References and copies

[Yermat]

Thomas Philips a écrit :
> I want to represent an NxN matrix by a list containing N lists, each
> of which has N elements. Initially the elements are set to " ". For
> N=2, I write
> 
> 
>>>>x = [" "][:]*2 #assignment creates references, not copies!
>>>>x
> 
> [' ', ' ']
> 
>>>>y = [x[:]]*2 
>>>>[...]
> 
> I fail to see the error in my first attempt. Where is the fly in the
> ointment?
> 
> Thomas Philips

Your trboules come from this line: y = [x[:]]*2 because you do not copy 
the x twice but just assign a copy of  x twice...
see the following:

 >>> x = [" "]*2
 >>> x
[' ', ' ']
 >>> x = [" "]*2
 >>> y = [x]*2
 >>> y
[[' ', ' '], [' ', ' ']]
 >>> id(y[0])
8303600
 >>> id(y[1])
8303600
 >>>
 >>> y = [[" "] *2, [" "]*2]
 >>> id(y[0])
8306544
 >>> id(y[1])
8305424
 >>>

-- 
Yermat