#!/usr/bin/env python
from __future__ import with_statement

from contextlib import contextmanager

@contextmanager
def lock():
    yield

class Good(list):
    def __iter__(self):
        it = list.__iter__(self)
        while True:
            try:
                with lock():
                    try:
                        obj = it.next()
                    except StopIteration:
                        print "inner StopIteration"
                        raise
            except StopIteration:
                print "outer StopIteration"
                raise
            yield obj

class Bad(list):
    def __iter__(self):
        it = list.__iter__(self)
        while True:
            try:
                with lock():
                    #try:
                        obj = it.next()
                    #except StopIteration:
                        #print "inner StopIteration"
                        #raise
            except StopIteration:
                print "outer StopIteration"
                raise
            yield obj

print list(Good(range(5)))
print list(Bad(range(5)))  # Never finishes