Re: Class methods read-only by default?
Thank you both, Steven and Andrew, for the insightful explanation. I shall keep it in mind when thinking about classes methods and instances. Thank you again. Manu -- http://mail.python.org/mailman/listinfo/python-list
Re: Class methods read-only by default?
Emanuele D'Arrigo man...@gmail.com (ED) wrote: ED Hi Everybody! ED I just tried this: class C(object): ED ...def method(self): ED ...pass ED ... c = C() delattr(c, method) ED Traceback (most recent call last): ED File stdin, line 1, in module ED AttributeError: 'C' object attribute 'method' is read-only ED How come? Who told the class to make the method read-only? I didn't! Methods in a class are done with the descriptor protocol. All access to the method through an instance is executed via the descriptor. The delete calls the __delete__ method of the descriptor which isn't implemented for functions. See http://docs.python.org/reference/datamodel.html?highlight=descriptor#implementing-descriptors (Actually, IIRC, the function object is its own descriptor) class C(object): ... def method(self): ... pass ... c=C() C.__dict__['method'] function method at 0xd2330 C.__dict__['method'].__get__ method-wrapper '__get__' of function object at 0xd2330 C.__dict__['method'].__delete__ Traceback (most recent call last): File stdin, line 1, in module AttributeError: 'function' object has no attribute '__delete__' -- Piet van Oostrum p...@cs.uu.nl URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4] Private email: p...@vanoostrum.org -- http://mail.python.org/mailman/listinfo/python-list
Re: Class methods read-only by default?
Emanuele D'Arrigo wrote: Hi Everybody! I just tried this: class C(object): ...def method(self): ...pass ... c = C() delattr(c, method) Traceback (most recent call last): File stdin, line 1, in module AttributeError: 'C' object attribute 'method' is read-only How come? Who told the class to make the method read-only? I didn't! i'm just guessing, but i suspect this is because of how it's implemented. class methods don't exist in the instance. instead, they exist in the class (which is itself an object, hence metaclasses etc), and the instance forwards them to the class for evaluation (if the term vtable makes any sense to you then the vtable for class methods is owned by the class, not the instance). so you may be able to delete the method from the class, but then it will affect all instances. you can't delete it from one instance because it's not in that instance. as i said, just a guess, based on vague ideas about how python works. andrew -- http://mail.python.org/mailman/listinfo/python-list
Re: Class methods read-only by default?
On Thu, 02 Apr 2009 06:07:20 -0700, Emanuele D'Arrigo wrote: Hi Everybody! I just tried this: class C(object): ...def method(self): ...pass ... c = C() delattr(c, method) Traceback (most recent call last): File stdin, line 1, in module AttributeError: 'C' object attribute 'method' is read-only How come? Who told the class to make the method read-only? I didn't! Nobody told the class to make the method read-only, because it isn't. You are trying to delete an *instance* attribute (one that lives inside the instance dictionary) but the method is a *class* attribute (it lives inside the class dictionary). class C(object): ... def method(self): pass ... c = C() C.__dict__.keys() # class attributes ['__dict__', '__module__', '__weakref__', 'method', '__doc__'] c.__dict__.keys() # instance attributes [] To remove the method, this will work: delattr(C, 'method') # call on the class But beware, if method is defined in a superclass of C, then that won't work either. This is a little confusing, it is a definite Gotcha, because getattr succeeds. But notice: getattr(c, 'method') bound method C.method of __main__.C object at 0x82ec14c See how it tells you that c.method is actually C.method? -- Steven -- http://mail.python.org/mailman/listinfo/python-list
