Python dot-equals (syntax proposal)

[Peter Otten]

Alf P. Steinbach wrote:

> <test lang="py3">
> >>> t = ([], [], [])
> >>> t
> ([], [], [])
> >>> t[0] += ["blah"]
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: 'tuple' object does not support item assignment
> >>> t
> (['blah'], [], [])
> >>> _
> </test>
> 
> Yep, it matters.
> 
> Is this change-but-raise-exception a bug?

No.

a[0] += b

translates to

a.__setitem__(0, a.__getitem__(0).__iadd__(b))

assuming a[0] has an __iadd__() method. It should be obvious that only the 
the last operation, the outer a.__setitem__(...), will fail here.
A possible fix might be a changed order of evaluation:

_internal_set = a.__setitem__
_internal_set(0, a.__getitem__(0).__iadd__(b))

I don't know if there are arguments against this other than increased 
compiler complexity.

Or one could argue that

a += b

should have been implemented as

a = a + b

or

a = a.__add__(b)

which is currently used as the fallback when there is no __iadd__() method 
and which gives a more intuitive behaviour at the cost of a greater 
overhead. But it's a little late for that discussion, for that language.

Peter