On Tue, Dec 30, 2014 at 09:41:33PM -0800, Alex Kleider wrote:
> In the process of doing so I discovered that list multiplication does
> not at all behave the way I expected (as demonstrated by the
> 'bad_flip_2_D' function.)
Well, don't keep us in suspense. How did you expect it to behave, and
how does it actually behave?
My guess is that you expected this:
py> grid = [[0]*5]*4 # make a 4x5 grid
py> print(grid)
[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
Looks good so far! But:
py> grid[1][1] = 99 # Adjust a single cell.
py> print(grid)
[[0, 99, 0, 0, 0], [0, 99, 0, 0, 0], [0, 99, 0, 0, 0], [0, 99, 0, 0, 0]]
while you expected:
[[0, 0, 0, 0, 0], [0, 99, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
Am I right?
The problem is that you guessed that list multiplication *copies* the
items. It does not. It just repeats them. So:
[a, b]*3
returns
[a, b, a, b, a, b]
but NOT
[copy(a), copy(b), copy(a), copy(b), copy(a), copy(b)]
If the list items are immutable, like ints or strings, the difference
doesn't matter. You can't modify immutable objects in-place, so you
never notice any difference between copying them or not.
Aside: the copy module actually implements copying for some immutable
objects by returning the original!
py> import copy
py> x = 2.3456
py> y = copy.copy(x)
py> y is x
True
py> x = []
py> y = copy.copy(x)
py> y is x
False
Floats are immutable and cannot be modified, so there is nothing you can
do to change float x. Making an actual copy is a waste of time, so
copy.copy just returns the same float object. But lists are mutable and
can be modified, so copy.copy has to actually build a new list.
But I digress. Back to list multiplication.
Since list multiplication doesn't *copy* the items, it just repeats
them, we can see what is happening if we expand the code a little with a
temporary variable:
Before:
grid = [[0]*5]*4 # make a 4x5 grid
grid[1][1] = 99 # Adjust a single cell.
Expanded:
x = [0]*5 # Make a single list of five immutable integers.
grid = [x]*4 # Like [x, x, x, x]
So here we have a list of four references to the same list, not four
different lists! If you change any one of those references, or indeed
the original "x" reference *in-place*, they all change. They all change
because in fact there is no "all", there is only one!
grid[1][1] = 99
print(x)
-> prints [0, 99, 0, 0, 0]
The solution to this is not to use list multiplication unless you know
the items are immutable. Instead, a list comprehension solves the
problem nicely:
py> grid = [[0]*5 for i in range(4)]
py> grid[1][1] = 99
py> print(grid)
[[0, 0, 0, 0, 0], [0, 99, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
--
Steven
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