> Firstly, I don't think it makes any sense to set encoding information
> globally for the Popen object. As a simple example, consider using
> Python to write a test suite for the iconv command line tool: there's
> only one Popen instance (for the iconv call), but different encodings
> for stdin and stdout.

Isn't that the exception rather than the rule? I think it actually makes
sense, in at least 99.83% of cases ;-), to have a common encoding
setting for all streams.
(I'm not sure about the "errors" setting, though: should we use strict
for stdin/stdout and backslashreplace for stderr, as the interpreter
does?)

Perhaps the common case should be made extra easy.