# assignment with [:]

Discussion in 'Python' started by Ben Bush, Dec 28, 2008.

1. ### Ben BushGuest

Hi,

I saw this line of code on a recent post:

a1[:] = [x*3 for x in a1]

Could somebody tells me what the [:] means? I can't find it anywhere.
See context below if needed:

On Dec 26, 4:46 pm, Tim Chase <> wrote:
> > What does *not* work is
> > 3 * [0,1,2]
> > As you know, this gives
> > [0,1,2,0,1,2,0,1,2]
> > What I am hoping for is
> > [0,3,6]
> > I see that I can use
> > numpy.multiply(3,range(3))
> > but this seems overkill to me. Can you tell I am coming to Python from
> > Matlab?

>
> The common way to do this is just
>
> a1 = [0,1,2]
> a2 = [x * 3 for x in a1]
>
> or, if you need a1 to be done in place:
>
> a1[:] = [x*3 for x in a1]
>
> -tkc

Ben Bush, Dec 28, 2008

2. ### Robert KernGuest

Ben Bush wrote:
> Hi,
>
> I saw this line of code on a recent post:
>
> a1[:] = [x*3 for x in a1]
>
> Could somebody tells me what the [:] means? I can't find it anywhere.

It's a slice assignment. When both the start and stop arguments are omitted, it
refers to the entire sequence. In this case, it means to replace the entire
contents of the list a1 with the value on the right-hand-side.

"a[2:4] = ..." would replace the 3rd and 4th items in the list with the sequence
on the right-hand-side, for instance.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
an underlying truth."
-- Umberto Eco

Robert Kern, Dec 28, 2008

3. ### James StroudGuest

Ben Bush wrote:
> On Dec 26, 4:46 pm, Tim Chase <> wrote:
>>> What does *not* work is
>>> 3 * [0,1,2]
>>> As you know, this gives
>>> [0,1,2,0,1,2,0,1,2]
>>> What I am hoping for is
>>> [0,3,6]
>>> I see that I can use
>>> numpy.multiply(3,range(3))
>>> but this seems overkill to me. Can you tell I am coming to Python from
>>> Matlab?

>> The common way to do this is just
>>
>> a1 = [0,1,2]
>> a2 = [x * 3 for x in a1]
>>
>> or, if you need a1 to be done in place:
>>
>> a1[:] = [x*3 for x in a1]

There is some subtlety here. The latter says to empty the list assigned
to the name "a1" and refill it with the products. Other references to
the same list will now reflect this operation. This procedure is not
equivalent to reassigning the name "a1". For example:

py> a = [1, 2, 3]
py> a1 = a
py> a1[:] = [x*3 for x in a1]
py> a1
[3, 6, 9]
py> a1
[3, 6, 9]

Whereas:

py> a = [1, 2, 3]
py> a1 = a
py> a1
[1, 2, 3]
py> a1 = [x*3 for x in a1]
py> a1
[3, 6, 9]
py> a
[1, 2, 3]

James

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

http://www.jamesstroud.com

James Stroud, Dec 28, 2008
4. ### James StroudGuest

James Stroud wrote:
> py> a = [1, 2, 3]
> py> a1 = a
> py> a1[:] = [x*3 for x in a1]
> py> a1
> [3, 6, 9]
> py> a1
> [3, 6, 9]

This should have been:

py> a = [1, 2, 3]
py> a1 = a
py> a1[:] = [x*3 for x in a1]
py> a
[3, 6, 9]
py> a1
[3, 6, 9]

James

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

http://www.jamesstroud.com

James Stroud, Dec 28, 2008