python bug in this list implementation?

Fredrik Lundh fredrik at
Wed Dec 28 08:50:47 CET 2005

Chris Smith wrote:

> I've been working on some multi-dimensional lists and I've encountered some
> very strange behaviour in what appears to be simple code, I'm using python
> 2.4.2 and IDLE. If anyone can tell me why it's behaving so strange please
> let me know, any improvements to my general coding style are also
> appreciated.
> code below:
> import sys
> import copy
> grid = []
> oGrid = []
> sGrid = []
> def createGrid():
>     f = open(r"...sudoku.txt", "rb") ## see attached for the file.
>     for line in f:
>         aLine = line.strip().split(',')
>         if aLine != [""]:
>             for i in xrange(len(aLine)):
>                 aLine[i] = int(aLine[i])
>             grid.append(aLine)

at this point, grid contains a list of lists.

>     oGrid = copy.deepcopy(grid)

if you assign to a name inside a function, that name is considered to be
*local*, unless you specify otherwise.  in other words, this doesn't touch
the *global* (module-level) oGrid variable.

>     sGrid.append(copy.deepcopy(grid))

here you add a list of lists to a list.  the result is a list with a single item.

> def printGrid():
>     print "original grid:"
>     for line in oGrid:
>         print line    #why doesn't this print anything?

because the *global* oGrid is still empty.

>     print "S grid:"
>     for line in sGrid:
>         print line  #this prints the grid but the formatting is all over the
> place.

because sGrid contains a single item; a copy of your original grid.

>     print "Iteration grid: "
>     for line in grid:
>         print line  #works fine!

as expected.

I suggest reading up on list methods and global variables in your favourite
python tutorial.

also read:


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