I have a problem with creating 2 dimensional array - python-3.x

Question

I defined a function that changes list (basically moves the last item to the start of list) and then i tried to make a 2d list with this fuction.
Here's some code:
prevRow = ["blue", "yellow", "red", "black", "white"]
def nextRow():
prevRow.insert((0, prevRow.pop()))
print(prevRow)
return prevRow
tablePreset = [["blue", "yellow", "red", "black", "white"], nextRow(), nextRow(), nextRow(), nextRow()]
print(tablePreset)
prevRow = ["blue", "yellow", "red", "black", "white"]
tablePreset = [["blue", "yellow", "red", "black", "white"]]
def nextRow():
prevRow.insert((0, prevRow.pop()))
print(prevRow)
return prevRow
for _ in range(4):
tablePreset.append(nextRow())
print(tablePreset)
in both cases i got the
['white', 'blue', 'yellow', 'red', 'black']
['black', 'white', 'blue', 'yellow', 'red']
['red', 'black', 'white', 'blue', 'yellow']
['yellow', 'red', 'black', 'white', 'blue']
[['blue', 'yellow', 'red', 'black', 'white'], ['yellow', 'red', 'black', 'white', 'blue'], ['yellow', 'red', 'black', 'white', 'blue'], ['yellow', 'red', 'black', 'white', 'blue'], ['yellow', 'red', 'black', 'white', 'blue']]
I don't know why but even though i called function 4 times, every return in the list is the same as the last value (print inside the function is for debugging purposes).
I'll be very greatfull if someone helps me :)

Answer 1

You need to make a copy of the list every time it's passed to the function nextRow (see code below). Some more information here (related topic).
prevRow = ["blue", "yellow", "red", "black", "white"]
tablePreset = [["blue", "yellow", "red", "black", "white"]]
def nextRow(prevRow):
prevRow_copy = prevRow.copy()
prevRow_copy.insert(0, prevRow_copy.pop())
return prevRow_copy
for _ in range(4):
prevRow = nextRow(prevRow)
tablePreset.append(prevRow)
print(tablePreset)
# >> out:
# [['blue', 'yellow', 'red', 'black', 'white'],
# ['white', 'blue', 'yellow', 'red', 'black'],
# ['black', 'white', 'blue', 'yellow', 'red'],
# ['red', 'black', 'white', 'blue', 'yellow'],
# ['yellow', 'red', 'black', 'white', 'blue']]
Another small example illustrating the importance of the copy:
a = []
b = [1,2,3]
a.append(b)
b.pop()
a.append(b)
b.pop()
a.append(b)
print(a)
# >> out:
# [[1], [1], [1]]
a = []
b = [1,2,3]
a.append(b.copy())
b.pop()
a.append(b.copy())
b.pop()
a.append(b.copy())
print(a)
# >> out:
# [[1, 2, 3], [1, 2], [1]]