[Python-Dev] Customizing the binding of attributes (original) (raw)

Guido van Rossum guido@python.org
Fri, 24 Aug 2001 06:03:34 -0400

From: "Guido van Rossum" <guido@python.org> > I'm not sure what you are asking about, but let me give an example. > You can write an auxiliary class that "describes" an instance > attribute. Instances of this "descriptor" class are stored in a class > dict. The descriptor class implements a method get which is > called in two situations: > > (a) when a class attribute is retrieved, .get(None, ) > is called; this can return an unbound method > > (b) when an instance attribute is retrieved, .get(, <class)_ _> is called; this can return a bound method > > Does this help?

From: "Thomas Heller" <thomas.heller@ion-tof.com>

Yes, thanks. Exactly what I meant. You only forgot to mention that the descriptor class must derive from object: is this intended?

Yes, almost all new features require you to switch to new-style classes; the classic class implementation is left unchanged (as much as possible) for compatibility. In particular, classic classes can't define get to the effect above.

These descriptors semi-work as class attributes of classic classes, but the corresponding set feature does not, because classic classes always store in dict upon assignment; new-style classes first look for a descriptor implementing set (really the C dispatch function tp_descr_set) and use that to override the default assignment.

I needed this to write

class X: method = myinstancemethod(func) instead of class X: pass X.method = new.instancemethod(func, None, X) where myinstancemethod is the descriptor class, and func is any callable object (maybe a function implemented in C). Two additional comments: - Shouldn't (my)instancemethod be a builtin? Similar to staticmethod and classmethod?

Yes. I just checked this in. The new name is 'getset' unless you have a better idea.

- The following seems to be a bug (remember, I'm on Windows):

C:>c:\python22\python.exe Python 2.2a2 (#22, Aug 22 2001, 01:24:03) [MSC 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information. >>> class myinstancemethod(object): ... def init(self, func): ... self.func = func ... def get(self, inst, klass=None): ... import new ... return new.instancemethod(self.func, inst, klass) ... >>> class X: ... meth = myinstancemethod(lambda *x: x) ... >>> X().meth <bound method X. of <_main_.X instance at 007C7F04>> >>> X().meth() (<_main_.X instance at 007E80E4>,) >>> X.meth Traceback (most recent call last): File "", line 1, in ? SystemError: NULL object passed to PyBuildValue

Yup. Thanks! Try this patch:

*** typeobject.c 2001/08/22 19:24:42 2.43 --- typeobject.c 2001/08/24 10:05:25

*** 2806,2811 **** --- 2806,2815 ---- Py_INCREF(self); return self; }

--Guido van Rossum (home page: http://www.python.org/~guido/)