In python, append child to root.children(list), and child.children(list) changed?

Question

code:

class Node:
    def __init__(self, key, children=[]):
        self.key = key
        self.children = children

    def __repr__(self):
        return self.key

execute:

root = Node("root")
child = Node("child")
root.children.append(child)
print child.children
print root.children[0].children

result:

[child]
[child]

This is really weird, why?

Python's version is 2.7.2.

Mutable object as default value! `def __init__(self, key, children=None): \\ children=children or []` — Aleksei astynax Pirogov, Jun 25 '12 at 10:13
No, see this question: http://stackoverflow.com/q/1132941/623518 — Chris, Jun 25 '12 at 10:15
@LeoDT: Not a bug. More like surprising feature. If you think about it hard enough, it makes sense why it would work like that. — cha0site, Jun 25 '12 at 10:19
The default value is a property of the _function object_. If you change it, you change it on the function object, so you change it for all calls to the function. — Katriel, Jun 25 '12 at 10:34

Mariusz Jamro · Accepted Answer · 2012-06-25T10:36:48.317

5

You shouldn't use mutable objects as default value of arguments (unless you exactly know what you're doing). See this article for explanation.

Instead use:

class Node:
    def __init__(self, key, children=None):
        self.key = key
        self.children = children if children is not None or []

edited Jun 25 '12 at 10:36

answered Jun 25 '12 at 10:17

Mariusz Jamro

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alternatively: `self.children = children if children is not None else []` or `self.children = [] if children is None else children`. – glglgl Jun 25 '12 at 10:25
1

`children or []` is not quite the same as `if children is not None`, because there are many false values which are not `None`. This may matter to you, or it may not, but the usual idiom is the latter. – Katriel Jun 25 '12 at 10:34

score 0 · Answer 2 · answered Jun 25 '12 at 10:19

class Node:
    def __init__(self, key,children): # don't use children =[] here, it has some side effects, see the example below
        self.key = key
        self.children =children

    def __repr__(self):
        return self.key

root = Node("root",[])
child = Node("child",[])
root.children.append(child)
print root.children
print root.children[0].key

output:

[child]
child

example:

def example(lis=[]):
    lis.append(1) #lis persists value
    print(lis)
example()
example()
example()

output :

[1]
[1, 1]
[1, 1, 1]

@Secator no it isn't , but `children or [] ` thing didn't came to my mind while I was writing this. — Ashwini Chaudhary, Jun 25 '12 at 10:24

In python, append child to root.children(list), and child.children(list) changed?

2 Answers2