6.8.1 Popen3 and Popen4 Objects 
Instances of the Popen3 and Popen4 classes have the
following methods:
- poll ()
 - 
Returns 
-1 if child process hasn't completed yet, or its return 
code otherwise.
 
- wait ()
 - 
Waits for and returns the return code of the child process.
 
The following attributes are also available: 
- fromchild
 - 
A file object that provides output from the child process.  For
Popen4 instances, this will provide both the standard output
and standard error streams.
 
- tochild
 - 
A file object that provides input to the child process.
 
- childerr
 - 
Where the standard error from the child process goes is
capturestderr was true for the constructor, or 
None.
This will always be None for Popen4 instances.
 
- pid
 - 
The process ID of the child process.
 
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