def prefix_dict_keys(dictionary, prefix, delimiter="/"):
    """Return a copy of a dictionary with a prefix added to every key.

    Parameters:
        dictionary (dict): Input dictionary.
        prefix (str): Prefix to add to every key.
        delimiter (str): String used to separate prefix and original key.

    Returns:
        dict

    Example:
        >>> prefix_dict_keys({'bar': 42}, prefix='foo', delimiter='.')
        {'foo.bar': 42}

    """
    return {delimiter.join((prefix, key)): val for key, val in dictionary.items()}