New vs Old Style Python Classes in C Extensions?

[Jeff Rush]

While I have a reasonable understanding of the differences in new-style versus
old-style classes, tonight while working a C extension module I realized I
don't know how to indicate which style my C extension module should appear as.

I'm following the Python docs for extended modules, but it doesn't say in
there anyplace I can find which style I'm creating. My clue that something
was wrong is when this:

from cextension import Context

class MyContext(Context):

def __init__(self):
super(Context, self).__init__()

repeatedly and reliably failed with a corrupted C data structure, while this:

class MyContext(Context):

def __init__(self):
Context.__init__()

worked without any problems. As I understand it, the former uses new-style
semantics while the latter uses old-style, and -thats- when I realized I have
no idea which my C extension implemented.

Any enlightenment?

-Jeff

 

[=?ISO-8859-1?Q?=22Martin_v=2E_L=F6wis=22?=]

from cextension import Context

class MyContext(Context):

def __init__(self):
super(Context, self).__init__()

repeatedly and reliably failed with a corrupted C data structure, while
this:

class MyContext(Context):

def __init__(self):
Context.__init__()

worked without any problems. As I understand it, the former uses
new-style semantics while the latter uses old-style,

This understanding is incorrect: Context is either a new-style
or an old-style class (if type(Context) is type, it's a new-style
class, if type(context) is named 'classobj', it's an old-style
class).

As you have implemented Context in an extension module, it likely
is a new-style class (i.e. a type).

Most likely, something is wrong with your tp_init slot. You
shouldn't be able to call Context.__init__(): that should
raise a type error, indicating that an object is needed
for the method __init__. That should hold whether Context
is a new-style or an old-style class.

HTH,
Martin

 

[Carl Banks]

While I have a reasonable understanding of the differences in new-style versus
old-style classes, tonight while working a C extension module I realized I
don't know how to indicate which style my C extension module should appear as.

I'm following the Python docs for extended modules, but it doesn't say in
there anyplace I can find which style I'm creating. My clue that something
was wrong is when this:

from cextension import Context

class MyContext(Context):

def __init__(self):
super(Context, self).__init__()

repeatedly and reliably failed with a corrupted C data structure, while this:

class MyContext(Context):

def __init__(self):
Context.__init__()

worked without any problems. As I understand it, the former uses new-style
semantics while the latter uses old-style, and -thats- when I realized I have
no idea which my C extension implemented.

Any enlightenment?

Short answer:

It has nothing to do with old-style classes, and is probably just due
to a mistake in your extension.

Longer answer:

C extention types have never implemented old-style classes. Before
Python 2.2, classes and types were different things. All class
instances were of the same type. However, C extension objects were a
different type, and weren't class instances at all.

Nowadays, new-style classes are types, but the converse isn't
necessary true. In particular, the typical way of defining old
extension types didn't work as a new-style type. So, as part of type-
class unification, they added some new fields (tp_members, tp_new, and
so on) and new idioms to replace some of the old idioms, effectively
creating "new-style types".

Unless you've adopted the new ways, your type isn't going to work as a
class. If you have adopted the new ways, then it's probably just a
mistake in your code.

I suspect you have adopted the new ways, because I'd expect Python to
raise TypeError when trying to subclass a function. So you probably
just have a bug.


Carl Banks