附加到列表复制最后一项python

Question

I am trying to use a path finding algorithm and I want it to print all the steps one by one, this is a simplified code with only the important parts. For some reason this prints out 7 same levels that are only the final step, but I need it to print out all of the steps. The problem seems to be in the appending part, but I don't know how to fix it.

level = [
    [">","#"," "," "],
    [" ","#","#"," "],
    [" "," ","#"," "],
    ["#"," "," "," "],
]

cycle = [[0,0],[1,0],[2,1],[3,2],[3,3],[2,3],[1,3],[0,2]]
output = []
for i in range(len(cycle)-1):
    level[cycle[i  ][0]][cycle[i  ][1]] = " "
    level[cycle[i+1][0]][cycle[i+1][1]] = ">"
    output.append(level)

for i in output:
    for ii in i:
        print(ii)
    print()

I need someone to solve this problem for me, as anything on this site doesn't work in my exact problem

Answer 1

import copy

level = [
    [">","#"," "," "],
    [" ","#","#"," "],
    [" "," ","#"," "],
    ["#"," "," "," "],
]

cycle = [[0,0],[1,0],[2,1],[3,2],[3,3],[2,3],[1,3],[0,2]]
output = []
for i in range(len(cycle)-1):
    level_copy = copy.deepcopy(level)  
    level_copy[cycle[i  ][0]][cycle[i  ][1]] = " "
    level_copy[cycle[i+1][0]][cycle[i+1][1]] = ">"
    output.append(level_copy)

for i in output:
    for ii in i:
        print(ii)
    print()

When you do level[cycle[i ][0]][cycle[i ][1]] = " " inside the loop, you are referring to the same level object defined in the first line. So you do end up putting level multiple times, but they all are referring to the same object and hence contain the value that was written in the last iteration of loop.

level = []
output = []
for i in range (2):
  if i == 0:
    level.append(1) 
    level.append(2)
  else
    level.append(24)
    level.append(25)
  output.append(level)

i = 0: Start: level=[], end: level=[1,2]
i = 1: Start: level=[1,2] end: level=[1,2,24,25] // Observe that it starts with state left at end of first iteration as we are still referring to the same object in memory referred to by variable level.