python-sdk/src/mcp/shared/session.py at main · PawitRoy/python-sdk · GitHub

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import logging

from collections.abc import Callable

from contextlib import AsyncExitStack

from datetime import timedelta

from types import TracebackType

from typing import Any, Generic, TypeVar

import anyio

import anyio.lowlevel

import httpx

from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream

from pydantic import BaseModel

from typing_extensions import Self

from mcp.shared.exceptions import McpError

from mcp.types import (

CancelledNotification,

ClientNotification,

ClientRequest,

ClientResult,

ErrorData,

JSONRPCError,

JSONRPCMessage,

JSONRPCNotification,

JSONRPCRequest,

JSONRPCResponse,

RequestParams,

ServerNotification,

ServerRequest,

ServerResult,

)

SendRequestT = TypeVar("SendRequestT", ClientRequest, ServerRequest)

SendResultT = TypeVar("SendResultT", ClientResult, ServerResult)

SendNotificationT = TypeVar("SendNotificationT", ClientNotification, ServerNotification)

ReceiveRequestT = TypeVar("ReceiveRequestT", ClientRequest, ServerRequest)

ReceiveResultT = TypeVar("ReceiveResultT", bound=BaseModel)

ReceiveNotificationT = TypeVar(

"ReceiveNotificationT", ClientNotification, ServerNotification

)

RequestId = str | int

class RequestResponder(Generic[ReceiveRequestT, SendResultT]):

"""Handles responding to MCP requests and manages request lifecycle.

This class MUST be used as a context manager to ensure proper cleanup and

cancellation handling:

Example:

with request_responder as resp:

await resp.respond(result)

The context manager ensures:

1. Proper cancellation scope setup and cleanup

2. Request completion tracking

3. Cleanup of in-flight requests

"""

def __init__(

self,

request_id: RequestId,

request_meta: RequestParams.Meta | None,

request: ReceiveRequestT,

session: """BaseSession[

SendRequestT,

SendNotificationT,

SendResultT,

ReceiveRequestT,

ReceiveNotificationT

]""",

on_complete: Callable[["RequestResponder[ReceiveRequestT, SendResultT]"], Any],

) -> None:

self.request_id = request_id

self.request_meta = request_meta

self.request = request

self._session = session

self._completed = False

self._cancel_scope = anyio.CancelScope()

self._on_complete = on_complete

self._entered = False # Track if we're in a context manager

def __enter__(self) -> "RequestResponder[ReceiveRequestT, SendResultT]":

"""Enter the context manager, enabling request cancellation tracking."""

self._entered = True

self._cancel_scope = anyio.CancelScope()

self._cancel_scope.__enter__()

return self

def __exit__(

self,

exc_type: type[BaseException] | None,

exc_val: BaseException | None,

exc_tb: TracebackType | None,

) -> None:

"""Exit the context manager, performing cleanup and notifying completion."""

try:

if self._completed:

self._on_complete(self)

finally:

self._entered = False

if not self._cancel_scope:

raise RuntimeError("No active cancel scope")

self._cancel_scope.__exit__(exc_type, exc_val, exc_tb)

async def respond(self, response: SendResultT | ErrorData) -> None:

"""Send a response for this request.

Must be called within a context manager block.

Raises:

RuntimeError: If not used within a context manager

AssertionError: If request was already responded to

"""

if not self._entered:

raise RuntimeError("RequestResponder must be used as a context manager")

assert not self._completed, "Request already responded to"

if not self.cancelled:

self._completed = True

await self._session._send_response( # type: ignore[reportPrivateUsage]

request_id=self.request_id, response=response

)

async def cancel(self) -> None:

"""Cancel this request and mark it as completed."""

if not self._entered:

raise RuntimeError("RequestResponder must be used as a context manager")

if not self._cancel_scope:

raise RuntimeError("No active cancel scope")

self._cancel_scope.cancel()

self._completed = True # Mark as completed so it's removed from in_flight

# Send an error response to indicate cancellation

await self._session._send_response( # type: ignore[reportPrivateUsage]

request_id=self.request_id,

response=ErrorData(code=0, message="Request cancelled", data=None),

)

@property

def in_flight(self) -> bool:

return not self._completed and not self.cancelled

@property

def cancelled(self) -> bool:

return self._cancel_scope.cancel_called

class BaseSession(

Generic[

SendRequestT,

SendNotificationT,

SendResultT,

ReceiveRequestT,

ReceiveNotificationT,

"""

Implements an MCP "session" on top of read/write streams, including features

like request/response linking, notifications, and progress.

This class is an async context manager that automatically starts processing

messages when entered.

"""

_response_streams: dict[

RequestId, MemoryObjectSendStream[JSONRPCResponse | JSONRPCError]

]

_request_id: int

_in_flight: dict[RequestId, RequestResponder[ReceiveRequestT, SendResultT]]

def __init__(

self,

read_stream: MemoryObjectReceiveStream[JSONRPCMessage | Exception],

write_stream: MemoryObjectSendStream[JSONRPCMessage],

receive_request_type: type[ReceiveRequestT],

receive_notification_type: type[ReceiveNotificationT],

# If none, reading will never time out

read_timeout_seconds: timedelta | None = None,

) -> None:

self._read_stream = read_stream

self._write_stream = write_stream

self._response_streams = {}

self._request_id = 0

self._receive_request_type = receive_request_type

self._receive_notification_type = receive_notification_type

self._read_timeout_seconds = read_timeout_seconds

self._in_flight = {}

self._exit_stack = AsyncExitStack()

async def __aenter__(self) -> Self:

self._task_group = anyio.create_task_group()

await self._task_group.__aenter__()

self._task_group.start_soon(self._receive_loop)

return self

async def __aexit__(

self,

exc_type: type[BaseException] | None,

exc_val: BaseException | None,

exc_tb: TracebackType | None,

) -> bool | None:

await self._exit_stack.aclose()

# Using BaseSession as a context manager should not block on exit (this

# would be very surprising behavior), so make sure to cancel the tasks

# in the task group.

self._task_group.cancel_scope.cancel()

return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)

async def send_request(

self,

request: SendRequestT,

result_type: type[ReceiveResultT],

) -> ReceiveResultT:

"""

Sends a request and wait for a response. Raises an McpError if the

response contains an error.

Do not use this method to emit notifications! Use send_notification()

instead.

"""

request_id = self._request_id

self._request_id = request_id + 1

response_stream, response_stream_reader = anyio.create_memory_object_stream[

JSONRPCResponse | JSONRPCError

](1)

self._response_streams[request_id] = response_stream

self._exit_stack.push_async_callback(lambda: response_stream.aclose())

self._exit_stack.push_async_callback(lambda: response_stream_reader.aclose())

jsonrpc_request = JSONRPCRequest(

jsonrpc="2.0",

id=request_id,

**request.model_dump(by_alias=True, mode="json", exclude_none=True),

)

# TODO: Support progress callbacks

await self._write_stream.send(JSONRPCMessage(jsonrpc_request))

try:

with anyio.fail_after(

None

if self._read_timeout_seconds is None

else self._read_timeout_seconds.total_seconds()

response_or_error = await response_stream_reader.receive()

except TimeoutError:

raise McpError(

ErrorData(

code=httpx.codes.REQUEST_TIMEOUT,

message=(

f"Timed out while waiting for response to "

f"{request.__class__.__name__}. Waited "

f"{self._read_timeout_seconds} seconds."

)

if isinstance(response_or_error, JSONRPCError):

raise McpError(response_or_error.error)

else:

return result_type.model_validate(response_or_error.result)

async def send_notification(self, notification: SendNotificationT) -> None:

"""

Emits a notification, which is a one-way message that does not expect

a response.

"""

jsonrpc_notification = JSONRPCNotification(

jsonrpc="2.0",

**notification.model_dump(by_alias=True, mode="json", exclude_none=True),

)

await self._write_stream.send(JSONRPCMessage(jsonrpc_notification))

async def _send_response(

self, request_id: RequestId, response: SendResultT | ErrorData

) -> None:

if isinstance(response, ErrorData):

jsonrpc_error = JSONRPCError(jsonrpc="2.0", id=request_id, error=response)

await self._write_stream.send(JSONRPCMessage(jsonrpc_error))

else:

jsonrpc_response = JSONRPCResponse(

jsonrpc="2.0",

id=request_id,

result=response.model_dump(

by_alias=True, mode="json", exclude_none=True

)

await self._write_stream.send(JSONRPCMessage(jsonrpc_response))

async def _receive_loop(self) -> None:

async with (

self._read_stream,

self._write_stream,

async for message in self._read_stream:

if isinstance(message, Exception):

await self._handle_incoming(message)

elif isinstance(message.root, JSONRPCRequest):

validated_request = self._receive_request_type.model_validate(

message.root.model_dump(

by_alias=True, mode="json", exclude_none=True

)

responder = RequestResponder(

request_id=message.root.id,

request_meta=validated_request.root.params.meta

if validated_request.root.params

else None,

request=validated_request,

session=self,

on_complete=lambda r: self._in_flight.pop(r.request_id, None),

)

self._in_flight[responder.request_id] = responder

await self._received_request(responder)

if not responder._completed: # type: ignore[reportPrivateUsage]

await self._handle_incoming(responder)

elif isinstance(message.root, JSONRPCNotification):

try:

notification = self._receive_notification_type.model_validate(

message.root.model_dump(

by_alias=True, mode="json", exclude_none=True

)

# Handle cancellation notifications

if isinstance(notification.root, CancelledNotification):

cancelled_id = notification.root.params.requestId

if cancelled_id in self._in_flight:

await self._in_flight[cancelled_id].cancel()

else:

await self._received_notification(notification)

await self._handle_incoming(notification)

except Exception as e:

# For other validation errors, log and continue

logging.warning(

f"Failed to validate notification: {e}. "

f"Message was: {message.root}"

)

else: # Response or error

stream = self._response_streams.pop(message.root.id, None)

if stream:

await stream.send(message.root)

else:

await self._handle_incoming(

RuntimeError(

"Received response with an unknown "

f"request ID: {message}"

)

async def _received_request(

self, responder: RequestResponder[ReceiveRequestT, SendResultT]

) -> None:

"""

Can be overridden by subclasses to handle a request without needing to

listen on the message stream.

If the request is responded to within this method, it will not be

forwarded on to the message stream.

"""

async def _received_notification(self, notification: ReceiveNotificationT) -> None:

"""

Can be overridden by subclasses to handle a notification without needing

to listen on the message stream.

"""

async def send_progress_notification(

self, progress_token: str | int, progress: float, total: float | None = None

) -> None:

"""

Sends a progress notification for a request that is currently being

processed.

"""

async def _handle_incoming(

self,

req: RequestResponder[ReceiveRequestT, SendResultT]

| ReceiveNotificationT

| Exception,

) -> None:

"""A generic handler for incoming messages. Overwritten by subclasses."""

pass

python-sdk/src/mcp/shared/session.py at main · PawitRoy/python-sdk · GitHub

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