Question

I’m trying to set a function to do something like this

   def __binaryTreeInsert(self, toInsert, currentNode=getRoot(), parentNode=None):

where current node starts as root, and then we change it to a different node in the method and recursivly call it again.

However, i cannot get the ‘currentNode=getRoot()’ to work. If i try calling the funcion getRoot() (as above) it says im not giving it all the required variables, but if i try to call self.getRoot() it complains that self is an undefined variable. Is there a way i can do this without having to specify the root while calling this method?

EDIT: The base case of this method is already

if currentNode == None:

so using that to set the root wouldn’t work

Answer 1

While arg=None is the idiomatic Python sentinel value for an non-supplied argument, it doesn’t have to be None. In Lua, for instance, the idiomatic non-supplied argument is an empty table. We can actually apply that to this case:

class Foo:
    sentinel = {}
    def bar(self, arg=sentinel):
        if arg is self.sentinel:
            print "You didn't supply an argument!"
        else:
            print "The argument was", arg

f = Foo()
f.bar(123)
f.bar()
f.bar(None)
f.bar({})

Output:

The argument was 123
You didn't supply an argument!
The argument was None
The argument was {}

This works for any case except explicitly passing Foo.sentinel, because Foo.sentinel is guaranteed to have a unique address — meaning, x is Foo.sentinel is only true when x is Foo.sentinel 🙂 Thus, due to the closure we’ve created around Foo.sentinel, there is only one object that can create an ambiguous situation, and it will never be used by accident.