Structure sub-subclass does not initialize with base class positional arguments

[jaraco]

BPO	5042
Nosy	@theller, @jaraco
Files	ctypes subclassing issues.py: problem description and demo workaround script init.c ctypes-structinit.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/theller'
closed_at = <Date 2009-09-18.19:48:44.414>
created_at = <Date 2009-01-24.16:04:18.593>
labels = ['ctypes', 'type-bug']
title = 'Structure sub-subclass does not initialize with base class positional arguments'
updated_at = <Date 2009-09-18.19:48:44.413>
user = 'https://github.com/jaraco'

bugs.python.org fields:

activity = <Date 2009-09-18.19:48:44.413>
actor = 'theller'
assignee = 'theller'
closed = True
closed_date = <Date 2009-09-18.19:48:44.414>
closer = 'theller'
components = ['ctypes']
creation = <Date 2009-01-24.16:04:18.593>
creator = 'jaraco'
dependencies = []
files = ['12845', '14920', '14921']
hgrepos = []
issue_num = 5042
keywords = ['patch']
message_count = 6.0
messages = ['80457', '92792', '92793', '92794', '92816', '92838']
nosy_count = 2.0
nosy_names = ['theller', 'jaraco']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue5042'
versions = ['Python 2.6', 'Python 3.1', 'Python 2.7', 'Python 3.2']

[jaraco]

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__)?

[theller]

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 ;-).

[jaraco]

Heh. Me neither.

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

[jaraco]

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?

[theller]

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

[theller]

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

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