Re: [pygame] Sprite Collision

[Kris Schnee <kschnee@xxxxxxxxxx>]

Ethan Glasser-Camp wrote:
> Ian Clark wrote:
> > On 4/30/07, Casey Duncan <casey@xxxxxxxxxxx> wrote:
> > > passible_at = [[True] * 100] * 100 # Create a 2 dimensional list with
> > > an element for each tile
> > This does not do what you think it would:
> >
> > Someone more skilled than I can explain *why* the first one does what
> > it does. I just know one way to get around it (and I'm sure there's a
> > more elegant solution).
>
> The expression
>
> [True]*100
>
> evaluates to a list that has 100 elements, each pointing to the same
> thing (the True object).

Aha! I've seen a problem like this before. I called it the "stripey doom 
error," because when I did a tile-based game and changed one element, 
I'd see a whole stripe of tiles change instead of one. Python has other 
"sticky variable" problems sometimes if you don't think carefully about 
whether something is a reference to another variable. I would sometimes 
end up changing "constants" and having other problems.

>>> coords = [42,100]
>>> copy_of_coords = coords
>>> copy_of_coords[0] += 5000
>>> copy_of_coords
[5042, 100]
>>> coords
[5042, 100] ## Bad!

But:
>>> coords = [42,100]
>>> copy_of_coords = [coords[0],coords[1]]
>>> copy_of_coords[0] += 5000
>>> copy_of_coords
[5042, 100]
>>> coords
[42, 100] ## OK!

When all else fails, copy.deepcopy() seems to make Python understand, "I 
want this to have _the same value_ as X now has, not to be a _reference_ 
to X."

Kris