mode parameter in popen2-4

[Tobiah]

In the popen docs, a file like object is returned,
and may be read to or written to depending on the
'mode' parameter ('r', or 'w').

Why is the mode parameter needed for popen2 and greater,
when both a stdin and stdout object is returned?  One 
wouldn't want to 'append' to a stream.... is it because
binary mode can still be turned on and off this way?

Thanks,

Tobiah

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