Syntax Question - list multiplication

[Duncan Smith]

Roel Schroeven wrote:
> Pablo Torres schreef:
> 
>> Hi guys!
>>
>> I am working on Conway's Game of Life right now and I've run into a
>> little problem.
>> I represent dead cells with 0s and live ones with 1s. Check this out:
>>
>>     >>> grid = [[0] * 3] * 3
>>     >>> grid
>>     [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>     >>> grid[0][0] = 1
>>     [[1, 0, 0], [1, 0, 0], [1, 0, 0]]
>>
>> Now, that's not what I want. I just want the first element of the
>> first sublist to be a 1, not the first of every one. Here's what I
>> tried and didn't work:
>>
>> 0.  grid = [[0] * 3][:] * 3    then the same grid[0][0] = 1 statement
>> 1.  grid = [[0][:] * 3] * 3    followed by the same assignment
>> 2.  (grid[0])[0] = 1    with the original initialization of the grid
>>
>> So, that means that it isn't a problem with two different variables
>> refering to the same list (0. and 1. prove that). What I don't
>> understand is why 2. doesn't work either. I'm baby-feeding my
>> instructions to Python and the mistake is still there. Any ideas?
>>
>> Hope you can help. Thanks in advance,
> 
> 
> The multiplication operator doesn't make new copies; all elements
> reference the same object, as you can see with id():
> 
>>>> lst = [0] * 3
>>>> lst
> [0, 0, 0]
>>>> id(lst[0]), id(lst[1]), id(lst[2])
> (9788716, 9788716, 9788716)
> 

I think you probably meant something like,

>>> lst = [[0] * 3] * 3
>>> lst
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> id(lst[0]), id(lst[1]), id(lst[2])
(15937704, 15937704, 15937704)
>>>

i.e. a list of mutable types where the difference between multiple
objects and multiple references to the same object will be apparent.
With small integers you're likely to find you have multiple references
to the same integer objects however the list is constructed.

Duncan