Flatten nested dictionary

flatten.py

def flatten(dictionary, prefix = list(), sep='.'):
    """
    Trivial function designed to make nested JSON suitable for
    writing as CSV file.

    >>> flatten(
      {
        'foo': 'bar',
        'buz': {
          'foo1': 1,
          'foo2: {
            'a': True,
            'b': False
          }
        }
      }
    )
    {
      'foo': 'bar',
      'buz.foo1': 1,
      'buz.foo2.a': True,
      'buz.foo2.b': False
    }

    :param dictionary: incoming dictionary
    :param prefix: prefix of key in flattened dictionary, used for recursive calls
    :param sep: used to separate keys of different levels
    :return: dictionary with flat structure
    """
    result = dict()
    for key, value in dictionary.items():
        if isinstance(value, dict):
            result.update(flatten(value, prefix + [key]))
        else:
            result[sep.join(prefix + [key])] = value
    return result