assignment with [:]

[Ben Bush]

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:

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

 

[Robert Kern]

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
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

 

[James Stroud]

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]

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