def coerce(self, args):
        """Return tuple of ints (year, month, day)."""
        if tuple(args) == ("", "", "") and self.allowNone:
            return None

        try:
            year, month, day = map(positiveInt, args)
        except ValueError:
            raise InputError("Invalid date")
        if (month, day) == (2, 29):
            if not calendar.isleap(year):
                raise InputError("%d was not a leap year" % year)
            else:
                return year, month, day
        try:
            mdays = calendar.mdays[month]
        except IndexError:
            raise InputError("Invalid date")
        if day > mdays:
            raise InputError("Invalid date")
        return year, month, day
Return tuple of ints (year, month, day).
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/formmethod.py