[Python-Dev] setting class attributes from C?

2002年3月05日 08:04:27 -0500

> Has anyone tried setting class attributes on a new-style class in C?
> The file sandbox/datetime/datetime.py has code that does this:
>> class datetime(basetime):
> ...
>> datetime.min = datetime(...)
> datetime.max = datetime(...)
>> I can easily add descriptors in the C version so that min and max are
> defined on instances, but using PyObject_SetAttrString() using the new
> class:
>> tmp = create_datetime(...);
> if (datetime_min == NULL)
> return;
> if (PyObject_SetAttrString((PyObject *) &PyDateTime_Type,
> "min", datetime_min) < 0)
> return;
>> produces this exception:
>> Traceback (most recent call last):
> File "test_cdatetime.py", line 9, in ?
> from _datetime import datetime, MINYEAR, MAXYEAR
> TypeError: can't set attributes of built-in/extension type 'datetime.datetime'
>> (where _datetime is the C extension that implements the C version of
> the type).

Type objects declared statically (as a pre-initialized static or
global C struct) are considered immutable.
If you want to modify the dict during module initialization in C, you
can access it as the tp_dict member of the type object. It is
initialized by PyType_Ready().
The subclass trick that Jeremy mentioned is good if you need to do it
in Python.
I think it works David Abraham because he has a custom metaclass; this
approach doesn't apply to adding types to the core language.
--Guido van Rossum (home page: http://www.python.org/~guido/)