##
8.18.1 Popen3 Objects

Instances of the `Popen3` class 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 of `Popen3` objects are also available:

`fromchild`
  A file object that provides output from the child process.

`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`.

`pid`
  The process ID of the child process.