How do you get sys.stdin filename?

This is probably scripting more than python? I don't know either, so what the hey. I can't get the name of the file that I pass as argument to a python script, say

python myscript.py < "input.para m"


From the script if I do

I get


I was expecting sys.stdin.name to give me "input.para m" instead. How do I get the file name? I'm basically trying to figure out if a file was passed as argument or not, and I want the file name to check if that file exists. If it wasn't passed and I try to read it, it waits for interactive input and I don't want that.

Any ideas?