Style question - defining immutable class data members

John Posner jjposner at
Mon Mar 16 00:07:51 CET 2009

Matthew Woodcraft said:

> I doubt it's because anyone particularly wanted this
> behaviour; it just
> falls out of the way '+=' is defined.
> At the point where 'inst.x += 1' is compiled,
> Python doesn't know
> whether 'inst.x' is going to turn out to be a class
> attribute or an
> instance attribute or a property. So really the only thing
> it can do is
> generate code to read inst.x in the usual way and then
> assign to inst.x
> in the usual way (where 'the usual way' for CPython
> is LOAD_ATTR and

That sounds reasonable to me.

Matthew Woodcraft also said:

>> Is there any actual advantage to self.attribute picking up
>> Class.attribute instead of raising a NameError?

> You use that any time you call an ordinary method using syntax like
> ''.

Yes, but that's performing a read (and call) operation on an attribute. My question concerned itself with potential confusion when you perform a write operation on an attribute.

Bruno Desthuilliers said:

>> My question is ... WHY does the interpreter  silently create the
>> instance attribute at this point,

> Becaause that's how you create instance attributes in Python. Why do you
> think 'self' - that is, a reference to some object - is mandatory in
> "methods" (really, functions) arguments list ?

> Or do you mean that the existence of a synonym class attribute should be
> checked on each instance variable assignement ? This would probably be a
> big performance hit, ...

Yes, Bruno, I'm persuaded by this argument. Ideally, I'd like the interpreter to prevent the programmer from shooting him/herself in the foot, but it's not worth the performance hit.

> and it would make per-instance method overloading
> impossible (remember that OOP is about objects, not classes).

Yes again. While I was writing the above comment ("write operation on an attribute"), it *did* cross my mind that you might want to write self.attribute, even if it names a method, not a data-item.

Summary: I no longer suspect that "Python is broken". I *do* think that there's a situation that is potentially quite confusing:

 * In the statement "self.x = self.x + 1", the two "self.x" names can sometimes refer to different objects.

 * Even worse, in the equivalent statement "self.x += 1", the single name "self.x" can sometimes refer to two different objects!

I think this situation should be handled in documentation. (I'm a tech writer in my day job ... oh wait, I forgot ... I got laid off from my day job in December.) I'll look into what the standard Python doc set says on this matter.


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