fix(serializer): Fix None serialization without langchain installed (#955)

Co-authored-by: Alexandre Brasseur <34026822+abrasseu@users.noreply.github.com>

Author: hassiebp

langfuse/serializer.py

@@ -78,7 +78,7 @@ def default(self, obj: Any):
                 return str(obj)
 
             # if langchain is not available, the Serializable type is NoneType
-            if Serializable is not None and isinstance(obj, Serializable):
+            if Serializable is not type(None) and isinstance(obj, Serializable):
                 return obj.to_json()
 
             # 64-bit integers might overflow the JavaScript safe integer range.

tests/test_serializer.py

@@ -5,7 +5,9 @@
 from pathlib import Path
 from pydantic import BaseModel
 import json
+import pytest
 import threading
+import langfuse.serializer
 from langfuse.serializer import (
     EventSerializer,
 )
@@ -162,6 +164,12 @@ def test_none():
     assert serializer.encode(None) == "null"
 
 
+def test_none_without_langchain(monkeypatch: pytest.MonkeyPatch):
+    monkeypatch.setattr(langfuse.serializer, "Serializable", type(None), raising=True)
+    serializer = EventSerializer()
+    assert serializer.encode(None) == "null"
+
+
 def test_slots():
     class SlotClass:
         __slots__ = ["field"]