Avoid raise-catch strategy in BaseChannel.checkpoint()

- This mirrors the work done earlier on BaseChannel.get()
- Comparing to a sentinel value is significantly faster than raising and catching an exception

libs/checkpoint/langgraph/checkpoint/base/__init__.py

@@ -124,6 +124,7 @@ def copy_checkpoint(checkpoint: Checkpoint) -> Checkpoint:
     )
 
 
+# Kept for backwards compat, newer versions of LangGraph no longer use this.
 def create_checkpoint(
     checkpoint: Checkpoint,
     channels: Optional[Mapping[str, ChannelProtocol]],

libs/langgraph/bench/sequential.py

@@ -34,7 +34,7 @@ if __name__ == "__main__":
 
     import uvloop
 
-    graph = create_sequential(2000).compile()
+    graph = create_sequential(3000).compile()
     input = {"messages": []}  # Empty list of messages
     config = {"recursion_limit": 20000000000}
 

libs/langgraph/langgraph/channels/any_value.py

@@ -55,3 +55,6 @@ class AnyValue(Generic[Value], BaseChannel[Value, Value, Value]):
 
     def is_available(self) -> bool:
         return self.value is not MISSING
+
+    def checkpoint(self) -> Value:
+        return self.value

libs/langgraph/langgraph/channels/base.py

@@ -3,6 +3,7 @@ from typing import Any, Generic, Sequence, TypeVar
 
 from typing_extensions import Self
 
+from langgraph.constants import MISSING
 from langgraph.errors import EmptyChannelError, InvalidUpdateError
 
 Value = TypeVar("Value")
@@ -33,7 +34,10 @@ class BaseChannel(Generic[Value, Update, C], ABC):
         """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."""
-        return self.get()
+        try:
+            return self.get()
+        except EmptyChannelError:
+            return MISSING
 
     @abstractmethod
     def from_checkpoint(self, checkpoint: C) -> Self:

libs/langgraph/langgraph/channels/binop.py

@@ -90,3 +90,6 @@ class BinaryOperatorAggregate(Generic[Value], BaseChannel[Value, Value, Value]):
 
     def is_available(self) -> bool:
         return self.value is not MISSING
+
+    def checkpoint(self) -> Value:
+        return self.value

libs/langgraph/langgraph/channels/ephemeral_value.py

@@ -59,3 +59,6 @@ class EphemeralValue(Generic[Value], BaseChannel[Value, Value, Value]):
 
     def is_available(self) -> bool:
         return self.value is not MISSING
+
+    def checkpoint(self) -> Value:
+        return self.value

libs/langgraph/langgraph/channels/last_value.py

@@ -61,3 +61,6 @@ class LastValue(Generic[Value], BaseChannel[Value, Value, Value]):
 
     def is_available(self) -> bool:
         return self.value is not MISSING
+
+    def checkpoint(self) -> Value:
+        return self.value

libs/langgraph/langgraph/channels/untracked_value.py

@@ -31,7 +31,7 @@ class UntrackedValue(Generic[Value], BaseChannel[Value, Value, Value]):
         return self.typ
 
     def checkpoint(self) -> Value:
-        raise EmptyChannelError()
+        return MISSING
 
     def from_checkpoint(self, checkpoint: Value) -> Self:
         empty = self.__class__(self.typ, self.guard)

libs/langgraph/langgraph/pregel/__init__.py

@@ -50,7 +50,6 @@ from langgraph.checkpoint.base import (
     BaseCheckpointSaver,
     CheckpointTuple,
     copy_checkpoint,
-    create_checkpoint,
     empty_checkpoint,
 )
 from langgraph.constants import (
@@ -91,6 +90,7 @@ from langgraph.pregel.algo import (
     local_write,
     prepare_next_tasks,
 )
+from langgraph.pregel.checkpoint import create_checkpoint
 from langgraph.pregel.debug import tasks_w_writes
 from langgraph.pregel.io import map_input, read_channels
 from langgraph.pregel.loop import AsyncPregelLoop, StreamProtocol, SyncPregelLoop

libs/langgraph/langgraph/pregel/checkpoint.py

@@ -0,0 +1,37 @@
+from datetime import datetime, timezone
+from typing import Mapping, Optional
+
+from langgraph.channels.base import BaseChannel
+from langgraph.checkpoint.base import LATEST_VERSION, Checkpoint
+from langgraph.checkpoint.base.id import uuid6
+from langgraph.constants import MISSING
+
+
+def create_checkpoint(
+    checkpoint: Checkpoint,
+    channels: Optional[Mapping[str, BaseChannel]],
+    step: int,
+    *,
+    id: Optional[str] = None,
+) -> Checkpoint:
+    """Create a checkpoint for the given channels."""
+    ts = datetime.now(timezone.utc).isoformat()
+    if channels is None:
+        values = checkpoint["channel_values"]
+    else:
+        values = {}
+        for k in channels:
+            if k not in checkpoint["channel_versions"]:
+                continue
+            v = channels[k].checkpoint()
+            if v is not MISSING:
+                values[k] = v
+    return Checkpoint(
+        v=LATEST_VERSION,
+        ts=ts,
+        id=id or str(uuid6(clock_seq=step)),
+        channel_values=values,
+        channel_versions=checkpoint["channel_versions"],
+        versions_seen=checkpoint["versions_seen"],
+        pending_sends=checkpoint.get("pending_sends", []),
+    )

libs/langgraph/langgraph/pregel/loop.py

@@ -38,7 +38,6 @@ from langgraph.checkpoint.base import (
     CheckpointTuple,
     PendingWrite,
     copy_checkpoint,
-    create_checkpoint,
     empty_checkpoint,
 )
 from langgraph.constants import (
@@ -88,6 +87,7 @@ from langgraph.pregel.algo import (
     should_interrupt,
     task_path_str,
 )
+from langgraph.pregel.checkpoint import create_checkpoint
 from langgraph.pregel.debug import (
     map_debug_checkpoint,
     map_debug_task_results,