`recursiveFormat()` – Recursively apply `string.format()` to all strings in a data structure.

recursiveFormat.py

def recursiveFormat(args, **kwargs):
    """
    Recursively apply `string.format()` to all strings in a data structure.

    This is intended to be used on a data structure that may contain
    format strings just before it is passed to `json.dump()` or `dumps()`.
    
    Ideally, I'd like to build this into a subclass of `json.JsonEncoder`,
    but it's tricky to separate out string handling in that class.  I'll
    continue to think about it.

    :param args: data structure; mostly dictionaries and lists
    :param kwargs: values to be used in `string.format()`
    :return: a new data structure with formatted strings
    """
    if isinstance(args, Mapping):
        return {key: recursive_format(value, **kwargs)
                for (key, value) in args.items()}
    elif isinstance(args, MutableSequence):
        return [recursive_format(value, **kwargs)
                for value in args]
    else:
        return args.format(**kwargs)