def cmdify(self, extra_args=None):
        """Encode into a cmd-executable string.

        This re-implements CreateProcess's quoting logic to turn a list of
        arguments into one single string for the shell to interpret.

        * All double quotes are escaped with a backslash.
        * Existing backslashes before a quote are doubled, so they are all
          escaped properly.
        * Backslashes elsewhere are left as-is; cmd will interpret them
          literally.

        The result is then quoted into a pair of double quotes to be grouped.

        An argument is intentionally not quoted if it does not contain
        whitespaces. This is done to be compatible with Windows built-in
        commands that don't work well with quotes, e.g. everything with `echo`,
        and DOS-style (forward slash) switches.

        The intended use of this function is to pre-process an argument list
        before passing it into ``subprocess.Popen(..., shell=True)``.

        See also: https://docs.python.org/3/library/subprocess.html
        """
        parts = list(self._parts)
        if extra_args:
            parts.extend(extra_args)
        return " ".join(
            arg if not next(re.finditer(r'\s', arg), None)
            else '"{0}"'.format(re.sub(r'(\\*)"', r'\1\1\\"', arg))
            for arg in parts
        )