[lxml-dev] [lxml][objectify] optimization of recursive object dumping

[Stefan Behnel]

Hi Holger,

Holger Joukl worote:
> I'm experimenting with a custom objectified datetime class based on
> Python's
> datetime that employs the dateutil.parser module to detect if some element
> value
> is in a valid datetime format, i.e. the parse function from dateutil.parser
> is used to implement the type_check for the PyType type registry.
> 
> Invoking this parse method is quite expensive, so I want this to happen
> rarely. As I am using "recursive element dumping" as default I found that
> for every __str__ call .pyval of the ObjectifiedDataElements in a tree is
> accessed, which in turn triggers parsing for my custom datetime class.

But that should only happen for normal text content (well, and dates). Numbers
should always be parsed first.


> As I don't really see a way to avoid this I propose the introduction of
> an additional property "_pyval_repr" that can be overridden in subclasses,
> which makes it possible to simply return element.text, if getting .pyval
> is expensive.

Hmmm, I don't really like the idea of adding a new Python method only to
optimise the debug output (which is what dump() is essentially meant for). I
understand that you use this as default, but I don't think many people will
rely on the performance of this function...

Have you considered switching from "dump() by default" to "implement __str__()
for all data types by hand"? There are not that many standard types...

On the other hand, what if we did something like this:

  cdef object _dump(_Element element, int indent):
    indentstr = "    " * indent
    if isinstance(element, ObjectifiedDataElement):
        value = str(element)
    else:
        ...

Would that help?

Stefan