2D List - Python - weird loop behaviour (does not stop when told to) - python-3.x

Question

My having troubles understanding why is my code not working like I think it should.
This function is supposed to fill up a 2 dimensional list with 1 instead of 0 given some parameters:
x = to tell it where to start to fill up on the x axis
y = to tell it where to start to fill up on the y axis
length = length of the rectangle
width = width of the rectangle
rotation = boolean to tell if the rectangle should be drawn
vertically (if True) or horizontally (if False)
So I call drawLoop() that will call drawVertical() that calls drawHorizon() afterward
This function is aimed at receiving multiple rectangle, but my problem lies when the first is added.
Here is the code:
# Create a 2D list of 120 * 60
thisBox = [["0"] * 120] * 60
# Filler function
def drawLoop(x, y, length, width, rotation):
def drawHorizon(x, y, length, width, rotation, row):
drawIndexH = 0
while drawIndexH < len(row):
if rotation == False and drawIndexH >= x and drawIndexH < x + length:
row[drawIndexH] = "1"
drawIndexH += 1
elif rotation == True and drawIndexH >= x and drawIndexH < x + width:
row[drawIndexH] = "1"
drawIndexH += 1
else:
return
def drawVertical(x, y, length, width, rotation):
drawIndexV = 0
while drawIndexV < len(thisBox):
if rotation == False and drawIndexV >= y and drawIndexV < y + width:
drawHorizon(x, y, length, width, rotation, thisBox[drawIndexV])
drawIndexV += 1
elif rotation == True and drawIndexV >= y and drawIndexV < y + length:
drawHorizon(x, y, length, width, rotation, thisBox[drawIndexV])
drawIndexV += 1
else:
drawIndexV += 1
# Launch vertical drawing
drawVertical(x, y, length, width, rotation)
# Launch main function
drawLoop(0, 0, 70, 50, False)
As of now, the 120 * 60 space is empty, so by calling the main function with drawloop(0, 0, 70, 50, False)on the last line, I'm supposed to see a 70 * 50 rectangle drawn at the position (0, 0). So that out of the 7200 0 (120 * 60) I should see only 3700 left (7200 - (70 * 50))
So the function is divided into 2 other functions: drawVertical(x, y, length, width, rotation) that will draw vertically and drawHorizon(x, y, length, width, rotation, row) that will draw horizontally first.
But somehow, at the first iteration of drawVertical(...), all the rows are being filled up in one iteration and do not stop at the exit condition: it should stop at y = 50 (the width) when if rotation == False and drawIndexV >= y and drawIndexV < y + width: of drawVertical(...) because drawIndexV < y + width should stop at 50. But it does not and I have no clue why.
Even if I tell drawVertical(...) to stop the loop at the first iteration with while drawIndexV < 2:, all the rows are being filled up.
So horizontally I have the expected result, but I never have it vertically. Can anybody spot my mistake? Many thanks in advance!
Ben.

Answer 1

The problem is indeed that
thisBox = [["0"] * 120] * 60
creates a list with 60 times the same element, a list with 120 "0", as the following code snippet shows:
for r in range(60):
print(id(thisBox[r]))
for which a sample output is:
140706579889408
140706579889408
140706579889408
140706579889408
140706579889408
140706579889408
...
Updating any element on any row will update the same element on every row, since every row is the same unique list object.
To avoid the issue, one needs to ensure that each of the enclosed lists (the 60 lists, each of which contains 120 "0") is a separate list object, distinct from all the other enclosed lists, i.e., has its own id.
If familiar with numpy (numpy.zeros), and depending on the exact requirements, resorting to numpy arrays could be a solution. If using numpy is an instance of "shooting a bird with a cannonball", one alternative is using list comprehensions to initialise the list:
thisBox = [["0" for c in range(120)] for r in range(60)]
Running the same code as before confirms that each list of 120 "0" now has its own id, i.e., each row is a separate list:
for r in range(60):
print(id(thisBox[r]))
140185522518784
140185522481600
140185522519680
140185522482560
140185503364672
...
(Would have added the solution described above, using list comprehensions, as a comment, but was barred from doing so because of insufficient "reputation" points)

Answer 2

I have found my problem (Merci Philippe!!!)
When I do this:
# Create a 2D list of 120 * 60
thisBox = [["0"] * 120] * 60
I'm actually creating a list of the same element 60 times. They all have the same memory allocation. So if I modify one, I modify them all. That's why one iteration of drawVertical(...) modified the whole 60 rows.