Description of type/class unification: Unifying types and classes in Python

There are two primary objectives:

  1. expose the necessary methods for making an existing class 'new-style'
  2. generating a wrapper class for subclasses in python to implement

To do this there are two scripts:

They each have a simple input language for determing exactly what to implement. Note that these .expose files are hand generated.

For example, looking at a partial listing of list.expose:

type_name: list
type_class: PyList
# exposed methods
expose_meth: :- append o
expose_meth: :i count o
expose_meth: pop i?
expose_meth: :b __nonzero__

So the type_name is 'list'.

>>> type([])
<type 'list'>

It is intended to expose methods for PyList.

Running produces some Java code.

$ python list.expose > list.txt

Opening the file list.txt in your favorite editor you'll see the Java code. This code should then be pasted into the class PyList at the top of the file. This will result in a slew of compiler problems.

The problem is PyList doesn't have any of the methods. The generated code expected 'list_append' but PyList has only 'append' so the compiler complains. This is intended. Now for the boring part. For each method exposed, we need to create a new method. For example:

public void append(PyObject o) {

final void list_append(PyObject o) {
    list[length-1] = o;

Notice the new method is final and package protected. So follow the pattern for each method that needs to be exposed.

The special method __init__ should delegate to 'list_init' which needs to handle the constructor arguments of a list. If there is no argument, create a new list. If an argument, copy it's contents to a new list.

Make sure the class has a constructor which takes a PyType.

Finally, make sure the type is registered with __builtin__.

Run some quick tests:

>>> list()
>>> list([1,2,3])
[1, 2, 3]
>>> type([])
<type 'list'>
>>> list
<type 'list'>

After that was done, run the regrtest and the bugtests. The bugtests caught a bug in my original effort. I had forgotten to make the constructor with PyType argument so any list(arg) call failed quickly as the PyType instance was the argument to the PyList(PyObject) constuctor and since PyType is not iterable, the call failed. The tests were great in tracking this down.

Next steps:

NewStyleClasses (last edited 2008-11-15 09:15:59 by localhost)