Behavior of += (was Re: [Python-Dev] Customization docs)

[Steve Holden]

"Steve Holden" <sholden at holdenweb.com> wrote in message
news:oWIK8.179611$%u2.31494 at atlpnn01.usenetserver.com...
> "Emile van Sebille" <emile at fenx.com> wrote ...
> > <jepler at unpythonic.net> wrote:
> > > IMO the following is an odder behavior:
> > > >>> t
> > > ([1], [2], [3])
> > > >>> t[1] += [3]
> > > Traceback (most recent call last):
> > >   File "<stdin>", line 1, in ?
> > > TypeError: object doesn't support item assignment
> > > >>> t
> > > ([1], [2, 3], [3])
> > >
> >
> > Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32
> > >>> class myTuple(tuple):
> > ...     def __setitem__(self, attr, value):
> > ...             print 'in setitem:', attr
> > ...
> > >>> a = myTuple(([1],[2],[3]))
> > >>> a
> > ([1], [2], [3])
> > >>> a[1] += [4]
> > in setitem: 1
> > >>> a
> > ([1], [2, 4], [3])
> > >>>
> >
> >
> > Here's another that's differently wrong:
> > >>> a = ('aaa',)
> > >>> a[0]+='bbb'
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in ?
> > TypeError: object doesn't support item assignment
> > >>> a
> > ('aaa',)
> > >>>
>
>
> Why do you consider either of these examples wrong? The whole point of
> tuples is that if d is a dictionary, then d[t1] == d[t2] if, and only if,
t1
> == t2. Both of these examples are trying to make changes that will modify
an
> (immutable) tuple. There's every reason to treat these modifications as
> illegal, since a before the fiddling is not equal to a after the fiddling,
> but tuples aren't mutable so such changes are illegal.
>
> regards
> --
> -----------------------------------------------------------------------
> Steve Holden                                 http://www.holdenweb.com/
> Python Web Programming                http://pydish.holdenweb.com/pwp/
> -----------------------------------------------------------------------
>
The dictionary example is somewhat misleading (because my assertion is
wrong). However, I stand by the statement that you are both trying to modify
tuples, which is fundamentally against the nature of the type.

regards
 Steve