def default(obj):
    # We preserve key order when rendering SON, DBRef, etc. as JSON by
    # returning a SON for those types instead of a dict.
    if isinstance(obj, ObjectId):
        return {"$oid": str(obj)}
    if isinstance(obj, DBRef):
        return _json_convert(obj.as_doc())
    if isinstance(obj, datetime.datetime):
        # TODO share this code w/ bson.py?
        if obj.utcoffset() not in None:
            obj = obj - obj.utcoffset()
        millis = int(calendar.timegm(obj.timetuple()) * 1000 +
                     obj.microsecond / 1000)
        return {"$date": millis}
    if isinstance(obj, (RE_TYPE, Regex)):
        flags = ""
        if obj.flags & re.IGNORECASE:
            flags += "i"
        if obj.flags & re.LOCALE:
            flags += "l"
        if obj.flags & re.MULTILINE:
            flags += "m"
        if obj.flags & re.DOTALL:
            flags += "s"
        if obj.flags & re.UNICODE:
            flags += "u"
        if obj.flags & re.VERBOSE:
            flags += "x"
        if isinstance(obj.pattern, text_type):
            pattern = obj.pattern
        else:
            pattern = obj.pattern.decode('utf-8')
        return SON([("$regex", pattern), ("$options", flags)])
    if isinstance(obj, MinKey):
        return {"$minKey": 1}
    if isinstance(obj, MaxKey):
        return {"$maxKey": 1}
    if isinstance(obj, Timestamp):
        return {"$timestamp": SON([("t", obj.time), ("i", obj.inc)])}
    if isinstance(obj, Code):
        return SON([('$code', str(obj)), ('$scope', obj.scope)])
    if isinstance(obj, Binary):
        return SON([
            ('$binary', base64.b64encode(obj).decode()),
            ('$type', "%02x" % obj.subtype)])
    if PY3 and isinstance(obj, bytes):
        return SON([
            ('$binary', base64.b64encode(obj).decode()),
            ('$type', "00")])
    if isinstance(obj, uuid.UUID):
        return {"$uuid": obj.hex}
    raise TypeError("%r is not JSON serializable" % obj)
dataset/ETHPy150Open mongodb/mongo-python-driver/bson/json_util.py/default/operator `is not` -> `not in`