Update

libs/langgraph/langgraph/channels/any_value.py

@@ -33,7 +33,7 @@ class AnyValue(Generic[Value], BaseChannel[Value, Value, Value]):
     def from_checkpoint(self, checkpoint: Optional[Value]) -> Self:
         empty = self.__class__(self.typ)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             empty.value = checkpoint
         return empty
 

libs/langgraph/langgraph/channels/binop.py

@@ -75,7 +75,7 @@ class BinaryOperatorAggregate(Generic[Value], BaseChannel[Value, Value, Value]):
     def from_checkpoint(self, checkpoint: Optional[Value]) -> Self:
         empty = self.__class__(self.typ, self.operator)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             empty.value = checkpoint
         return empty
 

libs/langgraph/langgraph/channels/dynamic_barrier_value.py

@@ -3,6 +3,7 @@ from typing import Any, Generic, NamedTuple, Optional, Sequence, Type, Union
 from typing_extensions import Self
 
 from langgraph.channels.base import BaseChannel, Value
+from langgraph.constants import MISSING
 from langgraph.errors import EmptyChannelError, InvalidUpdateError
 
 
@@ -54,7 +55,7 @@ class DynamicBarrierValue(
     ) -> Self:
         empty = self.__class__(self.typ)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             names, seen = checkpoint
             empty.names = names if names is not None else None
             empty.seen = seen

libs/langgraph/langgraph/channels/ephemeral_value.py

@@ -33,7 +33,7 @@ class EphemeralValue(Generic[Value], BaseChannel[Value, Value, Value]):
     def from_checkpoint(self, checkpoint: Optional[Value]) -> Self:
         empty = self.__class__(self.typ, self.guard)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             empty.value = checkpoint
         return empty
 

libs/langgraph/langgraph/channels/last_value.py

@@ -37,7 +37,7 @@ class LastValue(Generic[Value], BaseChannel[Value, Value, Value]):
     def from_checkpoint(self, checkpoint: Optional[Value]) -> Self:
         empty = self.__class__(self.typ)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             empty.value = checkpoint
         return empty
 

libs/langgraph/langgraph/channels/named_barrier_value.py

@@ -3,6 +3,7 @@ from typing import Generic, Optional, Sequence, Type
 from typing_extensions import Self
 
 from langgraph.channels.base import BaseChannel, Value
+from langgraph.constants import MISSING
 from langgraph.errors import EmptyChannelError, InvalidUpdateError
 
 
@@ -38,7 +39,7 @@ class NamedBarrierValue(Generic[Value], BaseChannel[Value, Value, set[Value]]):
     def from_checkpoint(self, checkpoint: Optional[set[Value]]) -> Self:
         empty = self.__class__(self.typ, self.names)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             empty.seen = checkpoint
         return empty
 

libs/langgraph/langgraph/channels/topic.py

@@ -3,6 +3,7 @@ from typing import Any, Generic, Iterator, Optional, Sequence, Type, Union
 from typing_extensions import Self
 
 from langgraph.channels.base import BaseChannel, Value
+from langgraph.constants import MISSING
 from langgraph.errors import EmptyChannelError
 
 
@@ -55,7 +56,7 @@ class Topic(
     def from_checkpoint(self, checkpoint: Optional[list[Value]]) -> Self:
         empty = self.__class__(self.typ, self.accumulate)
         empty.key = self.key
-        if checkpoint is not None:
+        if checkpoint is not MISSING:
             if isinstance(checkpoint, tuple):
                 empty.values = checkpoint[1]
             else:

libs/langgraph/langgraph/pregel/manager.py

@@ -4,6 +4,7 @@ from typing import AsyncIterator, Iterator, Mapping, Union
 
 from langgraph.channels.base import BaseChannel
 from langgraph.checkpoint.base import Checkpoint
+from langgraph.constants import MISSING
 from langgraph.managed.base import (
     ConfiguredManagedValue,
     ManagedValueMapping,
@@ -36,7 +37,7 @@ def ChannelsManager(
     with ExitStack() as stack:
         yield (
             {
-                k: v.from_checkpoint(checkpoint["channel_values"].get(k))
+                k: v.from_checkpoint(checkpoint["channel_values"].get(k, MISSING))
                 for k, v in channel_specs.items()
             },
             ManagedValueMapping(
@@ -90,7 +91,7 @@ async def AsyncChannelsManager(
         yield (
             # channels: enter each channel with checkpoint
             {
-                k: v.from_checkpoint(checkpoint["channel_values"].get(k))
+                k: v.from_checkpoint(checkpoint["channel_values"].get(k, MISSING))
                 for k, v in channel_specs.items()
             },
             # managed: build mapping from spec to result

libs/langgraph/tests/test_channels.py

@@ -6,13 +6,14 @@ import pytest
 from langgraph.channels.binop import BinaryOperatorAggregate
 from langgraph.channels.last_value import LastValue
 from langgraph.channels.topic import Topic
+from langgraph.constants import MISSING
 from langgraph.errors import EmptyChannelError, InvalidUpdateError
 
 pytestmark = pytest.mark.anyio
 
 
 def test_last_value() -> None:
-    channel = LastValue(int).from_checkpoint(None)
+    channel = LastValue(int).from_checkpoint(MISSING)
     assert channel.ValueType is int
     assert channel.UpdateType is int
 
@@ -31,7 +32,7 @@ def test_last_value() -> None:
 
 
 def test_topic() -> None:
-    channel = Topic(str).from_checkpoint(None)
+    channel = Topic(str).from_checkpoint(MISSING)
     assert channel.ValueType is Sequence[str]
     assert channel.UpdateType is Union[str, list[str]]
 
@@ -55,7 +56,7 @@ def test_topic() -> None:
 
 
 def test_topic_accumulate() -> None:
-    channel = Topic(str, accumulate=True).from_checkpoint(None)
+    channel = Topic(str, accumulate=True).from_checkpoint(MISSING)
     assert channel.ValueType is Sequence[str]
     assert channel.UpdateType is Union[str, list[str]]
 
@@ -73,7 +74,7 @@ def test_topic_accumulate() -> None:
 
 
 def test_binop() -> None:
-    channel = BinaryOperatorAggregate(int, operator.add).from_checkpoint(None)
+    channel = BinaryOperatorAggregate(int, operator.add).from_checkpoint(MISSING)
     assert channel.ValueType is int
     assert channel.UpdateType is int
 

libs/langgraph/tests/test_pregel.py

@@ -1310,8 +1310,8 @@ def test_pending_writes_resume(
             },
             "channel_values": {
                 "value": 1,
-                "branch:to:one": "__start__",
-                "branch:to:two": "__start__",
+                "branch:to:one": None,
+                "branch:to:two": None,
             },
         },
         metadata={
@@ -1363,8 +1363,8 @@ def test_pending_writes_resume(
         parent_config=None,
         pending_writes=UnsortedSequence(
             (AnyStr(), "value", 1),
-            (AnyStr(), "branch:to:one", "__start__"),
-            (AnyStr(), "branch:to:two", "__start__"),
+            (AnyStr(), "branch:to:one", None),
+            (AnyStr(), "branch:to:two", None),
         ),
     )
 

libs/langgraph/tests/test_pregel_async.py

@@ -2146,8 +2146,8 @@ async def test_pending_writes_resume(
                 },
                 "channel_values": {
                     "value": 1,
-                    "branch:to:one": "__start__",
-                    "branch:to:two": "__start__",
+                    "branch:to:one": None,
+                    "branch:to:two": None,
                 },
             },
             metadata={
@@ -2201,8 +2201,8 @@ async def test_pending_writes_resume(
             parent_config=None,
             pending_writes=UnsortedSequence(
                 (AnyStr(), "value", 1),
-                (AnyStr(), "branch:to:one", "__start__"),
-                (AnyStr(), "branch:to:two", "__start__"),
+                (AnyStr(), "branch:to:one", None),
+                (AnyStr(), "branch:to:two", None),
             ),
         )