Script can't find input file despite being in the same directory

[Robocop]

I have a simple little script that reads in postscript code, appends
it, then writes it to a new postscript file.  Everything worked fine a
month ago, but after rearranging my directory tree a bit my script
fails to find the base postscript file.

The line in question is:

  for line in fileinput.input(['base.ps']):
    output.write(line)

I'm kind of at a loss as the script is executing in the same directory
as base.ps, yet it can't find it.  I'm relatively new to python
scripting, so i'm expecting it's just something i haven't learned
about python that is causing the problem.  Any suggestions would be
greatly appreciated.