@r.david.murray: did you try running my test?  I think it shows that we are pretty darned close to fully portable.  I believe we could fix Popen to make it fully portable pretty easily.  In fact, there may be a pure-python fix.  Documenting the differences would also not be hard.  I would discourage you from relying *solely* on a description such as "uses execvpe on POSIX" to describe the semantics.  Aside from being a nonportable description, it doesn't help anyone who isn't familiar with the POSIX system calls.