Implied instance attribute creation when referencing a class attribute

[Chris Mellon]

On 16 Jan 2006 14:11:25 -0800, Russell Warren <russandheather at gmail.com> wrote:
> I just ran across a case which seems like an odd exception to either
> what I understand as the "normal" variable lookup scheme in an
> instance/object heirarchy, or to the rules regarding variable usage
> before creation.  Check this out:
>
> >>> class foo(object):
> ...   I = 1
> ...   def __init__(self):
> ...     print self.__dict__
> ...     self.I += 1
> ...     print self.__dict__
> ...
> >>> a=foo()
> {}
> {'I': 2}
> >>> foo.I
> 1
> >>> a.I
> 2
> >>> del a.I
> >>> a.I
> 1
> >>> del a.I
> Traceback (most recent call last):
>   File "<string>", line 1, in <string>
> AttributeError: I
> >>> non_existent_var += 1
> Traceback (most recent call last):
>   File "<string>", line 1, in <string>
> NameError: name 'non_existent_var' is not defined
>
>
> In this case, 'self.I += 1' clearly has inserted a surprise
> behind-the-scenes step of 'self.I = foo.I', and it is this which I find
> interesting.
>
> As I understand it, asking for self.I at this point should check
> self.__dict__ for an 'I' entry, and if it doesn't find it, head on up
> to foo.__dict__ and look for it.
>
> So... I initially *thought* there were two possibilities for what would
> happen with the 'self.I += 1':
>   1. 'self.I += 1' would get a hold of 'foo.I' and increment it
>   2. I'd get an AttributeError
>
> Both were wrong.  I thought maybe an AttributeError because trying to
> modify 'self.I' at that point in the code is a bit fuzzy... ie: am I
> really trying to deal with foo.I (in which case, I should properly use
> foo.I) or am I trying to reference an instance attribute named I (in
> which case I should really create it explicitly first or get an error
> as with the non_existent_var example above... maybe with 'self.I =
> foo.I').
>
> Python is obviously assuming the latter and is "helping" you by
> automatically doing the 'self.I = foo.I' for you.  Now that I know this
> I (hopefully) won't make this mistake again, but doing this seems
> equivalent to taking my 'non_existent_var += 1' example above and
> having the interpreter interpret as "oh, you must want to deal with an
> integer, so I'll just create one for you with 'non_existent_var = 0'
> first".  Fortunately this is not done, so why do it with the instance
> attribute reference?
>
> Does anyone have any solid reasoning behind the Python behavior?  It
> might help drive it home more so than just taking it as "that's the way
> it is" and remembering it.
>
> It gets even more confusing for me because the behaviour could be
> viewed as being opposite when dealing with mutable class members.  eg:
>
> >>> class foo(object):
> ...   M = [1,2,3]
> ...   def __init__(self):
> ...     self.M.append(len(self.M) + 1)
> ...     print self.M
> ...
> >>> a=foo()
> [1, 2, 3, 4]
> >>> foo.M
> [1, 2, 3, 4]
> >>> del a.M
> Traceback (most recent call last):
>   File "<string>", line 1, in <string>
> AttributeError: 'foo' object attribute 'M' is read-only
>
> By opposite I mean that with immutable objects, a sloppy self.I
> reference doesn't get you to the base class object, whereas with a
> mutable one you do get to the base object (although I do recognize that
> in both cases if you just remember that the interpreter will always
> stuff in a 'self.x = BaseClass.x' it works as expected in both the
> immutable and mutable case).
>
> After all that, I guess it boils down to me thinking that the code
> *should* interpret the attempted instance modification with one of the
> two possibilities I mentioned above (although after typing this I'm now
> leaning more towards an AttributeError rather than allowing 'self.I' to
> be synonymous with 'foo.I' if no local override).

I can see how this can be confusing, but I think the confusion here is
yours, not Pythons ;)

self.I += 10 is an *assignment*. Like any assignment, it causes the
attribute in question to be created. You wouldn't be suprised if you'd
done self.I='foo' and had it "create" the instance attribute, would
you?

If you write out the longhand for += it becomes totally obvious what
is happening and why it makes sense:

temp = self.I (ends up returning value bound to foo.I)
temp = temp+10 #if temp were mutable, this would modify it instead of
creating a new object
self.I = temp (binds name "I" in namespace "self" to the value bound
to temp, which will shadow the class attribute)

Note that nowhere is Python "inserting self.I = foo.I" for you - you
are (implicitly) retrieving the class attribute, and then inserting
that value into the instance attribute.

So your case 1 is actually exactly what is happening! Python is
getting a hold of foo.I and incrementing it (which you can see
happening when you use a mutable object). The bit you're missing is
after that, where you're then binding that value into the instance
attribute.


>
> Russ
>
> PS: Apologies if I mangled the "proper" terminology for talking about
> this... hopefully it makes sense.
>
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