Return size of binary tree

[ichabod801]

You could use preorder, and get the len() of the list returned by preorder. If you don't want to build that whole list, you could write a method to just calculate the number of nodes:

def __len__(self):
    if self.is_empty():
        return 0
    else:
        return 1 + len(self.lc()) + len(self.rc())

Note that by naming it __len__, it will be used by the built-in len() function, which is what I do with the calls to the children on line 5.