When trying to create a sub-subclass of a ctypes.Structure, the behavior
is not as expected.

An executable example that describes the problem in detail and
demonstrates the issue is available at
http://paste.turbogears.org/paste/29555 and also attached.

The failing behavior is near the beginning, following the description.

Three unsuccessful workarounds follow.

The expected behavior is found at the end, where a final workaround has
been implemented.

Can this behavior be improved in subsequent releases of ctypes?  If not,
can ctypes at least support the workaround natively (perhaps by
providing a method in the Structure class that performs the workaround
and can be called by a sub-subclass during __init__)?

The problem is the implementation of the current __init__ method, in
Modules/_ctypes/_ctypes.c, line 4024.  Rewritten in Python, and slightly
simplified it looks like this:

    def __init__(self, *args, **kw):
        """The current BUGGY __init__ method"""
        names = [f[0] for f in self._fields_]
        for n, v in zip(names, args):
            setattr(self, n, v)
        for n, v in kw.iteritems():
            setattr(self, n, v)

It assigns positional parameters to the names found in the _fields_ list
of the subclass, but it should look up all the _fields_ definitions in
all the superclasses, too.  Working pseudo Python code:

    def __init__(self, *args, **kw):
        """This is how the Structure's __init__method SHOULD be"""
        if args:
            names = []
            for tp in self.__class__.mro()[2::-1]:
                for n, _ in tp._fields_:
                    names.append(n)
            for n, v in zip(names, args):
                setattr(self, n, v)
        for n, v in kw.iteritems():
            setattr(self, n, v)

Now, I don't really want to write the above code in C ;-).

Heh.  Me neither.

Is it possible to use something like Cython to generate the C code?

That was easier than I thought. The generated code is _ugly_ but it did
seem to be able to generate it.

I created init.pyx

{{{
class X(object):
	def __init__(self, *args, **kw):
		"""This is how the Structure's __init__method SHOULD be"""
		if args:
			names = []
			for tp in self.__class__.mro()[2::-1]:
				for n, _ in tp._fields_:
					names.append(n)
			for n, v in zip(names, args):
				setattr(self, n, v)
		for n, v in kw.iteritems():
			setattr(self, n, v)
}}}

Then, ran cython init.pyx and got the attached file.

Is that any help?

> Is that any help?
Not really ;-).

Here is a patch I just wrote - it must still be checked for correctness,
and tests for it must be added.  I hope the comment in the code explains
how it works.  ctypes-structinit.patch

I've commited a slightly changed patch plus a test.

trunk: 74917, py3k: 74918, release26-maint: 74919, release31-maint: 74920.