multiple index inconsistency

[Mark McEahern]

[bert]
> The following line of code
> >>> a = 3*[range(3)]
> produces
> [[0, 1, 2], [0, 1, 2], [0, 1, 2]]
> 
> If I then write, say, 
> >>> a[0][1] = 3.7
> I get
> [[0, 3.7, 2], [0, 3.7, 2], [0, 3.7, 2]]
> and not
> [[0, 3.7, 2], [0, 1, 2], [0, 1, 2]]
> 
> Now if I write
> >>> a = [[0, 1, 2], [0, 1, 2], [0, 1, 2]]
> instead, and then 
> >>> a[0][1] = 3.7
> I get the result I expected (and wanted):
> [[0, 3.7, 2], [0, 1, 2], [0, 1, 2]]
> Why the inconsistency? Is this a bug or am I missing something?

Short answer:  lists are mutable.  ints aren't.

So when you do this:

  a = 3 * [range(3)]

you're basically doing this:

  l = range(3)
  a = 3 * [l]

The result is effectively:

  [[l], [l], [l]]

when you modify one of the lists, "all of them" are affected.

A better approach:

  x = 3
  y = 3
  a = [range(x) for x in range(y)]

(I assume x and y are different in theory.)

Now, try this:

  for x in a:
    print id(x)

and compare that to the list generated by 'a = 3 * [range(3)]'

Cheers,

// mark
-