redis/redis-py · error · ConnectionError

protocol must be either 2 or 3

Error message

protocol must be either 2 or 3

What it means

Raised as ConnectionError in Connection.__init__ after successfully parsing protocol as an int, when the value is outside the supported range (p < 2 or p > 3). redis-py supports only RESP2 and RESP3, so any other integer (0, 1, 4, ...) is rejected.

Source

Thrown at redis/asyncio/connection.py:704

        self.redis_connect_func = redis_connect_func
        self._reader: Optional[asyncio.StreamReader] = None
        self._writer: Optional[asyncio.StreamWriter] = None
        self._socket_read_size = socket_read_size
        self._active_read_timeout = None
        self._connect_callbacks: List[weakref.WeakMethod[ConnectCallbackT]] = []
        self._buffer_cutoff = 6000
        self._re_auth_token: Optional[TokenInterface] = None
        self._should_reconnect = False

        try:
            p = int(protocol)
        except TypeError:
            p = DEFAULT_RESP_VERSION
        except ValueError:
            raise ConnectionError("protocol must be an integer")
        else:
            if p < 2 or p > 3:
                raise ConnectionError("protocol must be either 2 or 3")
        self.protocol = p
        self.legacy_responses = legacy_responses
        if parser_class != _AsyncHiredisParser:
            # The Python parsers are protocol-specific; hiredis supports both.
            if self.protocol == 3 and parser_class == _AsyncRESP2Parser:
                parser_class = _AsyncRESP3Parser
            elif self.protocol == 2 and parser_class == _AsyncRESP3Parser:
                parser_class = _AsyncRESP2Parser
        self.set_parser(parser_class)

        # HIMPORT client-side state. `himport_registry` is the shared client-level
        # registry (empty if unconfigured) and persists across reconnects.
        self.himport_registry = himport_registry
        self._reset_himport_state()

        AsyncMaintNotificationsAbstractConnection.__init__(
            self,
            maint_notifications_config,

View on GitHub (pinned to da03cdc7e8)

Solutions

1. Use protocol=2 (RESP2) or protocol=3 (RESP3) — the only supported values.
2. Omit the argument to accept the library default (currently RESP3).
3. Validate user-supplied config before passing it in (see validationCode).

Example fix

// before
client = Redis(url='...', protocol=30)  # raises [96]
// after
client = Redis(url='...', protocol=3)

if protocol not in (2, 3):
    raise ValueError(f'protocol must be 2 or 3, got {protocol}')

from redis.exceptions import ConnectionError
try:
    client = Redis(protocol=protocol)
except ConnectionError as e:
    if 'protocol must be either 2 or 3' in str(e):
        client = Redis(protocol=3)

When it happens

Trigger: Passing protocol=1, protocol=4, protocol=0, or a negative int to Redis()/Connection(); a computed protocol value that lands outside 2..3.

Common situations: Typos in config (protocol=30), arithmetic that produces a bad value, or copy-paste from docs that used an unsupported version.

Related errors

• protocol must be an integer
• 'username' and 'password' cannot be passed along with 'crede
• Maintenance notifications are only supported with hiredis an
• Invalid RESP version
• Invalid Database

AI-assisted analysis of redis/redis-py@da03cdc7e8 (2026-08-04). Data as JSON: /data/errors/170de481b66b5878.json. Report an issue: GitHub.