How to not mutate nested lists in Python?

Question

My code:

 class World:
 def __init__(self, _map, pos):
 self.orig_map = _map
 self.map = self.orig_map[:]
 self.orig_pos = pos
 self.pos = list(self.orig_pos)
 def reset(self):
 self.map = self.orig_map[:]
 self.pos = list(self.orig_pos)
 def left(self):
 if self.pos[1]>0:
 self.pos[1]-=1
 def right(self):
 if not self.pos[1]+1>=len(self.map[0]):
 self.pos[1]+=1
 def up(self):
 if self.pos[0]>0:
 self.pos[0]-=1
 def down(self):
 if not self.pos[0]+1>=len(self.map):
 self.pos[0]+=1
 def eat(self):
 if self.map[self.pos[0]][self.pos[1]]==1:
 self.map[self.pos[0]][self.pos[1]]=0
 return True

What is supposed to happen:

>>> w=World([[0,0,0],[0,1,0],[0,0,0]],(0,0))
>>> w.right()
>>> w.down()
>>> w.eat()
True
>>> w.reset()
>>> w.map
>>> [[0, 0, 0], [0, 1, 0], [0, 0, 0]]

What happens:

>>> w=World([[0,0,0],[0,1,0],[0,0,0]],(0,0))
>>> w.right()
>>> w.down()
>>> w.eat()
True
>>> w.reset()
>>> w.map
>>> [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

Where it probably goes wrong: self.map = self.orig_map[:]

The above would have worked (tried and tested) for a single list, however, it doesn't seem to work for nested lists.

• Ma0: deep_copy it. See this
• Ilja Everilä: Did you mean w.orig_map in the last step? You could in this case use the copy module's deepcopy function.
• sbrm1: @IljaEverilä No, I meant w.map. However, at that step, they should be equal (value wise) but be different objects.
• Ilja Everilä: Right, missed the reset() call.
• Chris_Rands: Depending on exactly what self.orig_map contains, deepcopy may not work stackoverflow.com/questions/1601269/…

Answer 1

By saying self.map = self.orig_map[:] you are indeed making a copy of self.orig_map. However, this is a shallow copy, and the elements within self.map will still be the same objects as the elements within self.orig_map.

What you need to do instead is make self.map a deep copy of self.orig_map in your __init__. E.g.

import copy
...
 self.map = copy.deepcopy(self.orig_map)

Answer 2

You should use deepcopy

import copy
cop2 = copy.deepcopy(origin) 

It will recursively copy your object.