Set constants _EOF and _INTR.
    
    This avoids doing potentially costly operations on module load.

def _make_eof_intr():
    """Set constants _EOF and _INTR.
    
    This avoids doing potentially costly operations on module load.
    """
    global _EOF, _INTR
    if (_EOF is not None) and (_INTR is not None):
        return

    # inherit EOF and INTR definitions from controlling process.
    try:
        from termios import VEOF, VINTR
        fd = None
        for name in 'stdin', 'stdout':
            stream = getattr(sys, '__%s__' % name, None)
            if stream is None or not hasattr(stream, 'fileno'):
                continue
            try:
                fd = stream.fileno()
            except ValueError:
                continue
        if fd is None:
            # no fd, raise ValueError to fallback on CEOF, CINTR
            raise ValueError("No stream has a fileno")
        intr = ord(termios.tcgetattr(fd)[6][VINTR])
        eof = ord(termios.tcgetattr(fd)[6][VEOF])
    except (ImportError, OSError, IOError, ValueError, termios.error):
        # unless the controlling process is also not a terminal,
        # such as cron(1), or when stdin and stdout are both closed.
        # Fall-back to using CEOF and CINTR. There
        try:
            from termios import CEOF, CINTR
            (intr, eof) = (CINTR, CEOF)
        except ImportError:
            #                         ^C, ^D
            (intr, eof) = (3, 4)
    
    _INTR = _byte(intr)
    _EOF = _byte(eof)