On 6 July 2012 13:35, Campbell Barton <ideasman42(a)gmail.com> wrote:
> While this isnt impossible - its definetly not intended, when python
> runs PyType_Ready() it converts the PyGetSetDef's into PyObject's
> attribute descriptors which are stored in the PyType's dictionary.
> so strongly advice against trying to manipulate that data at low
> level...
Campbell,
That's what I thought indeed.
> - you could look at assigning properties to a class as can be done in python...
>
> MyClass.foo = property(getx, setx, delx, "This is a property")
>
> ... doing in C will be more verbose of course but it should work fine.
It is a very interesting option, though I'm running out of either C API fu or
imagination how to make such thing at C API level...without fiddling
with PyGetSetDef table.
If Python sources make use of it, I could grasp it easily, but what to
look there at?
Would you mind pointing where to learn some details?
Best regards,
--
Mateusz Loskot, http://mateusz.loskot.net
While this isnt impossible - its definetly not intended, when python
runs PyType_Ready() it converts the PyGetSetDef's into PyObject's
attribute descriptors which are stored in the PyType's dictionary.
so strongly advice against trying to manipulate that data at low
level... some alternatives could be....
- define pygetattro and manage the lookups on each call - typical way
to handle such cases.
- you could look at assigning properties to a class as can be done in python...
MyClass.foo = property(getx, setx, delx, "This is a property")
... doing in C will be more verbose of course but it should work fine.
On Thu, Jul 5, 2012 at 1:42 PM, Mateusz Loskot <mateusz(a)loskot.net> wrote:
> Hi,
>
> I have a type object implemented using plain Python 3 C API.
> The type has number of attributes - tp_getset slot set with typical
> table of geters and seters.
> I'd like to find out best way to make certain attributes read-only at run-time.
>
> The table I assign to tp_getset looks like this:
>
> PyGetSetDef Foo_getseters[] =
> {
> {"text", (getter)Foo_get_text, (setter)Foo_set_text,
> PyDoc_STR("Foo text"), NULL },
> {NULL}
> };
>
> Now, at some point at run-time, I'd like to disallow assignment to
> .text property
> Obvious way to do that is to check desired condition in Foo_set_text()
> and set AttributeError.
>
> I wonder if it would be possible and sensible to modify the tp_getset slot and
> simply set the Foo_set_text entry in Foo_getseters to NULL.
>
> Is it possible?
> Is there any better way to make an attribute read-only at run-time?
>
> Best regards,
> --
> Mateusz Loskot, http://mateusz.loskot.net
> _______________________________________________
> capi-sig mailing list
> capi-sig(a)python.org
> http://mail.python.org/mailman/listinfo/capi-sig
--
- Campbell
Hi,
I'm reading the PEP 253 [1] on subtyping and there are plenty of good
recommendations on how to structure the types, call tp_new and tp_init
slots, etc.
But, it lacks an important note on passing arguments from sub to super type.
It seems the PEP is unfinished as per the note:
(XXX There should be a paragraph or two about argument passing here.)
So, I'm trying to extrapolate some strategies well known from the Python
classes [2] subtyping, especially techniques that each level
strips-off arguments, etc.
I'm looking for techniques to achieve similar effect to this, but
in using plain Python C API (3.x):
class Shape:
def __init__(self, shapename, **kwds):
self.shapename = shapename
super().__init__(**kwds)
class ColoredShape(Shape):
def __init__(self, color, **kwds):
self.color = color
super().__init__(**kwds)
What would be the equivalent in C API?
Also, how to deal with similar situation but with arguments
specific to derived class expected in different order, it is given
at the end of the args tuple (or kwds dict, I assume principle would be same).
Here is some (pseudo-)code that illustrates the situation:
class Base:
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z
class Derived(Base):
def __init__(self, x, y, a):
self.a = a
super().__init__(x, y, None):
Note, if the 'a' was expected first:
Derived.__init__(self, a, x, y)
it would be similar situation to the Shape and ColoredShape above.
It would be easier to deal with, I assume.
[1] http://www.python.org/dev/peps/pep-0253/
[2] http://rhettinger.wordpress.com/2011/05/26/super-considered-super/
Best regards,
--
Mateusz Loskot, http://mateusz.loskot.net
Hi,
I have a type object implemented using plain Python 3 C API.
The type has number of attributes - tp_getset slot set with typical
table of geters and seters.
I'd like to find out best way to make certain attributes read-only at run-time.
The table I assign to tp_getset looks like this:
PyGetSetDef Foo_getseters[] =
{
{"text", (getter)Foo_get_text, (setter)Foo_set_text,
PyDoc_STR("Foo text"), NULL },
{NULL}
};
Now, at some point at run-time, I'd like to disallow assignment to
.text property
Obvious way to do that is to check desired condition in Foo_set_text()
and set AttributeError.
I wonder if it would be possible and sensible to modify the tp_getset slot and
simply set the Foo_set_text entry in Foo_getseters to NULL.
Is it possible?
Is there any better way to make an attribute read-only at run-time?
Best regards,
--
Mateusz Loskot, http://mateusz.loskot.net
