Advanced topics =============== Converter contexts ------------------ Converter contexts can be used to pass around information in recursive calls to the :meth:`convert` and :meth:`mapped` methods. A :class:`~ll.xist.xsc.Converter` object will be passed in all calls, so this object is the place to store information. However if each element, procinst and entity class decided on its own which attributes names to use, name collisions would be inevitable. To avoid this, the following system is used. When a class wants to store information in a converter, it has to define a :class:`Context` class (normally derived from the :class:`Context` class of its base class). The constructor must initialize the context object to a initial state. You can get the context object for a certain class by treating the converter as a dictionary with the class (or an instance) as the key like this: .. sourcecode:: python :caption: Defining and using a converter context from ll.xist import xsc class counter(xsc.Element): class Context(xsc.Element.Context): def __init__(self): xsc.Element.Context.__init__(self) self.count = 0 def convert(self, converter): context = converter[self] node = xsc.Text(context.count) context.count += 1 return node Chaining pools and extending namespaces --------------------------------------- When using :class:`ll.xist.xsc.Pool` objects it's possible to do some sort of "namespace subclassing". Registering a module in a pool not only registers the element, procinst and entity classes in the pool for parsing, but each attribute of the module (as long as it's weak referencable) is available as an attribute of the pool itself: .. sourcecode:: python :caption: Pool attributes from ll.xist import xsc from ll.xist.ns import html pool = xsc.Pool(html) print(pool.img) This outputs ````. It's possible to chain pools together. When an attribute isn't found in the first pool, it will be looked up in a second pool (the so called base pool): .. sourcecode:: python :caption: Pool chaining from ll.xist import xsc from ll.xist.ns import html, svg hpool = xsc.Pool(html) spool = xsc.Pool(svg, hpool) print(spool.img) Here the :obj:`hpool` (containing the :mod:`~ll.xist.ns.html` namespace) will be used when the attribute can't be found in :obj:`spool`. So this will again give the output ````. It's possible to get automatic pool chaining. If a module has an attribute :attr:`__bases__` (which must be a sequence of modules), they will be wrapped in a pool automatically and used as the base pools for the pool created for the first module. This makes it possible to "overwrite" element classes in existing namespaces. For example to replace the :class:`~ll.xist.ns.html.a` class in :mod:`ll.xist.ns.html`, put the following into a module :mod:`html2`: .. sourcecode:: python :caption: Automatic pool chaining (:file:`html2.py`) from ll.xist.ns import html __bases__ = [html] class a(html.a): xmlns = html.xmlns def convert(self, converter): node = html.a(self.content, self.attrs, target="_top") return node.convert(converter) Now you can use the module in a pool: .. sourcecode:: python :caption: Using a pool chain from ll.xist import xsc import html2 pool = xsc.Pool(html2) print(pool.a, pool.b) This outputs: .. sourcecode:: pycon Note that such a chained pool can of course be used when parsing XML. The parser will recursively search for the first class that has the appropriate name when instantiating the tree nodes. Conversion targets ------------------ The :obj:`converter` argument passed to the :meth:`convert` method has an attribute :attr:`target` which is a module or pool and specifies the target namespace to which :obj:`self` should be converted. You can check which conversion is wanted by checking e.g. the :attr:`xmlns` attribute. Once this is determined you can use element classes from the target to create the required XML object tree. This makes it possible to customize the conversion by passing a chained pool to the :meth:`convert` method that extends an existing namespace. The following example shows how an element be converted to two different targets: .. sourcecode:: python :caption: Using conversion targets from ll.xist import xsc from ll.xist.ns import html, fo class bold(xsc.Element): def convert(self, converter): if converter.target.xmlns == html.xmlns: node = converter.target.b(self.content) elif converter.target.xmlns == fo.xmlns: node = converter.target.inline(self.content, font_weight="bold") else: raise TypeError(f"unsupported conversion target {converter.target!r}") return node.convert(converter) The default target for conversion is :mod:`ll.xist.ns.html`. Other targets can be specified via the :obj:`target` argument in the :class:`Converter` constructor or the :meth:`conv` method: .. sourcecode:: pycon >>> from ll.xist.ns import html, fo >>> import foo # This is the code from above >>> print(foo.bold("foo").conv().string()) foo >>> print(foo.bold("foo").conv(target=html).string()) foo >>> print(foo.bold("foo").conv(target=fo).string()) foo Validation and content models ----------------------------- When generating HTML you might want to make sure that your generated code doesn't contain any illegal element nesting (i.e. something bad like ``

foo

`` in HTML). The module :mod:`ll.xist.ns.html` does this automatically: .. sourcecode:: pycon >>> from ll.xist.ns import html >>> node = html.p(html.p(u"foo")) >>> print(node.string()) /Users/walter/checkouts/LivingLogic.Python.xist/src/ll/xist/sims.py:222: \ WrongElementWarning: element \ may not contain element warnings.warn(WrongElementWarning(node, child, self.elements))

foo

For your own elements you can specify the content model too. This is done by setting the class attribute :attr:`model` inside the element class. :attr:`model` must be an object that provides a :meth:`checkvalid` method. This method will be called during parsing or publishing with the element as an argument. When invalid content is detected, the Python warning framework should be used to issue a warning. The module :mod:`ll.xist.sims` contains several classes that provide simple validation methods: * :class:`ll.xist.sims.Empty` can be used to ensure that the element doesn't have any content (like ``br`` and ``img`` in HTML). * :class:`ll.xist.sims.Any` does allow any content. * :class:`ll.xist.sims.NoElements` will warn about elements from the same namespace (elements from other namespaces will be OK). * :class:`ll.xist.sims.NoElementsOrText` will warn about elements from the same namespace and non-whitespace text content. * :class:`ll.xist.sims.Elements` will only allow the elements specified in the constructor. * :class:`ll.xist.sims.ElementsOrText` will only allow the elements specified in the constructor and text. None of these classes will check the number of child elements or their order. For more info see the :mod:`ll.xist.sims` module.