understanding binary search trees

binserarchtree.py

class BinSearchTreeElement(object):
    def __init__(self, data, leftTree=None, rightTree=None):
        self.data = data
        self.left = leftTree
        self.right = rightTree

    def add(self, element):
        if element <= self.data:
            self._insertElement(self.left, element)
        else:
            self._insertElement(self.right, element)

    def _insertElement(self, tree, element):
            if isinstance(tree, type(None)):
                tree = BinSearchTreeElement(element)
            else:
                tree.add(element)

    def search(self, searchFor):
        if searchFor == self.data:
            return id(self)
        elif searchFor <= self.data:
            searchtree = self.left
        else:
            searchtree = self.right
        if isinstance(searchtree, type(None)):
            return None
        elif searchtree.data == searchFor:
            return id(searchtree.data)
        else:
            searchtree.search(searchFor)
class TreeEmptyException(Exception):
    pass

if __name__ == "__main__":
    root = BinSearchTreeElement(2)
    # build tree
    root.add(2)
    root.add(3)
    root.add(9)
    root.add(2)
    root.add(4)
    root.add(7)

    searchElement = 7
    print "Suche nach %s ergab %s" % (searchElement, root.search(searchElement))