Filtering through an external process

[Paul Rubin]

Anyone know if there's code around to filter text through an external
process?  Sort of like the Emacs "filter-region" command.  For
example, say I have a program that reads input in English and outputs
it in Pig Latin.  I want my Python script to call the program, pipe
some input into it and read the output:

     english = "hello world"
     pig_latin = ext_filter("pig_latin", english) 

should set pig_latin to "ellohay orldway".  

Note that you can't just call popen2, jam the english through it and
then read the pig latin, because the subprocess can block if you give
it too much input before reading the output, and in general there's no
way to know how much buffering the subprocess is willing to do.  So a
proper solution has to use asynchronous i/o and keep polling the
output side, or else separate threads for reading and writing.

This is something that really belongs in the standard library.  I've
needed it several times and rather than going to the trouble of coding
and debugging it, I've always ended up using a temp file instead,
which is a kludge.