What causes "A Stream Socket was expected, got {sock!r}" in python/cpython?

Create the socket as SOCK_STREAM: socket.socket(socket.AF_INET, socket.SOCK_STREAM).

python/cpython · error · ValueError

A Stream Socket was expected, got {sock!r}

Error message

A Stream Socket was expected, got {sock!r}

What it means

Raised by create_connection's sock branch when the supplied socket is not of type SOCK_STREAM. asyncio's TCP path (and its historical AF_UNIX tolerance) requires a stream socket; datagram, raw, or packet sockets cannot back a streaming transport, so it rejects them with a ValueError naming the socket.

Source

Thrown at Lib/asyncio/base_events.py:1201

                            ', '.join(str(exc) for exc in exceptions)))
                    else:
                        # No exceptions were collected, raise a timeout error
                        raise TimeoutError('create_connection failed')
                finally:
                    exceptions = None

        else:
            if sock is None:
                raise ValueError(
                    'host and port was not specified and no sock specified')
            if sock.type != socket.SOCK_STREAM:
                # We allow AF_INET, AF_INET6, AF_UNIX as long as they
                # are SOCK_STREAM.
                # We support passing AF_UNIX sockets even though we have
                # a dedicated API for that: create_unix_connection.
                # Disallowing AF_UNIX in this method, breaks backwards
                # compatibility.
                raise ValueError(
                    f'A Stream Socket was expected, got {sock!r}')

        transport, protocol = await self._create_connection_transport(
            sock, protocol_factory, ssl, server_hostname,
            ssl_handshake_timeout=ssl_handshake_timeout,
            ssl_shutdown_timeout=ssl_shutdown_timeout)
        if self._debug:
            # Get the socket from the transport because SSL transport closes
            # the old socket and creates a new SSL socket
            sock = transport.get_extra_info('socket')
            logger.debug("%r connected to %s:%r: (%r, %r)",
                         sock, host, port, transport, protocol)
        return transport, protocol

    async def _create_connection_transport(
            self, sock, protocol_factory, ssl,
            server_hostname, server_side=False,
            ssl_handshake_timeout=None,

View on GitHub (pinned to bc6749cc3b)

Solutions

Create the socket as SOCK_STREAM: socket.socket(socket.AF_INET, socket.SOCK_STREAM).
For datagram needs use loop.create_datagram_endpoint instead of create_connection.
For AF_UNIX streams prefer loop.create_unix_connection.

Example fix

// before
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
await loop.create_connection(proto, None, None, sock=sock)

// after
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
await loop.sock_connect(sock, addr)
transport, proto = await loop.create_connection(
    proto, None, None, sock=sock)

Defensive patterns

Strategy: type-guard

Validate before calling

if sock is not None and sock.type != socket.SOCK_STREAM:
    raise ConfigError(f'expected SOCK_STREAM, got type {sock.type}')

Type guard

import socket

def is_stream_socket(sock: socket.socket) -> bool:
    return (sock.type & socket.SOCK_STREAM) == socket.SOCK_STREAM

Prevention

Create sockets with SOCK_STREAM for create_connection; use create_datagram_endpoint for UDP.
Centralize socket creation in one factory so the type is never wrong by accident.
Log sock.type when adopting external sockets to catch mismatches early.

When it happens

Trigger: loop.create_connection(proto, None, None, sock=s) where s was created with socket.SOCK_DGRAM or SOCK_RAW. Also sockets typed with flags that change the effective type check (e.g. SOCK_NONBLOCK is fine, but SOCK_DGRAM is not).

Common situations: Copy-pasted socket setup from a UDP code path; test doubles that fabricate sockets with the wrong type; passing a packet socket intended for a custom protocol into asyncio's stream machinery.

Related errors

AI-assisted analysis of python/cpython@bc6749cc3b (2026-08-14). Data as JSON: /api/errors/529d1664c78493d7. Report an issue: GitHub.