Python Variable Scope, variable referenced before assignment

Question

I am testing a very simple python script. What the function does is not important to my question, my question is the following code somehow gives me:

File "test.py", line 13, in traverse
    if root.val > minVal and ans > root.val:
UnboundLocalError: local variable 'ans' referenced before assignment

But I do have ans defined, why does it not complain about minVal but does about ans?

class Test(object):

    def findSecondMinimumValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        minVal = root.val
        ans = float("inf")

        def traverse(root):
            if not root:
                return

            if root.val > minVal and ans > root.val:
                ans = root.val

            traverse(root.left)
            traverse(root.right)

        traverse(root)
        return ans if ans != float("inf") else -1

You assign to `ans = ...` as a result, Python sees `ans ` in the `traverse` function as local. Note that this assignment has no use here, since you only set a reference. — Willem Van Onsem, Sep 20 '17 at 08:11

score 1 · Answer 1 · answered Sep 20 '17 at 08:14

1

Inside your function traverse you are defining a new variable ans, which has nothing to do with the ans you defined above. if you want to use the same variable put nonlocal ans at the top of your traverse function.

answered Sep 20 '17 at 08:14

MegaIng

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I got it now, thank you! – user2002692 Sep 20 '17 at 08:33

Python Variable Scope, variable referenced before assignment

1 Answers1