inserting into a list

[John Salerno]

Let me apologize in advance for what I'm sure is an achingly simple
question, but I just can't find the answer in either of my Python books.
I've tried a few tests with the interactive prompt, but they don't work
either.

All I'm trying to do is insert an item into a list, like so:

L = [1, 2, 4]

and I want to insert the integer 3 into the position L[2], so that the
list reads [1, 2, 3, 4]

I've tried all kinds of combinations of slicing assignment, but I always
get:

TypeError: can only assign an iterable

Can someone please embarrass me with the simple answer? :)

[Christoph Haas]

Re: inserting into a list

On Tuesday 07 March 2006 16:18, John Salerno wrote:[color=blue]
> Let me apologize in advance for what I'm sure is an achingly simple
> question, but I just can't find the answer in either of my Python books.
> I've tried a few tests with the interactive prompt, but they don't work
> either.
>
> All I'm trying to do is insert an item into a list, like so:
>
> L = [1, 2, 4]
>
> and I want to insert the integer 3 into the position L[2], so that the
> list reads [1, 2, 3, 4][/color]

Either

L[2:2]=[3]

or

L.insert(2,3)

Kindly
Christoph
--
~
~
".signature " [Modified] 1 line --100%-- 1,48 All

[Diez B. Roggisch]

Re: inserting into a list

John Salerno wrote:
[color=blue]
> Let me apologize in advance for what I'm sure is an achingly simple
> question, but I just can't find the answer in either of my Python books.
> I've tried a few tests with the interactive prompt, but they don't work
> either.
>
> All I'm trying to do is insert an item into a list, like so:
>
> L = [1, 2, 4]
>
> and I want to insert the integer 3 into the position L[2], so that the
> list reads [1, 2, 3, 4]
>
> I've tried all kinds of combinations of slicing assignment, but I always
> get:
>
> TypeError: can only assign an iterable
>
> Can someone please embarrass me with the simple answer? :)[/color]
[color=blue][color=green][color=darkred]
>>> l = [1,2,3]
>>> l.insert(2, 10)
>>> l[/color][/color][/color]
[1, 2, 10, 3][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Embarrasing enough?

Diez

[John Salerno]

Re: inserting into a list

Diez B. Roggisch wrote:
[color=blue][color=green][color=darkred]
>>>> l = [1,2,3]
>>>> l.insert(2, 10)
>>>> l[/color][/color]
> [1, 2, 10, 3]
>
> Embarrasing enough?[/color]

Actually, I was trying to figure it out with the slice technique
instead. But yeah, as Christopher's example showed, it's not hard. But I
didn't realize you had to assign a list item to the slice, so I was doing:

L[2:2] = 3

among other things, but they all involved '= 3', not '= [3]'

[John Salerno]

Re: inserting into a list

Christoph Haas wrote:
[color=blue]
> L[2:2]=[3][/color]

I'm still a little confused about this. If what I'm inserting is just an
integer, why wouldn't

L[2:2] = 3

work? What if you wanted to insert an actual list into that slot? Would
you have to wrap it in double brackets?

[Diez B. Roggisch]

Re: inserting into a list

John Salerno wrote:
[color=blue]
> Christoph Haas wrote:
>[color=green]
>> L[2:2]=[3][/color]
>
> I'm still a little confused about this. If what I'm inserting is just an
> integer, why wouldn't
> L[2:2] = 3
>
> work?[/color]

Because a slice represents a list - even if it is a one-elemented one. So,
replacing it you need another list.
[color=blue]
> What if you wanted to insert an actual list into that slot? Would
> you have to wrap it in double brackets?[/color]

Why don't you just _try_ that? It would have been way faster than to ask
questions you can easily answer yourself.

Diez

[Mel Wilson]

Re: inserting into a list

John Salerno wrote:[color=blue]
> Christoph Haas wrote:[color=green]
>> L[2:2]=[3][/color][/color]
[ ... ]
What if you wanted to insert an actual list into that
slot? Would[color=blue]
> you have to wrap it in double brackets?[/color]

Yep.

It's a strong-typing thing. Slices of lists are lists, and
therefore what you assign to one has got to be a list, or
convertible to a list (a tuple would work.)

Python 2.4.2 (#1, Jan 23 2006, 21:24:54)
[GCC 3.3.4] on linux2
Type "help", "copyright" , "credits" or "license" for more
information.[color=blue][color=green][color=darkred]
>>> a=[1,3,4]
>>> a[2:3][/color][/color][/color]
[4][color=blue][color=green][color=darkred]
>>> a[2:2][/color][/color][/color]
[][color=blue][color=green][color=darkred]
>>> a[1:1]=[2]
>>> a[/color][/color][/color]
[1, 2, 3, 4][color=blue][color=green][color=darkred]
>>> a[1:2][/color][/color][/color]
[2]


Mel.

[Warby]

Re: inserting into a list

It makes sense because a slice IS a list, so you should assign a list
to it. Yours is just a special case in which the target slice has a
length of zero. It's still a list, just an empty one:
[color=blue][color=green][color=darkred]
>>> L = [1,2,4]
>>> print L[2:2][/color][/color][/color]
[]

As for your question, yes:
[color=blue][color=green][color=darkred]
>>> L = [1,2,4]
>>> L[2:2] = [[3]]
>>> print L[/color][/color][/color]
[1, 2, [3], 4]

Cheers! :)

[John Salerno]

Re: inserting into a list

Diez B. Roggisch wrote:
[color=blue]
> Why don't you just _try_ that? It would have been way faster than to ask
> questions you can easily answer yourself.[/color]

I did try it, but I was still hoping for an explanation, which I've also
gotten from you guys, some in nicer terms than others.

[John Salerno]

Re: inserting into a list

Warby wrote:[color=blue]
> It makes sense because a slice IS a list, so you should assign a list
> to it. Yours is just a special case in which the target slice has a
> length of zero. It's still a list, just an empty one:
>[color=green][color=darkred]
>>>> L = [1,2,4]
>>>> print L[2:2][/color][/color]
> []
>
> As for your question, yes:
>[color=green][color=darkred]
>>>> L = [1,2,4]
>>>> L[2:2] = [[3]]
>>>> print L[/color][/color]
> [1, 2, [3], 4]
>
> Cheers! :)
>[/color]

Thanks guys! What I wasn't realizing was that a slice is a list, so I
needed a list. :)

[Diez B. Roggisch]

Re: inserting into a list

John Salerno wrote:
[color=blue]
> Diez B. Roggisch wrote:
>[color=green]
>> Why don't you just _try_ that? It would have been way faster than to ask
>> questions you can easily answer yourself.[/color]
>
> I did try it, but I was still hoping for an explanation, which I've also
> gotten from you guys, some in nicer terms than others.[/color]

You got an explanation to your first question from me.

But you obviously _didn't_ try your second one. Which would be a no-brainer
as Warby's reply shows, and given that you already knew how to insert a
single element list I consider it being not so nice of _you_ not to do so,
but instead post before you tried.

Diez

[John Salerno]

Re: inserting into a list

Diez B. Roggisch wrote:[color=blue]
> John Salerno wrote:
>[color=green]
>> Diez B. Roggisch wrote:
>>[color=darkred]
>>> Why don't you just _try_ that? It would have been way faster than to ask
>>> questions you can easily answer yourself.[/color]
>> I did try it, but I was still hoping for an explanation, which I've also
>> gotten from you guys, some in nicer terms than others.[/color]
>
> You got an explanation to your first question from me.
>
> But you obviously _didn't_ try your second one. Which would be a no-brainer
> as Warby's reply shows, and given that you already knew how to insert a
> single element list I consider it being not so nice of _you_ not to do so,
> but instead post before you tried.
>
> Diez[/color]

Actually, I did try the second one too, but I'm not trying to get into a
fight here, so let's just forget it. I appreciate the fact that you
responded at all, and with answers to my questions.

[James Stroud]

Re: inserting into a list

John Salerno wrote:[color=blue]
> Diez B. Roggisch wrote:
>[color=green]
>> Why don't you just _try_ that? It would have been way faster than to ask
>> questions you can easily answer yourself.[/color]
>
>
> I did try it, but I was still hoping for an explanation, which I've also
> gotten from you guys, some in nicer terms than others.[/color]

People who answer questions on this list have forgotten how unintuitive
intuitive can be. In other words, they have found that the intuitive way
to do things in python is usually the right way, which may not be the
case in other languages. Thus, your instinct to see if your instincts
are correct rings as laziness here, when in fact you are just being
rigorous.

Here is one my favorite examples of python intuitiveness:

if something is not something_else:
do_whatever()

Who would have thunk it?

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[John Salerno]

Re: inserting into a list

James Stroud wrote:
[color=blue]
> Here is one my favorite examples of python intuitiveness:
>
> if something is not something_else:
> do_whatever()
>
> Who would have thunk it?[/color]

That's actually one of the things that first surprised me about Python,
that you can actually say "is not" in a programming language, and it
reads like a sentence! :) (Not to mention 'and' and 'or')

[Steven D'Aprano]

Re: inserting into a list

On Tue, 07 Mar 2006 12:26:00 -0800, James Stroud wrote:
[color=blue]
> John Salerno wrote:[color=green]
>> Diez B. Roggisch wrote:
>>[color=darkred]
>>> Why don't you just _try_ that? It would have been way faster than to ask
>>> questions you can easily answer yourself.[/color]
>>
>>
>> I did try it, but I was still hoping for an explanation, which I've also
>> gotten from you guys, some in nicer terms than others.[/color]
>
> People who answer questions on this list have forgotten how unintuitive
> intuitive can be. In other words, they have found that the intuitive way
> to do things in python is usually the right way, which may not be the
> case in other languages. Thus, your instinct to see if your instincts
> are correct rings as laziness here, when in fact you are just being
> rigorous.[/color]

Not rigorous. Perhaps thorougher.

Had the OP worded the question more rigorously, we wouldn't be having this
argument:

"I wanted to see what happens if you try to insert a list into a list
using slicing, and discovered that this works:

L[2:2] = [ [1,2,3] ]

Now I don't understand the reasoning behind this. Can somebody explain the
rationale between needing to wrap objects in a list in order to insert
using slices?"

To which the answer would be, so it is consistent with other slice
assignment:

L[2:10] = [1, 2, 3, 4]

Retrieving a slice returns a list. Assigning to a slice requires a list.
Making an exception for the special case of L[x:x] goes against
the philosophy of the Python language: Python generally doesn't accept
that special cases are special enough to break the rules.

Half the battle is asking the right question. The other half of the battle
is asking the right question in the right way.



--
Steven.