[Python-Dev] PEP 203 Augmented Assignment

Thomas Wouters thomas@xs4all.net
Sun, 30 Jul 2000 19:09:41 +0200

On Sun, Jul 30, 2000 at 12:29:58PM -0500, Guido van Rossum wrote:

> > So the most preferred way is to pass a single argument which is either:
> > 
> > - A single object (directly what was passed in) for a single index.
> > - A slice-object if any kind of single-slice was passed in, which holds the
> >   zero-to-three objects that the slice was 'created' with.
> > - A tuple of zero or more of either of the above two.

> (Make that a tuple of two or more!)  Indexing like a[] is illegal, and
> indexing like a[i] passes a non-tuple.

No sir:

>>> class X:
...     def __getitem__(self, val):
...             return val
>>> x = X()
>>> x[()]
>>> x[1,]

> > While I'm writing this down, I'm not sure if it's such a good idea. Isn't
> > this placing a tad too much into one function ? It might require some severe
> > logic to support this all, especially if you give 'special' meanings to some
> > indexes. And we're inserting a new catch-all method -- a set of them,
> > actually: get, set and del -- and that while Paul is trying to solve the
> > other catch-all Python has, __getattr__/__setattr__.

> Actually, __getitem__ *already* has to deal with -- the only case it
> doesn't get currently is the step-less single slice, which gets passed
> to __getslice__ -- and dies if it's not there.

It should, but it doesn't, in most cases. God knows the code I wrote that
had getitem and getslice didn't take slice objects into account, mostly
because I didn't know how they worked ! Not to mention slice-tuples, which I
didn't know were possible until you mentioned it ;)

> > And lets not forget the convenience of writing those methods: __getitem__ is
> > intuitive, both in name and semantics. So is __getslice__. The passing of a
> > slice object to __getitem__ is a tad less intuitive, and a tuple of
> > index/slice objects even less.

> Don't worry, that's already the case.

I know, I was arguing for an improvement, here :)

> > I'm tempted to suggest a single change: when __getslice__ is not defined,
> > pass a slice object (with no step, as if the slice had a trailing ':') to
> > __getitem__, and document it properly.
> Great!  That's *exactly* what I've been proposing.

[snip more approvals]

> > I suspect this is all for 2.1 or later, though.

> Actually, the actual change (fall back on __*item__ with a slice
> object when __*slice__ doesn't exist) should be simple to add for
> 2.0.  Just submit to the SF PM!

That's what I thought. I'll see about that inbetween augmented assignment
(which is not easy to do right, in retrospect. I almost uploaded a patch
that evaluated 'x' in 'x += 1' twice ;) unless someone beats me to it, of

Thomas Wouters <thomas@xs4all.net>

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