msgpack serializer fails on pathlib.PurePath and range objects

[AdeevMardia2008]

Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangGraph rather than my code.
• The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package).
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Related Issues / PRs

#8345

Reproduction Steps / Example Code (Python)

import pathlib
from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer

s = JsonPlusSerializer()

# Fails: PurePosixPath is a PurePath but not a Path
p = pathlib.PurePosixPath("/foo/bar")
s.dumps(p)

# Fails: range has no serializer
r = range(0, 10, 2)
s.dumps(r)

Error Message and Stack Trace (if applicable)

Description

• I'm using LangGraph's msgpack checkpointer to serialize state.
• I expect pathlib.PurePath subclasses (PurePosixPath, PureWindowsPath) and range objects to serialize/deserialize correctly.
• Instead, they raise a serialization error. PurePath subclasses are missed because the check uses pathlib.Path instead of pathlib.PurePath. range objects have no serialization support at all.

System Info

langgraph latest

[shreyas1903]

Hi, I'd like to work on this if a maintainer can approve/assign it (looks like Adeev Mardia (@AdeevMardia2008) 's PR #8345 was auto-closed only because it wasn't yet linked to an approved issue, not because of the fix itself).

The fix is small and contained to _msgpack_default in libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py:

1. PurePath check too narrow — isinstance(obj, pathlib.Path) only matches concrete, instantiable paths (PosixPath/WindowsPath), missing PurePosixPath/PureWindowsPath. Widening to isinstance(obj, pathlib.PurePath) (the abstract base for the whole hierarchy) covers all subtypes without changing behavior for existing Path objects.

2. range has no serializer at all — needs a new branch using the existing EXT_CONSTRUCTOR_POS_ARGS ext mechanism, e.g.:

   elif isinstance(obj, range):
       return ormsgpack.Ext(
           EXT_CONSTRUCTOR_POS_ARGS,
           _msgpack_enc(("builtins", "range", (obj.start, obj.stop, obj.step))),
       )

Deserialization falls out for free since EXT_CONSTRUCTOR_POS_ARGS already reconstructs via getattr(importlib.import_module(module), name)(*args).

Happy to open a PR with both fixes plus round-trip tests for PurePosixPath, PureWindowsPath, and range (including non-default start/stop/step) once this issue is approved. Let me know if that sounds good.

[sergioperezcheco]

Hi! I have a fix ready for this issue — it changes isinstance(obj, pathlib.Path) to isinstance(obj, pathlib.PurePath) in _msgpack_default and adds range serialization with tests. Could I be assigned to submit a PR?