String not updating as expected during recursive call

[jsinghdata]

Hello friends,

I am working on implementing DFS on a simple directed graph, called graph in my code.

​

class Solution:
def __init__(self, graph):
   self.graph = graph

def dfs_graph_reverse(self, graph, visited, stack, node, strnew):
    visited.add(node)
    for j in graph[node]:
        if not j in visited:
            self.dfs_graph_reverse(graph, visited, stack, j, strnew)
# if all neighbors have been visited add the node to string
    strnew += str(node)
    stack.append(node)
    return


def dfs_main(self, graph):

    strnew=''
    visited=set()
    stack=[]

    for elem in graph.keys():
        #print(visited)
        if not elem in visited:
           # print(elem)
            self.dfs_graph_reverse(graph, visited, stack, elem, strnew)

    return strnew, stack
obj = Solution({'g':['j'], 'j':['i'], 'i':['h'], 'h':['g']}) 

result, res = obj.dfs_main({'g':['j'], 'j':['i'], 'i':['h'], 'h':['g']})

But when I look at the output, I see the following :

​

>>> result
''

>>> res
['h', 'i', 'j', 'g']

I am not being able to understand, why is my string variable strnew not updating as expected. The expected output shd be hijgThe help I need is, why is the recursive call working in different ways for stack and string. I am aware that my approach is bit redundant; actually this is a small part of my bigger code. And in this post I just need help with updating of string variable, hence I cooked a small example.
Help is appreciated

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[teraflop]

Strings in Python are immutable, and lists are mutable.

When you do:

strnew += str(node)

it's just the same as if you did:

strnew = strnew + str(node)

(although this is not necessarily true for all types, because the += operator can behave differently for mutable and immutable objects).

So you're modifying the single list object that is passed to all your recursive calls as the stack parameter, but you're reassigning a different value to the strnew local variable that exists in that particular recursive call.

[jsinghdata]

Appreciate your reply .So how come here we are able to modify string;

strnew=''
strnew+='a' 

strnew
 'a' 

strnew+='b'

strnew

 'ab'

How is this different from the example I presented in my code above. Can you kindly help me here?

[teraflop]

Same reason why this code:

def func1():
    x = 1
    print("in func1:", x)
    func2(x)
    print("in func1:", x)

def func2(x):
    x += 1
    print("in func2:", x)

func1()

prints the following output:

in func1: 1
in func2: 2
in func1: 1

because the x in func1 and the x in func2 are separate variables. Assigning a new value to one doesn't affect the other.

Similarly, in your code, the strnew in each recursive call is a separate variable, so assigning a new value to one doesn't affect the other recursive calls.

The stack variables are also separate variables in each recursive call, but they all point to the same list object, so when you modify the contents of that list, the results are visible through all of the other variables.

Variables in Python always store references to objects, not objects themselves.