Problems with instantiating list in constructor

Question

class Tree:
    def __init__(self, label, children = []):
        self.label = label
        self.children = children

t1 = Tree("START")
t2 = Tree("END")
t1.children.append(t2)

print t2.children
[<__main__.Tree instance at 0x005421C0>]

Why isn't t2.children empty?

Check out the above link: everyone is surprised by this issue eventually. For suggestions of Pythonic ways to fix it, check out http://stackoverflow.com/questions/366422/what-is-the-pythonic-way-to-avoid-default-parameters-that-are-empty-lists — David Robinson, Oct 14 '13 at 04:12

score 0 · Answer 1 · answered Oct 14 '13 at 04:32

0

The problem is that the default value is executed only once - when the def statement is excuted. Thus, all instance objects' children are assigned the same list.

In short, the lists for t1 and t2 are the same list.

answered Oct 14 '13 at 04:32

Petar Ivanov

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Problems with instantiating list in constructor

1 Answers1