Element/LiveElement do not allow genshifile templates

[Cory Dodt]

Using docFactory = genshifile(..) in an Element or LiveElement doesn't work. txGenshi currently expects to be able to get a reference to the renderable by adapting the Nevow context object. Element and LiveElement don't use the context--that is pretty much the reason they exist.

Typical traceback that occurs when the rendering machinery finds an Element with a genshifile loader:

Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/Nevow-0.10.0-py2.6.egg/nevow/_flat.py", line 462, in _flattensome
    element = state.next()
  File "/usr/local/lib/python2.6/dist-packages/Nevow-0.10.0-py2.6.egg/nevow/_flat.py", line 424, in flatten
    raise FlattenerError(e, roots, extract_tb(exc_info()[2]))
nevow._flat.FlattenerError: Exception while flattening:
  <chichimec.example.RandomNumber object at 0x1baa850>
  File "/usr/local/lib/python2.6/dist-packages/Nevow-0.10.0-py2.6.egg/nevow/_flat.py", line 415, in flatten
    element = stack[-1].next()
  File "/usr/local/lib/python2.6/dist-packages/Nevow-0.10.0-py2.6.egg/nevow/_flat.py", line 334, in _flatten
    yield _flatten(request, renderable.render(request), [], renderable,
  File "/usr/local/lib/python2.6/dist-packages/Nevow-0.10.0-py2.6.egg/nevow/athena.py", line 1868, in render
    document = tags.invisible[Element.render(self, request)]
  File "/usr/local/lib/python2.6/dist-packages/Nevow-0.10.0-py2.6.egg/nevow/page.py", line 107, in render
    return docFactory.load(None, _getPreprocessors(self))
  File "/usr/local/lib/python2.6/dist-packages/txGenshi-0.0.1.0cdd1-py2.6.egg/txgenshi/loader.py", line 140, in load
    renderer = inevow.IRenderer(ctx)
TypeError: ('Could not adapt', None, <InterfaceClass nevow.inevow.IRenderer>)

[Cory Dodt (corydodt)]

I think I can implement this with PEP 252(?) descriptors:

class NiftyDateWidget(LiveElement):
    docFactory = loader.genshifile('blah.xhtml') # as usual

...

class genshifile(object):
    def __init__(self, filename):
        self.filename = filename
        self._cache = NOCACHE

    def __get__(self, instance, cls):
        if self._cache is NOCACHE:
            gf = _GenshiFile(filename)
            gf.object = instance
            self._cache = gf
        else:
            gf = self._cache
        return gf

Then, the class _GenshiFile is what genshifile used to be; but load is modified
to use self.object instead of the context.

[Cory Dodt (corydodt)]

Merged with approval - using descriptor-based approach.