Non-working example of removing a branch in an urwidtree widget

removetest.py

import sys

import urwid
from urwidtrees.tree import Tree
from urwidtrees.widgets import TreeBox
from urwidtrees.decoration import CollapsibleIndentedTree


# define selectable urwid.Text widgets to display paths
class FocusableText(urwid.WidgetWrap):
    """Widget to display paths lines"""
    def __init__(self, path):
        txt = path[-1]
        t = urwid.Text(txt, wrap="clip")
        w = urwid.AttrMap(t, 'body', 'focus')
        urwid.WidgetWrap.__init__(self, w)

    def selectable(self):
        return True

    def keypress(self, size, key):
        return key


def get_path(path, dictlike):
    for step in path:
        dictlike = dictlike[step]
    return dictlike


def del_path(path, dictlike):
    for step in path[:-1]:
        dictlike = dictlike[step]
    del dictlike[path[-1]]


class MyTree(Tree):

    def __init__(self, data):
        self.data = data
        self.root = ("root",)

    def __getitem__(self, pos):
        return FocusableText(pos)

    # helpers

    def _get_children(self, path):
        node = get_path(path, self.data)
        if node and isinstance(node, dict):
            children = node.keys()
            return [path + (child,) for child in sorted(children)]
        return []

    def _get_siblings(self, path):
        parent = self.parent_position(path)
        if parent is not None:
            return self._get_children(parent)
        return [path]

    # UrwidTrees Tree API

    def parent_position(self, path):
        print >>sys.stderr, "parent_position", path
        if path != self.root:
            return path[:-1]

    def first_child_position(self, path):
        try:
            children = self._get_children(path)
        except KeyError:
            return path
        if children:
            return children[0]

    def last_child_position(self, path):
        try:
            children = self._get_children(path)
        except KeyError:
            return path
        if children:
            return children[-1]

    def next_sibling_position(self, path):
        siblings = self._get_siblings(path)
        if path in siblings:
            myindex = siblings.index(path)
            if myindex + 1 < len(siblings):  # path is not the last entry
                return siblings[myindex + 1]

    def prev_sibling_position(self, path):
        siblings = self._get_siblings(path)
        if path in siblings:
            myindex = siblings.index(path)
            if myindex > 0:  # path is not the first entry
                return siblings[myindex - 1]


class MyTreeBox(TreeBox):

    def keypress(self, size, key):
        "Spicing up the keybindings!"
        if key == "delete":
            _, path = self.get_focus()
            self.set_focus(path[:-1])
            del_path(path, treedata)
            self.refresh()
        else:
            return TreeBox.keypress(self, size, key)


palette = [
    ('body', 'black', 'light gray'),
    ('focus', 'light gray', 'dark blue', 'standout'),
    ('bars', 'dark blue', 'light gray', ''),
    ('arrowtip', 'light blue', 'light gray', ''),
    ('connectors', 'light red', 'light gray', ''),
]


treedata = {"root": {"a": {"1": None, "2": None}, "b": None, "c": None}}

if __name__ == "__main__":

    tree = CollapsibleIndentedTree(MyTree(treedata))
    box = MyTreeBox(tree)

    loop = urwid.MainLoop(box, palette)

    loop.run()

your bug is in line 32: you forgot to return dictlike, so the global variable will not be amended.
It is true that the trace is not very telling here, but that's urwids fault i believe :)