demo.py

#!/usr/bin/env python
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)

See http://stackoverflow.com/questions/26221309/preorder-traversal-using-python-generators-with-a-mechanism-to-ignore-subtrees

There's a bug in this code. If there are two adjacent {c: 3} nodes (or whichever ones you want to filter out) in the object, then line 40 needs to throw() another IgnoreSubtree to the iterator. There's code that needs to be duplicated here.

My conclusion from all this is that subtree pruning should be done by passing in a callback, rather than throwing exceptions into the iterator.