Monday, May 19, 2008

Jython Notes

I've been getting back into the Jython codebase this last week. The last time I submitted a Jython patch was in the beginning of 2004, so it's been a while. Things have changed... Jython is finally requiring Java 5 for the next release, which means the usual improvements, but especially good use of annotations. Here's some notes from my puttering around (mostly dragging Jython's set module up to compatibility with CPython 2.5's):
  • Expect Eclipse to be slightly confused. (Lots of "errors.") This is normal. Use ant to build.
  • ant regrtest is handy. run it before you start making changes so you know what's already broken in trunk. (At least between releases, jython does not appear to be religious about "no tests shall fail." But as a new developer you should make "no additional tests should fail" your motto.)
  • Subjective impression: Jython re performace is a bit slow. Jython uses its own re implementation predating the Java regular expressions in jdk 1.4. But, the JRuby guys reported that the jdk implementation doesn't perform very well, so Jython hasn't been in a hurry to switch. The JRuby solution was to port the oniguruma re engine from C to Java. But, Ruby's strings are byte-based and mutable where Jython's are not, so using the JRuby engine isn't just a matter of dropping it in. Also, these string differences may be a source of the poor performance the ruby people saw, so independant testing is in order here.
  • All of the Derived classes (PySetDerived, PyLongDerived, etc.) just exist to let python code subclass builtin types. Those derived classes are generated by a .py script in src/templates
  • If you add a Java class that needs to be exposed to python using the @Expose annotations, you need to add the class name to CoreExposed.includes, or Jython will default to picking attributes via reflection and it usually guesses wrong.
  • Given a PyObject, you can (usually) easily instantiate another PyObject of the same class with pyobject.getType().__call__(). The only times this won't work is when your type's __new__ does something tricky, like how PyFrozenSet or PyTuple return a singleton for an empty frozenset or tuple.
Thanks to all the people in #jython who helped me out, especially Philip Jenvey!

1 comment:

Viagra said...

That issue that Jython will need Java for future releases will create a lot of problems for people with low speccied computers.