Python - Function has a list as argument. How to return another list without changing the first?

Question

I'm pretty new in Python (and programming as a whole). I'm pretty sure the answer to this is obvious, but I really don't know what to do.

def do_play(value, slot, board):
    temp=board
    (i,j) = slot
    temp[i][j] = value
    return temp

board is a list of lists. value is an integer. slot is and integer tuple.

What I am trying to do here is to

• feed the function board
• copy board to a new list called temp
• insert a new value in a specific location in temp
• return temp, leaving board unchanged

When I run this is the shell, both the the original list (board) and the new list (temp) change. = \

Any help would be appreciated.

Answer 1

Use copy.deepcopy() to copy the object.

Answer 2

Are you trying to copy board?

temp = board[:]

Or maybe this to copy the structure.

temp = [ r[:] for r in board ]

Answer 3

temp=board does not make a new board. It makes the temp variable reference the very same list as board. So changing temp[i][j] changes board[i][j] too.

To make a copy, use

import copy
temp=copy.deepcopy(board)

---

Note that temp=board[:] makes temp refer to a new list (different than board, but the contents (that is, the lists within the list) are still the same:

In [158]: board=[[1,2],[3,4]]    
In [159]: temp=board[:]    

Modifying temp modifies board too:

In [161]: temp[1][0]=100    
In [162]: temp
Out[162]: [[1, 2], [100, 4]]    
In [163]: board
Out[163]: [[1, 2], [100, 4]]

id shows the object's memory address. This shows temp and board are different lists:

In [172]: id(temp)
Out[172]: 176446508

In [173]: id(board)
Out[173]: 178068780   # The ids don't match

But this shows the second list inside temp is the very same list inside board:

In [174]: id(temp[1])
Out[174]: 178827948

In [175]: id(board[1])
Out[175]: 178827948    # The ids are the same

---

But if you use copy.deepcopy, then the lists within the list also get copied, which is what you need if modifying temp is to leave board unchanged:

In [164]: import copy    
In [165]: board=[[1,2],[3,4]]    
In [166]: temp=copy.deepcopy(board)    
In [167]: temp[1][0]=100    
In [168]: temp
Out[168]: [[1, 2], [100, 4]]    
In [169]: board
Out[169]: [[1, 2], [3, 4]]

Answer 4

temp=board

This doesn't copy list, it just makes one more reference to the same object. Use temp = board[:] instead.

Answer 5

Here temp is a reference to board, a shallow copy. I usually like to import the copy module (import copy) and use copy.deepcopy which makes temp separate from board. You'd call it something like this:

import copy
temp = copy.deepcopy(board)

Otherwise, you can just make a slice of board (which also makes a deep copy). I believe this should work, but I haven't tried it on a list of lists. You'd call it as so:

temp = board[:]