# surprising += for lists

Discussion in 'Python' started by Ulrich Eckhardt, Nov 4, 2012.

1. ### Ulrich EckhardtGuest

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]. My interpretations thereof is that the tuple (x, y) is 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.

Can anyone help me understand this?

Thanks!

Uli

Ulrich Eckhardt, Nov 4, 2012

2. ### Alec TaylorGuest

Quick aside, you can insert tuples without much effort: `points += ((3,5),)`

And also that I can't do the reverse, i.e.:
>>> foo = tuple()
>>> foo += [5,6]

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate tuple (not "list") to tuple

On Sun, Nov 4, 2012 at 10:57 PM, 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]. My interpretations thereof is that the tuple (x, y) is 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.
>
> Can anyone help me understand this?
>
> Thanks!
>
> Uli
>
>
> --
> http://mail.python.org/mailman/listinfo/python-list

Alec Taylor, Nov 4, 2012

3. ### Dave AngelGuest

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.

> 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

Dave Angel, Nov 4, 2012
4. ### Terry ReedyGuest

On 11/4/2012 7:45 AM, Dave Angel wrote:

> What I wonder about is why list's __add__ is so fussy.

Guido's reason is that it is not clear what the types of [1,2] + (3,4),
(1,2) + [3,4], [] + range(4), range(2) + [3,4], etcetera should be. Such
mixtures may be bugs. Seq.__add__ exists to implement '+'.

[].extend() will clearly be a list, and accepts any iterable.

I believe the same logic is used for set operations.

--
Terry Jan Reedy

Terry Reedy, Nov 4, 2012