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6.9.1 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 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 6.1.5 for the `W*()` family of functions.

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`
  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.

`pid`
  The process ID of the child process.