How can I know how much to read from a subprocess

[Lawrence D'Oliveiro]

In message <1190070896.332844.23080 at 22g2000hsm.googlegroups.com>, 
spam.noam at gmail.com wrote:

> ... how can I know if the process wrote something to its
> output, and how much it wrote?

Others have mentioned using one-byte reads. To tell if something is
available for reading, use select
<http://docs.python.org/lib/module-select.html>.