Watch for dict item setting in python

tracer dict

class TracerDict(object):
    """Watches changes on a dict for the keys passed in"""
    def __init__(self, check_keys):
        self.check_keys = check_keys
        self._dict = {}

    def __setitem__(self, key, value):
        if key in self.check_keys:
            import pdb; pdb.set_trace()
        self._dict[key] = value

    def __getitem__(self, key):
        return self._dict[key]