==============
lxml.objectify
==============

lxml supports an alternative API similar to the Amara_ bindery or
gnosis.xml.objectify_ through a custom Element implementation.  The main idea
is to hide the usage of XML behind normal Python objects, sometimes referred
to as data-binding.  It allows you to use XML as if you were dealing with a
normal Python object hierarchy.

Accessing the children of an XML element deploys object attribute access.  If
there are multiple children with the same name, slicing and indexing can be
used.  Python data types are extracted from XML content automatically and made
available to the normal Python operators.

This API is very different from the ElementTree API.  If it is used, it should
not be mixed with other element implementations, to avoid non-obvious
behaviour.

.. _Amara: http://uche.ogbuji.net/tech/4suite/amara/
.. _gnosis.xml.objectify: http://gnosis.cx/download/

.. contents::
..
   1   Setting up lxml.objectify
   2   Creating objectify trees
   3   Element access through object attributes
   4   Namespace handling
   5   ObjectPath
   6   Python data types
   7   Defining additional data classes
   8   Recursive string representation of elements
   9   What is different from ElementTree?
   10  Resetting the API


Setting up lxml.objectify
-------------------------

To make use of ``objectify``, you need both the ``lxml.etree`` module and
``lxml.objectify``::

    >>> from lxml import etree
    >>> from lxml import objectify

The next step is to create a parser that builds objectify documents.  The
objectify API is meant for data-centric XML (as opposed to document XML with
mixed content).  Therefore, we configure the parser to let it remove
whitespace-only text from the parsed document if it is not enclosed by an XML
element.  Note that this alters the document infoset, so if you consider the
removed spaces as data in your specific use case, you should go with a normal
parser and just set the element class lookup.  Most applications, however,
will work fine with the following setup::

    >>> parser = etree.XMLParser(remove_blank_text=True)

    >>> lookup = objectify.ObjectifyElementClassLookup()
    >>> parser.setElementClassLookup(lookup)

If you want additional support for `namespace specific classes`_, you can
register the objectify lookup as a fallback of the namespace lookup.  Note,
however, that you have to take care in this case, that the namespace classes
inherit from ``objectify.ObjectifiedElement``, not only from the normal
``lxml.etree.ElementBase``, so that they support the ``objectify`` API.  The
above setup code then becomes::

    >>> lookup = etree.ElementNamespaceClassLookup(
    ...                   objectify.ObjectifyElementClassLookup() )
    >>> parser.setElementClassLookup(lookup)

.. _`namespace specific classes`: element_classes.html#namespace-class-lookup


Creating objectify trees
------------------------

To create an ``objectify`` tree, you can either parse a document with the
parser you created::

    >>> from StringIO import StringIO
    >>> xml = StringIO('<test/>')
    >>> tree = etree.parse(xml, parser)
    >>> print isinstance(tree.getroot(), objectify.ObjectifiedElement)
    True

or you can call the ``makeelement()`` method of the parser to create a new
root element from scratch::

    >>> obj_el = parser.makeelement("test")
    >>> print isinstance(obj_el, objectify.ObjectifiedElement)
    True

New subelements will automatically inherit the setup.  However, all
independent elements that you create through the normal etree API will not be
associated with the parser and therefore not support the ``objectify`` API::

    >>> subel = etree.SubElement(obj_el, "sub")
    >>> print isinstance(subel, objectify.ObjectifiedElement)
    True

    >>> independent_el = etree.Element("new")
    >>> print isinstance(independent_el, objectify.ObjectifiedElement)
    False

The ``makeelement()`` method of the parser has the same signature as the
normal ``Element()`` factory known from lxml.etree and can therefore easily
replace the respective calls.

For convenience, ``objectify`` also replicates the standard factory
``Element()`` and the ``fromstring()`` function from ``lxml.etree`` using a
parser that is local to the ``objectify`` module.  So, after setting up the
parser based element lookup above, you can keep using the same API as in
``lxml.etree``, except that you have to import these functions from a
different module::

    >>> obj_el = objectify.Element("new")
    >>> print isinstance(obj_el, objectify.ObjectifiedElement)
    True

    >>> obj_el = objectify.fromstring("<test/>")
    >>> print isinstance(obj_el, objectify.ObjectifiedElement)
    True

You can change this parser with ``objectify.setDefaultParser(parser)``, which
also allows to add the above support for namespace specific element classes.


Element access through object attributes
----------------------------------------

The main idea behind the ``objectify`` API is to hide XML element access
behind the usual object attribute access pattern.  Asking an element for an
attribute will return the sequence of children with corresponding tag names::

    >>> root = objectify.Element("root")
    >>> b = etree.SubElement(root, "b")
    >>> print root.b[0].tag
    b
    >>> root.index(root.b[0])
    0
    >>> b = etree.SubElement(root, "b")
    >>> print root.b[0].tag
    b
    >>> print root.b[1].tag
    b
    >>> root.index(root.b[1])
    1

For convenience, you can omit the index '0' to access the first child::

    >>> print root.b.tag
    b
    >>> root.index(root.b)
    0
    >>> del root.b

Iteration and slicing also obey the requested tag::

    >>> x1 = etree.SubElement(root, "x")
    >>> x2 = etree.SubElement(root, "x")
    >>> x3 = etree.SubElement(root, "x")

    >>> [ el.tag for el in root.x ]
    ['x', 'x', 'x']

    >>> [ el.tag for el in root.x[1:3] ]
    ['x', 'x']

    >>> [ el.tag for el in root.x[-1:] ]
    ['x']

    >>> del root.x[1:2]
    >>> [ el.tag for el in root.x ]
    ['x', 'x']

If you want to iterate over all children or need to provide a specific
namespace for the tag, use the ``iterchildren()`` method.  Like the other
methods for iteration, it supports an optional tag keyword argument::

    >>> [ el.tag for el in root.iterchildren() ]
    ['b', 'x', 'x']

    >>> [ el.tag for el in root.iterchildren(tag='b') ]
    ['b']

    >>> [ el.tag for el in root.b ]
    ['b']

XML attributes are accessed as in the normal ElementTree API::

    >>> c = etree.SubElement(root, "c", myattr="someval")
    >>> print root.c.get("myattr")
    someval

    >>> root.c.set("c", "oh-oh")
    >>> print root.c.get("c")
    oh-oh

In addition to the normal ElementTree API for appending elements to trees,
subtrees can also be added by assigning them to object attributes.  In this
case, the subtree is automatically deep copied and the tag name of its root is
updated to match the attribute name::

    >>> el = objectify.Element("yet_another_child")
    >>> root.new_child = el
    >>> print root.new_child.tag
    new_child
    >>> print el.tag
    yet_another_child

    >>> root.y = [ objectify.Element("y"), objectify.Element("y") ]
    >>> [ el.tag for el in root.y ]
    ['y', 'y']

The latter is a short form for operations on the full slice::

    >>> root.y[:] = [ objectify.Element("y") ]
    >>> [ el.tag for el in root.y ]
    ['y']

You can also replace children that way::

    >>> child1 = etree.SubElement(root, "child")
    >>> child2 = etree.SubElement(root, "child")
    >>> child3 = etree.SubElement(root, "child")

    >>> el = objectify.Element("new_child")
    >>> subel = etree.SubElement(el, "sub")

    >>> root.child = el
    >>> print root.child.sub.tag
    sub

    >>> root.child[2] = el
    >>> print root.child[2].sub.tag
    sub

Note that special care must be taken when changing the tag name of an element::

    >>> print root.b.tag
    b
    >>> root.b.tag = "notB"
    >>> root.b
    Traceback (most recent call last):
      ...
    AttributeError: no such child: b
    >>> print root.notB.tag
    notB


Namespace handling
------------------

Namespaces are handled mostly behind the scenes.  If you access a child of an
Element without specifying a namespace, the lookup will use the namespace of
the parent::

    >>> root = objectify.Element("{ns}root")
    >>> b = etree.SubElement(root, "{ns}b")
    >>> c = etree.SubElement(root, "{other}c")

    >>> print root.b.tag
    {ns}b
    >>> print root.c
    Traceback (most recent call last):
        ...
    AttributeError: no such child: {ns}c

You can access elements with different namespaces via ``getattr()``::

    >>> print getattr(root, "{other}c").tag
    {other}c

For convenience, there is also a quick way through item access::

    >>> print root["{other}c"].tag
    {other}c

The same approach must be used to access children with tag names that are not
valid Python identifiers::

    >>> el = etree.SubElement(root, "{ns}tag-name")
    >>> print root["tag-name"].tag
    {ns}tag-name

    >>> new_el = objectify.Element("{ns}new-element")
    >>> el = etree.SubElement(new_el, "{ns}child")
    >>> el = etree.SubElement(new_el, "{ns}child")
    >>> el = etree.SubElement(new_el, "{ns}child")

    >>> root["tag-name"] = [ new_el, new_el ]
    >>> print len(root["tag-name"])
    2
    >>> print root["tag-name"].tag
    {ns}tag-name

    >>> print len(root["tag-name"].child)
    3
    >>> print root["tag-name"].child.tag
    {ns}child
    >>> print root["tag-name"][1].child.tag
    {ns}child

or for names that have a special meaning in lxml.objectify::

    >>> root = objectify.XML("<root><text>TEXT</text></root>")

    >>> print root.text.text
    Traceback (most recent call last):
      ...
    AttributeError: 'NoneType' object has no attribute 'text'

    >>> print root["text"].text
    TEXT


ObjectPath
----------

For both convenience and speed, objectify supports its own path language,
represented by the ``ObjectPath`` class::

    >>> root = objectify.Element("{ns}root")
    >>> b1 = etree.SubElement(root, "{ns}b")
    >>> c  = etree.SubElement(b1,   "{ns}c")
    >>> b2 = etree.SubElement(root, "{ns}b")
    >>> d  = etree.SubElement(root, "{other}d")

    >>> path = objectify.ObjectPath("root.b.c")
    >>> print path
    root.b.c
    >>> path.hasattr(root)
    True
    >>> print path.find(root).tag
    {ns}c

    >>> find = objectify.ObjectPath("root.b.c")
    >>> print find(root).tag
    {ns}c

    >>> find = objectify.ObjectPath("root.{other}d")
    >>> print find(root).tag
    {other}d

    >>> find = objectify.ObjectPath("root.{not}there")
    >>> print find(root).tag
    Traceback (most recent call last):
      ...
    AttributeError: no such child: {not}there

    >>> find = objectify.ObjectPath("{not}there")
    >>> print find(root).tag
    Traceback (most recent call last):
      ...
    ValueError: root element does not match: need {not}there, got {ns}root

    >>> find = objectify.ObjectPath("root.b[1]")
    >>> print find(root).tag
    {ns}b

    >>> find = objectify.ObjectPath("root.{ns}b[1]")
    >>> print find(root).tag
    {ns}b

Apart from strings, ObjectPath also accepts lists of path segments:

    >>> find = objectify.ObjectPath(['root', 'b', 'c'])
    >>> print find(root).tag
    {ns}c

    >>> find = objectify.ObjectPath(['root', '{ns}b[1]'])
    >>> print find(root).tag
    {ns}b

You can also use relative paths starting with a '.' that ignore the actual
root element and only inherit its namespace::

    >>> find = objectify.ObjectPath(".b[1]")
    >>> print find(root).tag
    {ns}b

    >>> find = objectify.ObjectPath(['', 'b[1]'])
    >>> print find(root).tag
    {ns}b

    >>> find = objectify.ObjectPath(".unknown[1]")
    >>> print find(root).tag
    Traceback (most recent call last):
      ...
    AttributeError: no such child: {ns}unknown

    >>> find = objectify.ObjectPath(".{other}unknown[1]")
    >>> print find(root).tag
    Traceback (most recent call last):
      ...
    AttributeError: no such child: {other}unknown

ObjectPath objects can be used to manipulate trees::

    >>> root = objectify.Element("{ns}root")

    >>> path = objectify.ObjectPath(".some.child.{other}unknown")
    >>> path.hasattr(root)
    False
    >>> path.find(root)
    Traceback (most recent call last):
      ...
    AttributeError: no such child: {ns}some

    >>> path.setattr(root, "my value") # creates children as necessary
    >>> path.hasattr(root)
    True
    >>> print path.find(root).text
    my value
    >>> print root.some.child["{other}unknown"].text
    my value

    >>> print len( path.find(root) )
    1
    >>> path.addattr(root, "my new value")
    >>> print len( path.find(root) )
    2
    >>> [ el.text for el in path.find(root) ]
    ['my value', 'my new value']

As with attribute assignment, ``setattr()`` accepts lists:

    >>> path.setattr(root, ["v1", "v2", "v3"])
    >>> [ el.text for el in path.find(root) ]
    ['v1', 'v2', 'v3']


Note, however, that indexing is only supported in this context if the children
exist.  Indexing of non existing children will not extend or create a list of
such children but raise an exception::

    >>> path = objectify.ObjectPath(".{non}existing[1]")
    >>> path.setattr(root, "my value")
    Traceback (most recent call last):
      ...
    TypeError: creating indexed path attributes is not supported

It is worth noting that ObjectPath does not depend on the ``objectify`` module
or the ObjectifiedElement implementation.  It can also be used in combination
with Elements from the normal lxml.etree API.


Python data types
-----------------

The objectify module knows about Python data types and tries its best to let
element content behave like them.  For example, they support the normal math
operators::

    >>> root = objectify.fromstring(
    ...             "<root><a>5</a><b>11</b><c>true</c><d>hoi</d></root>")
    >>> root.a + root.b
    16
    >>> root.a += root.b
    >>> print root.a
    16

    >>> root.a = 2
    >>> print root.a + 2
    4
    >>> print 1 + root.a
    3

    >>> print root.c
    True
    >>> root.c = False
    >>> if not root.c:
    ...     print "false!"
    false!

    >>> print root.d + " test !"
    hoi test !
    >>> root.d = "%s - %s"
    >>> print root.d % (1234, 12345)
    1234 - 12345


To see the data types that are currently used, you can call the module level
``dump()`` function that returns a recursive string representation for
elements::

    >>> root = objectify.fromstring("""
    ... <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    ...   <a attr1="foo" attr2="bar">1</a>
    ...   <a>1.2</a>
    ...   <b>1</b>
    ...   <b>true</b>
    ...   <c>what?</c>
    ...   <d xsi:nil="true"/>
    ... </root>
    ... """)

    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 1 [IntElement]
          * attr1 = 'foo'
          * attr2 = 'bar'
        a = 1.2 [FloatElement]
        b = 1 [IntElement]
        b = True [BoolElement]
        c = 'what?' [StringElement]
        d = None [NoneElement]
          * xsi:nil = 'true'

You can freely switch between different types for the same child::

    >>> root = objectify.fromstring("<root><a>5</a></root>")
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 5 [IntElement]

    >>> root.a = 'nice string!'
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 'nice string!' [StringElement]

    >>> root.a = True
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = True [BoolElement]

    >>> root.a = [1, 2, 3]
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 1 [IntElement]
        a = 2 [IntElement]
        a = 3 [IntElement]

    >>> root.a = (1, 2, 3)
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 1 [IntElement]
        a = 2 [IntElement]
        a = 3 [IntElement]

However, data elements continue to provide the objectify API.  This means that
sequence operations such as ``len()``, slicing and indexing (e.g. of strings)
cannot behave as the Python types.  Like all other tree elements, they show
the normal slicing behaviour of objectify elements::

    >>> root = objectify.fromstring("<root><a>test</a><b>toast</b></root>")
    >>> print root.a + ' me' # behaves like a string, right?
    test me
    >>> len(root.a) # but there's only one 'a' element!
    1
    >>> [ a.tag for a in root.a ]
    ['a']
    >>> print root.a[0].tag
    a

    >>> print root.a
    test
    >>> [ str(a) for a in root.a[:1] ]
    ['test']

If you need to run sequence operations on data types, you must ask the API for
the *real* Python value.  The string value is always available through the
normal ElementTree ``.text`` attribute.  Additionally, all data classes
provide a ``.pyval`` attribute that returns the value as plain Python type::

    >>> root = objectify.fromstring("<root><a>test</a><b>5</b></root>")
    >>> root.a.text
    'test'
    >>> root.a.pyval
    'test'

    >>> root.b.text
    '5'
    >>> root.b.pyval
    5

Note, however, that both attributes are read-only in objectify.  If you want
to change values, just assign them directly to the attribute::

    >>> root.a.text  = "25"
    Traceback (most recent call last):
      ...
    TypeError: attribute 'text' of 'StringElement' objects is not writable

    >>> root.a.pyval = 25
    Traceback (most recent call last):
      ...
    TypeError: attribute 'pyval' of 'StringElement' objects is not writable

    >>> root.a = 25
    >>> print root.a
    25

In other words, objectify data elements behave like immutable Python types.


How data types are matched
--------------------------

Objectify uses two different types of Elements.  Structural Elements (or tree
Elements) represent the object tree structure.  Data Elements represent the
data containers at the leafs.  You can explicitly create tree Elements with
the ``objectify.Element()`` factory and data Elements with the
``objectify.DataElement()`` factory.

When Element objects are created, lxml.objectify must determine which
implementation class to use for them.  This is relatively easy for tree
Elements and less so for data Elements.  The algorithm is as follows:

1. If an element has children, use the default tree class.

2. If an element is defined as xsi:nil, use the NoneElement class.

3. If a "Python type hint" attribute is given, use this to determine the element
   class, see below.
 
4. If an XML Schema xsi:type hint is given, use this to determine the element
   class, see below.

5. Try to determine the element class from the text content type by trial and
   error.

6. If the element is a root node then use the default tree class.

7. Otherwise, use the default class for empty data classes.

You can change the default classes for tree Elements and empty data Elements
at setup time.  The ``ObjectifyElementClassLookup()`` call accepts two keyword
arguments, ``tree_class`` and ``empty_data_class``, that determine the Element
classes used in these cases.  By default, ``tree_class`` is a class called
``ObjectifiedElement`` and ``empty_data_class`` is a ``StringElement``.

The "type hint" mechanism deploys an XML attribute defined as
``lxml.objectify.PYTYPE_ATTRIBUTE``.  It may contain any of the following
string values: int, long, float, str, unicode, none::

    >>> print objectify.PYTYPE_ATTRIBUTE
    {http://codespeak.net/lxml/objectify/pytype}pytype
    >>> ns, name = objectify.PYTYPE_ATTRIBUTE[1:].split('}')

    >>> root = objectify.fromstring("""\
    ... <root xmlns:py='%s'>
    ...   <a py:pytype='str'>5</a>
    ...   <b py:pytype='int'>5</b>
    ...   <c py:pytype='none' />
    ... </root>
    ... """ % ns)

    >>> print root.a + 10
    510
    >>> print root.b + 10
    15
    >>> print root.c
    None

Note that you can change the name and namespace used for this attribute
through the ``setPytypeAttributeTag(tag)`` module function, in case your
application ever needs to.  There is also a utility function ``annotate()``
that recursively generates this attribute for the elements of a tree::

    >>> root = objectify.fromstring("<root><a>test</a><b>5</b></root>")
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 'test' [StringElement]
        b = 5 [IntElement]

    >>> objectify.annotate(root)

    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        a = 'test' [StringElement]
          * py:pytype = 'str'
        b = 5 [IntElement]
          * py:pytype = 'int'

A second way of specifying data type information uses XML Schema types as
element annotations.  Objectify knows those that can be mapped to normal
Python types::

    >>> root = objectify.fromstring('''\
    ...    <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    ...      <d xsi:type="double">5</d>
    ...      <l xsi:type="long"  >5</l>
    ...      <s xsi:type="string">5</s>
    ...    </root>
    ...    ''')
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        d = 5.0 [FloatElement]
          * xsi:type = 'double'
        l = 5L [LongElement]
          * xsi:type = 'long'
        s = '5' [StringElement]
          * xsi:type = 'string'

For convenience, the ``DataElement()`` factory creates an Element with a
Python value in one step.  You can pass the required Python type name or the
XSI type name::

    >>> root = objectify.Element("root")
    >>> root.x = objectify.DataElement(5, _pytype="long")
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        x = 5L [LongElement]
          * py:pytype = 'long'

    >>> root.x = objectify.DataElement(5, _pytype="str", myattr="someval")
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        x = '5' [StringElement]
          * py:pytype = 'str'
          * myattr = 'someval'

    >>> root.x = objectify.DataElement(5, _xsi="integer")
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        x = 5 [IntElement]
          * py:pytype = 'int'
          * xsi:type = 'integer'

There is a side effect of the type lookup.  If you assign a string value using
attribute assignment and that string value turns out to be valid for any of
the type checks, you will end up with the resolved type instead of a
StringElement::

    >>> root = objectify.Element("root")
    >>> root.s = "5"
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        s = 5 [IntElement]

You can use the ``DataElement()`` factory to avoid this behaviour and thus
provide the type of a data element by hand::

    >>> root = objectify.Element("root")
    >>> root.s = objectify.DataElement(5,  _pytype="str")
    >>> print objectify.dump(root)
    root = None [ObjectifiedElement]
        s = '5' [StringElement]
          * py:pytype = 'str'

 
Defining additional data classes
--------------------------------

You can plug additional data classes into objectify that will be used in
exactly the same way as the predefined types.  Data classes can either inherit
from ``ObjectifiedDataElement`` directly or from one of the specialised
classes like ``NumberElement`` or ``BoolElement``.  The numeric types require
an initial call to the NumberElement method ``self._setValueParser(function)``
to set their type conversion function (string -> numeric Python type).  This
call should be placed into the element ``_init()`` method.

The registration of data classes uses the ``PyType`` class::

    >>> class ChristmasDate(objectify.ObjectifiedDataElement):
    ...     def callSanta(self):
    ...         print "Ho ho ho!"

    >>> def checkChristmasDate(date_string):
    ...     if not date_string.startswith('24.12.'):
    ...         raise ValueError # or TypeError

    >>> xmas_type = objectify.PyType('date', checkChristmasDate, ChristmasDate)

The PyType constructor takes a string type name, an (optional) callable type 
check and the custom data class.  If a type check is provided it must accept a 
string as argument and raise ValueError or TypeError if it cannot handle the
string value.

PyTypes are used if an element carries a ``py:pytype`` attribute denoting its
data type or, in absence of such an attribute, if the given type check callable
does not raise a ValueError/TypeError exception when applied to the element
text. 

If you want, you can also register this type under an XML Schema type name::

    >>> xmas_type.xmlSchemaTypes = ("date",)

XML Schema types will be considered if the element has an ``xsi:type``
attribute that specifies its data type.  The line above binds the XSD type
``date`` to the newly defined Python type.  Note that this must be done before
the next step, which is to register the type.  Then you can use it::

    >>> xmas_type.register()

    >>> root = objectify.fromstring(
    ...             "<root><a>24.12.2000</a><b>12.24.2000</b></root>")
    >>> root.a.callSanta()
    Ho ho ho!
    >>> root.b.callSanta()
    Traceback (most recent call last):
      ...
    AttributeError: no such child: callSanta

If you need to specify dependencies between the type check functions, you can
pass a sequence of type names through the ``before`` and ``after`` keyword
arguments of the ``register()`` method.  The PyType will then try to register
itself before or after the respective types, as long as they are currently
registered.  Note that this only impacts the currently registered types at the
time of registration.  Types that are registered later on will not care about
the dependencies of already registered types.

If you provide XML Schema type information, this will override the type check
function defined above::

    >>> root = objectify.fromstring('''\
    ...    <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    ...      <a xsi:type="date">12.24.2000</a>
    ...    </root>
    ...    ''')
    >>> print root.a
    12.24.2000
    >>> root.a.callSanta()
    Ho ho ho!

To unregister a type, call its ``unregister()`` method::

    >>> root.a.callSanta()
    Ho ho ho!
    >>> xmas_type.unregister()
    >>> root.a.callSanta()
    Traceback (most recent call last):
      ...
    AttributeError: no such child: callSanta

Be aware, though, that this does not immediately apply to elements to which
there already is a Python reference.  Their Python class will only be changed
after all references are gone and the Python object is garbage collected.


Recursive string representation of elements
-------------------------------------------

Normally, elements use the standard string representation for str() that is
provided by lxml.etree.  You can enable a pretty-print representation for
objectify elements like this::

    >>> objectify.enableRecursiveStr()

    >>> root = objectify.fromstring("""
    ... <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    ...   <a attr1="foo" attr2="bar">1</a>
    ...   <a>1.2</a>
    ...   <b>1</b>
    ...   <b>true</b>
    ...   <c>what?</c>
    ...   <d xsi:nil="true"/>
    ... </root>
    ... """)

    >>> print str(root)
    root = None [ObjectifiedElement]
        a = 1 [IntElement]
          * attr1 = 'foo'
          * attr2 = 'bar'
        a = 1.2 [FloatElement]
        b = 1 [IntElement]
        b = True [BoolElement]
        c = 'what?' [StringElement]
        d = None [NoneElement]
          * xsi:nil = 'true'

This behaviour can be switched off in the same way::

    >>> objectify.enableRecursiveStr(False)


What is different from ElementTree?
-----------------------------------

Such a different Element API obviously implies some side effects to the normal
behaviour of the rest of the API.

* len(<element>) returns the sibling count, not the number of children of
  <element>. You can retrieve the number of children with the
  ``countchildren()`` method. 

* Iteration over elements does not yield the children, but the siblings.  You
  can access all children with the ``iterchildren()`` method on elements or
  retrieve a list by calling the ``getchildren()`` method.

* The find, findall and findtext methods use a different implementation as
  they rely on the original iteration scheme.  This has the disadvantage that
  they may not be 100% backwards compatible, and the additional advantage that
  they now support any XPath expression.