TypeError: unbound method add() must be called with BinaryTree instance as first argument (got nothing instead)

[Peter Otten]

Dan Stromberg wrote:

> I'm getting the error in the subject, from the following code:
>     def add(self, key):
>         """
>         Adds a node containing I{key} to the subtree
>         rooted at I{self}, returning the added node.
>         """
>         node = self.find(key)
>         if not node:
>             node.key = key
>             # placeholder
>             node.left, node.right = self.__class__(parent=node),
> self.__class__(parent=node)
>             return (False, node)
>         else:
>             if random.random() < 0.5:
>                 print('node.left is %s' % node.left)
>                 return BinaryTree.add(self=node.left, key=key)
>             else:
>                 print('node.right is %s' % node.left)
>                 return BinaryTree.add(self=node.right, key=key)
> 
> The above add() method is part of a BinaryTree(object) class, whose
> subclass is RedBlackTree.
> 
> We need to explicitly call BinaryTree.add() with an explict self, to avoid
> inappropriately calling RedBlackTree.add().; BinaryTree.add() is being
> called with a RedBlackTree instance as self.
> 
> The debugging print and traceback look like:
> node.left is  0 -1 red
> Traceback (most recent call last):
>   File "app_main.py", line 51, in run_toplevel
>   File "test-red_black_tree_mod", line 328, in <module>
>     test()
>   File "test-red_black_tree_mod", line 316, in test
>     all_good &= test_duplicates()
>   File "test-red_black_tree_mod", line 194, in test_duplicates
>     tree.add(value)
>   File
> "/home/dstromberg/src/home-svn/red-black-tree-
mod/trunk/duncan/red_black_bag_mod.py",
> line 919, in add
>     (replaced, node) = super(RedBlackTree, self).add(key=key)
>   File
> "/home/dstromberg/src/home-svn/red-black-tree-
mod/trunk/duncan/red_black_bag_mod.py",
> line 376, in add
>     return BinaryTree.add(self=node.left, key=key)
> TypeError: unbound method add() must be called with BinaryTree instance as
> first argument (got nothing instead)
> 
> Why is it complaining that .add() is getting nothing, when node.left isn't
> None?  As you can see above the traceback, it's got a value represented by
> "node.left is  0 -1 red".
> 
> python 2.x, python 3.x and pypy all give this same error, though jython
> errors out at a different point in the same method.
> 
> Thanks!

I never ran into that, but apparently you cannot pass self as a keyword 
parameter:

>>> class A(object):
...     def add(self): pass
... 
>>> a = A()
>>> A.add(a)
>>> A.add(self=a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method add() must be called with A instance as first 
argument (got nothing instead)