def add_dicts(dict1, dict2):
    """Sum the values of two dictionaries.

    :param dict1:
      Dictionary (possibly empty) containing keys with floating point values.
    :param dict2:
      Dictionary (possibly empty) containing keys with floating point values.
    :returns:
      New dictionary, with all unique keys from dict1/2, and the sum of the 
      values from each key in both dictionaries.
      Example: 
        dict1 = {'W1':60.0,'W2':75.0}
        dict2 = {'W1':40.0,'W2':25.0,'W3':100.0}
        add_dicts(dict1,dict2) => {'W1':100.0,'W2':100.0,'W3':100.0}
    """
    result = {}
    for key, value in dict1.items():
        if key not in result:
            result[key] = value
        else:
            result[key] += value
    for key, value in dict2.items():
        if key not in result:
            result[key] = value
        else:
            result[key] += value
    return result