Emit ERROR message when app handles exception

When a FastAPI application built from a MLflow model handles an error,
emit a log message at `ERROR` level.

Relates to issue #15

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- Emit error message when an exception is handled by the built application
 
 ## [0.6.2] - 2023-08-23
 ### Fixed

fastapi_mlflow/applications.py

@@ -4,6 +4,7 @@
 Copyright (C) 2022, Auto Trader UK
 
 """
+import logging
 from inspect import signature
 
 from fastapi import FastAPI, Request
@@ -16,6 +17,7 @@
 
 def build_app(pyfunc_model: PyFuncModel) -> FastAPI:
     """Build and return a FastAPI app for the mlflow model."""
+    logger = logging.getLogger(__name__)
     app = FastAPI()
     predictor = build_predictor(pyfunc_model)
     response_model = signature(predictor).return_annotation
@@ -31,6 +33,8 @@ def build_app(pyfunc_model: PyFuncModel) -> FastAPI:
     def handle_serialisable_exception(
         _: Request, exc: DictSerialisableException
     ) -> ORJSONResponse:
+        nonlocal logger
+        logger.exception(exc.message)
         return ORJSONResponse(
             status_code=500,
             content=exc.to_dict(),

tests/conftest.py

@@ -109,10 +109,14 @@ def predict(
 
 
 class ExceptionRaiser(PythonModel):
+    """A PythonModle that always raises an exception."""
+
+    ERROR_MESSAGE = "I always raise an error!"
+
     def predict(
         self, context: PythonModelContext, model_input: pd.DataFrame
     ) -> pd.DataFrame:
-        raise ValueError("I always raise an error!")
+        raise ValueError(self.ERROR_MESSAGE)
 
 
 @pytest.fixture(scope="session")

tests/test_application.py

@@ -12,7 +12,7 @@
 import pytest as pytest
 from fastapi import FastAPI
 from fastapi.testclient import TestClient
-from mlflow.pyfunc import PyFuncModel  # type: ignore
+from mlflow.pyfunc import PyFuncModel, PythonModel  # type: ignore
 
 from fastapi_mlflow.applications import build_app
 
@@ -87,3 +87,22 @@ def test_built_application_handles_model_exceptions(
         "name": "ValueError",
         "message": "I always raise an error!",
     } == response.json()
+
+
+def test_built_application_logs_exceptions(
+    model_input: pd.DataFrame,
+    pyfunc_model_value_error: PyFuncModel,
+    python_model_value_error: PythonModel,
+    caplog: pytest.LogCaptureFixture,
+):
+    app = build_app(pyfunc_model_value_error)
+    client = TestClient(app, raise_server_exceptions=False)
+    df_str = model_input.to_json(orient="records")
+    request_data = f'{{"data": {df_str}}}'
+
+    _ = client.post("/predictions", content=request_data)
+
+    assert len(caplog.records) >= 1
+    log_record = caplog.records[-1]
+    assert log_record.name == "fastapi_mlflow.applications"
+    assert log_record.message == python_model_value_error.ERROR_MESSAGE