How to get leaf nodes of a tree using Python?

In python you can use an internal function to collect the leaf nodes and then return the list of them.

def get_leaf_nodes(self):
    leafs = []
    def _get_leaf_nodes( node):
        if node is not None:
            if len(node.children) == 0:
                leafs.append(node)
            for n in node.children:
                _get_leaf_nodes(n)
    _get_leaf_nodes(self.root)
    return leafs

If you want a more clean OOP approach you can create an extra private method for the collection of leafs:

def get_leaf_nodes(self):
    leafs = []
    self._collect_leaf_nodes(self.root,leafs)
    return leafs

def _collect_leaf_nodes(self, node, leafs):
    if node is not None:
        if len(node.children) == 0:
            leafs.append(node)
        for n in node.children:
            self._collect_leaf_nodes(n, leafs)

This is the way I'd do it in Java.

This method should be enough to get the leaves reachable from any node, if you call it with the root of your tree, you will obtain all of the leaves of the tree:

def get_leaves(node):
    if not node.children:
        yield node

    for child in node.children:
        for leaf in get_leaves(child):
             yield leaf