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from __future__ import annotations from abc import ABC, abstractmethod from collections.abc import Sequence from typing import Any, Generic, TypeVar from typing_extensions import Self from langgraph._internal._typing import MISSING from langgraph.errors import EmptyChannelError Value = TypeVar("Value") Update = TypeVar("Update") Checkpoint = TypeVar("Checkpoint") __all__ = ("BaseChannel",) class BaseChannel(Generic[Value, Update, Checkpoint], ABC): """Base class for all channels.""" __slots__ = ("key", "typ") def __init__(self, typ: Any, key: str = "") -> None: self.typ = typ self.key = key @property @abstractmethod def ValueType(self) -> Any: """The type of the value stored in the channel.""" @property @abstractmethod def UpdateType(self) -> Any: """The type of the update received by the channel.""" # serialize/deserialize methods def copy(self) -> Self: """Return a copy of the channel. By default, delegates to checkpoint() and from_checkpoint(). Subclasses can override this method with a more efficient implementation.""" return self.from_checkpoint(self.checkpoint()) def checkpoint(self) -> Checkpoint | Any: """Return a serializable representation of the channel's current state. Raises EmptyChannelError if the channel is empty (never updated yet), or doesn't support checkpoints.""" try: return self.get() except EmptyChannelError: return MISSING @abstractmethod def from_checkpoint(self, checkpoint: Checkpoint | Any) -> Self: """Return a new identical channel, optionally initialized from a checkpoint. If the checkpoint contains complex data structures, they should be copied.""" # read methods @abstractmethod def get(self) -> Value: """Return the current value of the channel. Raises EmptyChannelError if the channel is empty (never updated yet).""" def is_available(self) -> bool: """Return True if the channel is available (not empty), False otherwise. Subclasses should override this method to provide a more efficient implementation than calling get() and catching EmptyChannelError. """ try: self.get() return True except EmptyChannelError: return False # write methods @abstractmethod def update(self, values: Sequence[Update]) -> bool: """Update the channel's value with the given sequence of updates. The order of the updates in the sequence is arbitrary. This method is called by Pregel for all channels at the end of each step. If there are no updates, it is called with an empty sequence. Raises InvalidUpdateError if the sequence of updates is invalid. Returns True if the channel was updated, False otherwise.""" def consume(self) -> bool: """Notify the channel that a subscribed task ran. By default, no-op. A channel can use this method to modify its state, preventing the value from being consumed again. Returns True if the channel was updated, False otherwise. """ return False def finish(self) -> bool: """Notify the channel that the Pregel run is finishing. By default, no-op. A channel can use this method to modify its state, preventing finish. Returns True if the channel was updated, False otherwise. """ return False