[Python-Dev] INPLACE_ADD and INPLACE_MULTIPLY oddities in ceval.c (original) (raw)

Armin Rigo arigo at tunes.org
Thu Mar 30 09:56:00 CEST 2006

Hi Tim,

Oups, sorry. I only just realized my mistake and the meaning of your message.

On Thu, Mar 30, 2006 at 09:27:02AM +0200, Armin Rigo wrote:

>>> t = (1,2,3) >>> t += [4,5,6] TypeError: can only concatenate tuple (not "list") to tuple

>>> t += array([4,5,6]) TypeError: ... This is current behavior and it wouldn't change.

I'm pasting untested bits of code. Indeed, as you point out:

>>> t = (1,2,3)
>>> t += array([4,5,6])
>>> t
array([5, 7, 9])

and it would remain so after the fix. I still think the fix is a good thing, and the above is an issue at a different level. It's somehow the "fault" of list.iadd and list.imul, which are oddballs -- before the introduction of set objects, it was the single place in the whole library of built-in types where in-place behavior was different from normal behavior.

It would require an official language extension to say that for all sequences, += is supposed to accept any iterable (which may or may not be a good thing, I have no opinion here).

Otherwise, I'd just ignore the whole sub-issue, given that 'tuple += array' returning an array is just correct language-wise and doesn't look like a trap for bad surprises -- if the user expected a tuple but gets an array, most tuple-like operations will work just fine on the array, except hashing, which gives a clean TypeError.

A bientot,

Armin

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