Using self.xxxx as a default parameter – Python

Question

I’m trying to simplify one of my homework problems and make the code a little better. What I’m working with is a binary search tree. Right now I have a function in my Tree() class that finds all the elements and puts them into a list.

tree = Tree()
#insert a bunch of items into tree

then I use my makeList() function to take all the nodes from the tree and puts them in a list.
To call the makeList() function, I do tree.makeList(tree.root). To me this seems a little repetitive. I’m already calling the tree object with tree.so the tree.root is just a waste of a little typing.

Right now the makeList function is:

    def makeList(self, aNode):
        if aNode is None:
            return []
        return [aNode.data] + self.makeList(aNode.lChild) + self.makeList(aNode.rChild)

I would like to make the aNode input a default parameter such as aNode = self.root (which does not work) that way I could run the function with this, tree.makeList().

First question is, why doesn’t that work?
Second question is, is there a way that it can work? As you can see the makeList() function is recursive so I cannot define anything at the beginning of the function or I get an infinite loop.

EDIT
Here is all the code as requested:

class Node(object):
    def __init__(self, data):
        self.data = data
        self.lChild = None
        self.rChild = None

class Tree(object):
    def __init__(self):
        self.root = None

    def __str__(self):
        current = self.root

    def isEmpty(self):
        if self.root == None:
            return True
        else:
            return False

    def insert (self, item):
        newNode = Node (item)
        current = self.root
        parent = self.root

        if self.root == None:
            self.root = newNode
        else:
            while current != None:
                parent = current
                if item < current.data:
                    current = current.lChild
                else:
                    current = current.rChild

            if item < parent.data:
                parent.lChild = newNode
            else:
                parent.rChild = newNode

    def inOrder(self, aNode):
        if aNode != None:
            self.inOrder(aNode.lChild)
            print aNode.data
            self.inOrder(aNode.rChild)

    def makeList(self, aNode):
        if aNode is None:
            return []
        return [aNode.data] + self.makeList(aNode.lChild) + self.makeList(aNode.rChild)


    def isSimilar(self, n, m):
        nList = self.makeList(n.root)
        mList = self.makeList(m.root) 
        print mList == nList

Asked By: chrisheinze

||

Source

Answer 1

It doesn’t work because default arguments are evaluated at function definition time, not at call time:

def f(lst = []):
    lst.append(1)
    return lst

print(f()) # prints [1]
print(f()) # prints [1, 1]

The common strategy is to use a None default parameter. If None is a valid value, use a singleton sentinel:

NOTHING = object()

def f(arg = NOTHING):
    if arg is NOTHING:
        # no argument
    # etc.

Answered By: Fred Foo

Answer 2

larsmans answered your first question

For your second question, can you simply look before you leap to avoid recursion?

def makeList(self, aNode=None):
    if aNode is None:
        aNode = self.root
    treeaslist = [aNode.data]
    if aNode.lChild:
        treeaslist.extend(self.makeList(aNode.lChild))
    if aNode.rChild:
        treeaslist.extend(self.makeList(aNode.rChild))
    return treeaslist

Answered By: nofinator

Answer 3

If you want to treat None as a valid argument, you could use a **kwarg parameter.

def function(arg1, arg2, **kwargs):
    kwargs.setdefault('arg3', default)
    arg3 = kwargs['arg3']
    
    # Continue with function

function("amazing", "fantastic") # uses default
function("foo", "bar", arg3=None) # Not default, but None
function("hello", "world", arg3="!!!")

I have also seen ... or some other singleton be used like this.

def function(arg1, arg2=...):
    if arg2 is ...:
        arg2 = default

Answered By: killjoy

Using self.xxxx as a default parameter – Python

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