References error in Python

Question

The problem comes from the state variable (in args). It is modified in my code (after the new_state modifications). However, I've read that using list() could prevent this kind of problem (it looks like state and new_state have the same reference).

To sum up, if I display the value of state at the beginning of the function, and just before the return, the values are different (and I obviously don't want to change the value of this variable !). How can I solve this problem ?

def successor(self, state, numberClients, numberDepots, Q, dist_table):         
    succ_list = list() 
    for i in range(0, len(state)): 
        for j in range(0, len(state[i])): 
           switchIndex = 0 
           while switchIndex < length: 
              permutationIndex = 0 
              while permutationIndex < len(state[switchIndex]):                     
                  new_state = list(state) 
                  temp = new_state[switchIndex][permutationIndex] 
                  new_state[switchIndex][permutationIndex] = new_state[i][j] 
                  new_state[i][j] = temp 
                  if checkConst(new_state): # accept only in some cases (we don't care here)
                      succ_list.append(('act', new_state))             
                  permutationIndex += 1 
           switchIndex += 1 
    return succ_list

Pavel Anossov · Accepted Answer · 2013-03-16 17:07:35Z

2

It looks like state is a list of lists?

When you do new_state = list(state), you are copying the outer list, but the references inside the copy still point to the same inner lists.

Try this:

new_state = [list(e) for e in state]

answered Mar 16, 2013 at 17:07

Pavel Anossov

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GodMan · Accepted Answer · 2013-03-16 17:11:35Z

You have found the solution yourself. The references in the call: list(...) are copied to the new list. So essentially, it is not a deep copy but a shallow copy. The following code explains it.

>>> class A: pass
... 
>>> a=map(lambda x:A(), range(5))
>>> a
[<__main__.A instance at 0x1018d11b8>, <__main__.A instance at 0x1018f17a0>, <__main__.A instance at 0x101868950>, <__main__.A instance at 0x1018687a0>, <__main__.A instance at 0x101868830>]
>>> b=list(a)
>>> b
[<__main__.A instance at 0x1018d11b8>, <__main__.A instance at 0x1018f17a0>, <__main__.A instance at 0x101868950>, <__main__.A instance at 0x1018687a0>, <__main__.A instance at 0x101868830>]
>>> b[0].v=23
>>> a
[<__main__.A instance at 0x1018d11b8>, <__main__.A instance at 0x1018f17a0>, <__main__.A instance at 0x101868950>, <__main__.A instance at 0x1018687a0>, <__main__.A instance at 0x101868830>]
>>> a[0]
<__main__.A instance at 0x1018d11b8>
>>> a[0].v
23

Also, in order to prevent the problem, you can use:-

import copy

And, then, use new_state = copy.deepcopy(state) in your code instead of new_state = list(state).

The following code explains this:-

>>> import copy
>>> copy.deepcopy
<function deepcopy at 0x1004d71b8>


>>> a=map(lambda x:A(), range(5))
>>> b=copy.deepcopy(a)
>>> b[0].v=23
>>> a[0].v
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute 'v'

Ok deepcopy will do the trick, thanks guys ! It's a list of lists yeah.

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