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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.