`CharacterData`

* :class:`CDATASection`

* :class:`Notation`

* :class:`Entity`

* :class:`EntityReference`

* :class:`DocumentFragment`

Most of these reflect information in the XML document that is not of general
utility to most DOM users.

.. rubric:: Footnotes

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