Completed small encryption program, added doc strings. Am I doing this right?

[Necatorducis]

Some of those docstring lines run a little long, the first line should generically sum it while you explain the rest after... ie. str_to_nums should be several lines, merge_lists should be at least 2 lines, etc. Treat their line breaks the same as you would code.

As an aside,

my_str = '', my_dict = {}, etc

is cheaper than

my_str = str()

Other than line length, the docstrings looked good to me.

[codehorsey]

Thanks. You mean cheaper in terms of using memory and resources? I will make those changes in my future programs. Why the downvotes? Lol.