Currently Popen.communicate accept str is universal_newlines==True and bytes if universal_newlines==False.

I like to accept both str and bytes if universal_newlines is False and raise explicit exception for bytes if universal_newlines is True.

Looks like universal_newlines for bytes doesn't make sense, also we prefer to use str everywhere. Bytes should be used if it is really byte-stream or if unicode cannot be passed by some limitations of underlying OS etc.

There are couple dark corners:
1. What to do if stdin doesn't have `encoding` attribute? Convert to bytes using filesystemencoding? Raise explicit exception? Adding `encode` parameter for .communicate doesn't looks elegant.
2. How .communicate works on Windows? I see completely different implementation of this method for win32.

> What to do if stdin doesn't have `encoding` attribute? Convert to bytes using filesystemencoding?

If choosing an encoding, it probably makes sense to default to the same as for when universal_newlines=True, namely locale.getpreferredencoding(False).

> Adding `encode` parameter for .communicate doesn't looks elegant.

Also see issue 6135 (to allow setting encoding).

Uhum.. Universal Newlines is being deprecated.

No it isn't.  There's some issue about a 'U' parameter somewhere that is being deprecated, but that has nothing to do with this issue.

See also issue16903.