How to use the texttable.FallbackToText function in texttable

def _to_float(cls, x):
        if x is None:
            raise FallbackToText()
        try:
            return float(x)
        except (TypeError, ValueError):
            raise FallbackToText()

FMT = {
            'a':self._fmt_auto,
            'i':self._fmt_int,
            'f':self._fmt_float,
            'e':self._fmt_exp,
            't':self._fmt_text,
            }

        n = self._precision
        dtype = self._dtype[i]
        try:
            if callable(dtype):
                return dtype(x)
            else:
                return FMT[dtype](x, n=n)
        except FallbackToText:
            return self._fmt_text(x)