import collections

class SafeDict(dict):
    def __missing__(self, key):
        return '{' + key + '}'


# from https://stackoverflow.com/questions/6027558/flatten-nested-python-dictionaries-compressing-keys
def flatten(d, parent_key='', sep='/'):
    items = []
    for k, v in d.items():
        new_key = parent_key + sep + k if parent_key else k
        if isinstance(v, collections.MutableMapping):
            items.extend(flatten(v, new_key, sep=sep).items())
        else:
            items.append((new_key, v))
    return dict(items)