Instances of the Popen3 and Popen4 classes have the
Returns -1 if child process hasn't completed yet, or its return
Waits for and returns the status code of the child process. The
status code encodes both the return code of the process and
information about whether it exited using the exit()
system call or died due to a signal. Functions to help interpret the
status code are defined in the os module; see section
14.1.5 for the W*() family of functions.
The following attributes are also available:
A file object that provides output from the child process. For
Popen4 instances, this will provide both the standard output
and standard error streams.
A file object that provides input to the child process.
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