why this happend using model random?

[Robert Kern]

flyaflya wrote:
> Robert Kern wrote:
> 
>>flyaflya wrote:
>>
>>
>>>from random import *
>>>
>>>col = [0 for i in range(10)]
>>>a = [col for  i in range(10)]
>>
>>
>>This is the problem. The list "a" now has ten references to the same 
>>object "col". They are not copied.
>>
>>
>>>seed()
>>>for i  in range(10):
>>>     for j in  range(10):
>>>         a[i][j] = randint(0, 100)
>>
>>
>>So every time you index into "a[i][j]" you are always getting "col[j]".
>>
> 
> thanks,I see,I know about reference and copy,but when is it a copy?when 
> a reference?how can I get a copy when need?

Usually, references get passed around wherever possible. Just putting 
"col" into the list comprehension like you did will never copy.

If you need a copy of a list, you could do "col[:]" or "list(col)". More 
generally, see the "copy" module.

-- 
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
  Are the graves of dreams allowed to die."
   -- Richard Harter