file detection

[micseydel]

but assuming whatever is not int is a file i think is not good enough ... something might succeed with something else.

Why is that not good enough? If it quacks like a duck, treat it like a duck. If it doesn't quack, then just raise an exception.

I also don't understand what you mean about file descriptors. That sounds like C-talk. In Python, it's an object, not a numeric.