linux - How to check if command line argument is file or not in python?

I want to give input as

%>Python_script_name listed_files

How to check if exist as file or it is some input argument.

Please share the small script, by which I can check if command line argument is valid file or it is a simple variable to be processed.

Just to summarize, I need to process my script so as to take or a single argument at command line.

Please help.

Thanks in advance.