python string vs list odd behaviour

Question

If I try the following snippets of code on a binary tree, and try to print the arr and string later, arr gives me the correct result but string is empty. Any thoughts? Is it something to do with lists being passed by reference and strings passed by value?

def post_order(root, arr = []):
    if(root is not None):
        post_order(root.left, arr)
        post_order(root.right, arr)
        arr.append(root.value)

def post_order1(root, string = ''):
    if(root is not None):
        post_order1(root.left, string)
        post_order1(root.right, string)
        string += str(root.value)

# assume I've made my binary tree
arr, string = [], ''
post_order(root, arr)
post_order1(root, string)
print arr, string
# arr holds the correct post-order sequence
# string is empty

strings are immutable so string += str(root.value) might not be doing much, do you mean to += before you call post_order1 passing the new string and then returning it? I think you may fare better with a class making string an attribute — Padraic Cunningham
– Padraic Cunningham, Commented Apr 25, 2016 at 19:34
Related: In Python, why can a function modify some arguments as perceived by the caller, but not others? — jfs
– jfs, Commented Apr 25, 2016 at 19:37
You're not including sufficient information. Include you inputs, desired and existing outputs. Also, you should really return those functions so that the variables are garbage collected. — Pouria
– Pouria, Commented Apr 25, 2016 at 19:40

Brian · Accepted Answer · 2016-04-25 19:34:35Z

2

In Python, lists are mutable and strings are immutable. This means that a list can be modified, but a string cannot. Strings can only be reassigned.

In your function, you are modifying the list using .append(), but you are only reassigning your string +=

answered Apr 25, 2016 at 19:34

Brian

1,67512 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Danny Liu Over a year ago

Ahh that makes a lot of sense! Thanks Brian :)

kofemann · Accepted Answer · 2016-04-25 19:34:24Z

1

Arr is an array, which you extend. String passed to post_order1 is an immutable object and a copy is created when updated. As a result, the original string stays unchanged.

answered Apr 25, 2016 at 19:34

kofemann

4,4731 gold badge38 silver badges43 bronze badges

Comments

nino_701 · Accepted Answer · 2016-04-25 22:35:12Z

-1

You should rectify your code like this:

def post_order1(root, string = ''):
    if not root : return string

left  = post_order1(root.left, string)
right = post_order1(root.right, left)
return right + str(root.value)

edited Apr 25, 2016 at 22:35

answered Apr 25, 2016 at 19:48

nino_701

6903 silver badges13 bronze badges

1 Comment

nino_701 Over a year ago

I corrected the code in the post. Thank you @Danny Liu

Collectives™ on Stack Overflow

python string vs list odd behaviour

3 Answers 3

1 Comment

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related