A file object that provides error output from the child process, if capturestderr was true for the constructor, otherwise None. This will always be None for Popen4 instances.
The process ID of the child process.
Any time you are working with any form of inter-process communication, control flow needs to be carefully thought out. This remains the case with the file objects provided by this module (or the os module equivalents).
When reading output from a child process that writes a lot of data to standard error while the parent is reading from the child’s standard output, a deadlock can occur. A similar situation can occur with other combinations of reads and writes. The essential factors are that more than _PC_PIPE_BUF bytes are being written by one process in a blocking fashion, while the other process is reading from the first process, also in a blocking fashion.
There are several ways to deal with this situation.
The simplest application change, in many cases, will be to follow this model in the parent process:
import popen2
r, w, e = popen2.popen3('python slave.py')
e.readlines()
r.readlines()
r.close()
e.close()
w.close()
with code like this in the child:
import os
import sys
# note that each of these print statements
# writes a single long string
print >>sys.stderr, 400 * 'this is a test\n'
os.close(sys.stderr.fileno())