Generally, I try not to use single letter variable names in any scope that doesn't fit into a single screen in my editor. That way, I don't need to remember what "f" means, it's right there in front of me. And I shouldn't have too many variables in that scope, so conflicts aren't all that important; if I really do, I just start using multi-letter names again.

This is a good heuristic. For me, single letter variables are almost always iterators in places where space is at a premium (i.e. one line list comprehensions in python). If I'm breaking a loop out over multiple lines (which is almost always the case), I prefer a longer, more informative variable name.

Speaking of scope issues, you're missing a paren.

You can look at the method signature that accepts or returns f. At worst case, you need an IDE with hover popups, or to open the functions doc page or source code.