Not serializing lambdas in ParsingSettings.parse_pdf serializer, with…

… a test

src/paperqa/settings.py

@@ -320,6 +320,7 @@ def _custom_serializer(
             info.mode == "json"
             and hasattr(self.parse_pdf, "__module__")
             and hasattr(self.parse_pdf, "__name__")
+            and self.parse_pdf.__name__ != "<lambda>"
         ):
             # If going to JSON, and we can get a FQN, do so for JSON compliance
             data["parse_pdf"] = f"{self.parse_pdf.__module__}.{self.parse_pdf.__name__}"

tests/test_paperqa.py

@@ -3291,6 +3291,9 @@ async def test_timeout_resilience() -> None:
     assert not llm_results
 
 
+TEST_STUB_LAMBDA = lambda: 1  # noqa: E731
+
+
 def test_parse_pdf_string_resolution() -> None:
     # Test with a valid string FQN
     pymupdf_str = Settings(
@@ -3314,7 +3317,7 @@ def test_parse_pdf_string_resolution() -> None:
     )
     assert "parse_pdf" not in pypdf_str.model_dump()["parsing"]
 
-    # Test directly passing a parser
+    # Test directly passing a normal parser
     pymupdf_fn = Settings(parsing=ParsingSettings(parse_pdf=pymupdf_parse_pdf_to_pages))
     assert pymupdf_fn.parsing.parse_pdf == pymupdf_parse_pdf_to_pages
     assert (
@@ -3323,6 +3326,12 @@ def test_parse_pdf_string_resolution() -> None:
     )
     assert "parse_pdf" not in pymupdf_fn.model_dump()["parsing"]
 
+    # Test directly passing a lambda parser
+    lambda_fn = Settings(parsing=ParsingSettings(parse_pdf=TEST_STUB_LAMBDA))
+    assert lambda_fn.parsing.parse_pdf == TEST_STUB_LAMBDA
+    assert "parse_pdf" not in lambda_fn.model_dump(mode="json")["parsing"]
+    assert "parse_pdf" not in lambda_fn.model_dump()["parsing"]
+
     # Test a nonexistent FQN
     with pytest.raises(ValueError, match="Failed to locate"):
         Settings(parsing=ParsingSettings(parse_pdf="nonexistent.module.function"))