You misunderstand: I am not reading $PWD. I need to call a C program as
a subprocess, which is written by a third party and which determines its
current working directory by reading $PWD. os.chdir will not have any
effect on this script, nor will passing "cwd" to "subprocess.call". I
have to write os.environ["PWD"] = os.getcwd() in my code before it will
work.

Not only that, but of course I have to know about $PWD and the fact that
some people use it. Otherwise it just seems like Python isn't correctly
passing on the current working directory.