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| Date | Sun, 04 Nov 2012 07:45:45 -0500 |
| From | Dave Angel <d@davea.name> |
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| To | Ulrich Eckhardt <doomster@knuut.de> |
| Subject | Re: surprising += for lists |
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On 11/04/2012 06:57 AM, Ulrich Eckhardt wrote: > Hi everybody! > > I was just smacked by some very surprising Python 2.7 behaviour. I was > assembling some 2D points into a list: > > points = [] > points += (3, 5) > points += (4, 6) > > What I would have expected is to have [(3, 5), (4, 6)], instead I got [3, > 5, 4, 6]. mylist += is equivalent to mylist.extend. And as you say, what you wanted was append. > My interpretations thereof is that the tuple (x, y) is iterable, You're confusing cause and effect. If it weren't iterable, it'd be an error. It would NOT just somehow change to be equivalent to append. >>> points.extend(4) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'int' object is not iterable > so the elements are appended one after the other. Actually, I should have > used points.append(), but that's a different issue. > > Now, what really struck me was the fact that [] + (3, 5) will give me a > type error. Here I wonder why the augmented assignment behaves so much > different. What I wonder about is why list's __add__ is so fussy. > Can anyone help me understand this? > > Thanks! > > Uli > > I'd also point out that when using the extend() function call, we'd have to spell it: points.extend((3,5)) The extra parentheses are to make it clear to the compiler that this is a single argument, a tuple, and not two arguments. And similarly, points.append((3,5)) to get your original desired behavior. -- DaveA
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surprising += for lists Ulrich Eckhardt <doomster@knuut.de> - 2012-11-04 12:57 +0100 Re: surprising += for lists Alec Taylor <alec.taylor6@gmail.com> - 2012-11-04 23:06 +1100 Re: surprising += for lists Dave Angel <d@davea.name> - 2012-11-04 07:45 -0500 Re: surprising += for lists Terry Reedy <tjreedy@udel.edu> - 2012-11-04 12:57 -0500
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