demo.py

#!/usr/bin/env python
import sys

class IgnoreSubtree(Exception):
    pass

def preorder_traversal(obj):
    try:
        yield obj
    except IgnoreSubtree:
        return
    else:
        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:
                        try:
                            iterator.throw(e)
                        except StopIteration:
                            pass


from nose.tools import *

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

    iterator = preorder_traversal(obj)
    out = []
    for o in iterator:
        out.append(o)
        if o == {'c': 3}:
            try:
                val = iterator.throw(IgnoreSubtree)
            except StopIteration:
                pass
            else:
                out.append(val)

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


def test_ignore_subtree2():
    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}:
            try:
                val = iterator.throw(IgnoreSubtree)
            except StopIteration:
                pass
            else:
                out.append(val)

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