Re: Class variable inheritance

[HPJ]

> Makes sense to me. To step through what's happening:
>
> >>> A.n, B.n
> (0, 0)
>
> Here, the lookup on B.n fails (that is, B itself has no variable n),
> and thus falls back to A.n

See, this is what tripped me up, right at the beginning. I thought B
would inherit (as in copy) the variable n from A.

Can you point out to me which part (or parts) of the Language
Reference says this is the way it's supposed to be?
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Re: Class variable inheritance

[HPJ]

> I could, but I will let you read and find what it says about class
> attributes.

You think I would have asked specifically about the Language Reference
if I hadn't read it and failed to find what I was looking for?

The closest thing I was able to find was section 3.2. "The standard
type hierarchy", subsection "Custom classes", where it says:

"When the attribute name is not found [in its namespace dictionary],
the attribute search continues in the base classes. This search of the
base classes uses the C3 method resolution order [...]"

That tells me how the lookup works, which I already knew. But it
doesn't tell me what happens when a class is inherited.

Now, one could argue if the attributes were copied, there would be no
need for a lookup in the base classes, so derivatively it says that's
not the case. To this I would say yes, there still would because after
they've been copied through inheritance, attributes can still be
removed via "del". Anyway, this passage, which is the only one I could
find, doesn't provide a direct and conclusive answer to my question.
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Re: Class variable inheritance

[HPJ]

> would you expect the B class to have a copy of the foo method?

Sorta. I would expect B to have a copy of the "foo" attribute, which
then refers to the same method as A.foo. So the method itself will be
be copied, but its address stored separately in A.foo and B.foo.
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Re: Class variable inheritance

[HPJ]

> Conceptually, Python checks for the presence of B.foo, and if it's
> not there it checks for foo's presence in the base classes.

Yes, I have no problem believing you guys that this is what Python
does. Still, my question remains about where in the Language Reference
this is specified. And if the answer is nowhere, than the LR needs to
be amended, for obviously the way inheritance is done is no small
matter and its understanding should not be left to the user's own
intuition.
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Re: Class variable inheritance

[HPJ]

> http://docs.python.org/reference/datamodel.html#new-style-and-classic...
> - search for 'method resolution order' for other hits in that document.

First of all, that's the LR for Python 2, I'm with Python 3. Second of
all, there's one single passage containing the phrase "method
resolution order" in the Python 3 LR, and it's one that I've already
quoted -- and discarded -- previously in this thread.
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Re: Class variable inheritance

[HPJ]

And by the way, the reason I've come across this problem at all is
because I have something like this:

class A:
  class X:
n = 'a'
  x = X()

class B(A):
  class X(A.X):
n = 'b'
# x = X()

The line commented out was originally not there, but I found out I had
to add it if I wanted B().x.n to be 'b' instead of 'a'.

I might be wrong, but aren't there some (non-obscure) OOP language in
which the equivalent code (without the out-commented line) would have
made B().x an object of type B.X instead of A.X?
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Re: Class variable inheritance

[HPJ]

> Maybe.  For some languages this might be an obvious behavior, but
> Python would have different circumstances so it isn't so obvious (or
> possible) there.

Which means this topic definitely needs to be handled in detail by the
LR, and preferably also in the Tutorial. Otherwise there is no way
anyone coming to Python could know which of the many ways of
inheritance Python follows.
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