demo.py

class IgnoreSubtree(Exception):
    pass

def preorder_traversal(obj):
    try:
        yield obj
    except IgnoreSubtree:
        return

    if isinstance(obj, dict):
        for k, v in obj.iteritems():
            iterator = preorder_traversal(v)
            for o in iterator:
                try:
                    yield o
                except IgnoreSubtree as e:
                  pass


from nose.tools import *

def test_ignore_subtree():
    obj = {'a': {'c': 3, 'd': 4}, 'b': 2}
    eq_([obj, {'c': 3, 'd': 4}, 3, 4, 2],
            list(preorder_traversal(obj)))

    iterator = preorder_traversal(obj)
    out = []
    for o in iterator:
        out.append(o)
        if o == {'c': 3, 'd': 4}:
            iterator.throw(IgnoreSubtree)

    eq_([obj, {'c': 3, 'd': 4}, 2], out)

    # Expected vs. actual:
    # [{'a': {'c': 3, 'd': 4}, 'b': 2}, {'c': 3, 'd': 4}, 2]
    # [{'a': {'c': 3, 'd': 4}, 'b': 2}, {'c': 3, 'd': 4}, 4, 2]

See http://stackoverflow.com/a/26222805/388951