Testing Prediction Endpoints

Testing endpoints that perform machine learning predictions requires specific strategies past standard API logic validation. TestClient is often used for general endpoint testing, but prediction endpoints involve external dependencies like loaded models and potentially complex inference logic, making direct testing sometimes impractical or slow. Techniques are presented to effectively unit and integration test ML prediction endpoints.

The primary goal when testing prediction endpoints is not typically to re-validate the model's accuracy (which is usually handled during the ML training and evaluation phases) but rather to ensure the API surrounding the model works correctly. We want to verify:

Input Handling: Does the endpoint correctly receive, parse, and validate input data using the defined Pydantic models?
Integration with Prediction Logic: Does the endpoint correctly call the underlying function or method responsible for making predictions?
Output Formatting: Does the endpoint return the prediction results in the expected format, adhering to the response model?
Error Handling: Does the endpoint gracefully handle errors during prediction (e.g., invalid input shapes, unexpected model behavior)?

Using TestClient for Prediction Endpoints

As with other endpoints, FastAPI's TestClient is the primary tool for testing prediction endpoints. You simulate HTTP requests (e.g., POST requests with prediction input) and assert the expected HTTP status codes and response bodies.

# Example structure (assuming pytest and TestClient fixture 'client')

from fastapi import status
from pydantic import BaseModel

# Assume these are defined elsewhere
# from my_app.schemas import PredictionInput, PredictionOutput
# from my_app.main import app

class PredictionInput(BaseModel):
    feature1: float
    feature2: str

class PredictionOutput(BaseModel):
    prediction: float
    probability: float | None = None

# In your test file (e.g., test_predictions.py)
def test_predict_endpoint_success(client):
    # Define valid input data based on PredictionInput schema
    input_data = {"feature1": 10.5, "feature2": "categoryA"}

    response = client.post("/predict", json=input_data)

    assert response.status_code == status.HTTP_200_OK

    # Assuming the endpoint returns data matching PredictionOutput
    response_data = response.json()
    assert "prediction" in response_data
    # Further assertions based on expected output structure...
    # For example, check type:
    assert isinstance(response_data["prediction"], float)

def test_predict_endpoint_invalid_input(client):
    # Input data missing a required field
    invalid_input_data = {"feature1": 10.5}

    response = client.post("/predict", json=invalid_input_data)

    # FastAPI/Pydantic automatically handle validation errors
    assert response.status_code == status.HTTP_422_UNPROCESSABLE_ENTITY

Handling Model Dependencies in Tests

The main challenge arises from the ML model itself. Loading and running a real model during unit tests can be slow, resource-intensive, and introduce external factors (like file paths) that complicate testing. We need ways to isolate the API logic from the actual model inference for faster, more reliable unit tests.

Mocking the Prediction Logic

Mocking involves replacing the actual prediction function or model object with a substitute (a "mock") during the test run. This mock can be programmed to return predefined outputs, allowing you to test the API's behavior without executing the real model inference.

FastAPI's dependency injection system provides an elegant way to achieve this using app.dependency_overrides. You can override the dependency that provides the model or the prediction function itself.

Let's assume your prediction endpoint uses a dependency to get the prediction result:

# In your application (e.g., my_app/predictor.py)
def get_model():
    # Logic to load your actual ML model
    # ...
    # return loaded_model
    pass # Placeholder

def perform_prediction(data: PredictionInput, model = Depends(get_model)):
    # Actual prediction logic using the loaded model
    # prediction_result = model.predict(processed_data)
    # return {"prediction": prediction_result, "probability": 0.95} # Example
    # For demonstration, return a fixed structure
    return {"prediction": data.feature1 * 2, "probability": 0.9}

# In your main FastAPI file (e.g., my_app/main.py)
from fastapi import FastAPI, Depends
from .schemas import PredictionInput, PredictionOutput
from .predictor import perform_prediction

app = FastAPI()

@app.post("/predict", response_model=PredictionOutput)
async def predict(data: PredictionInput, result: dict = Depends(perform_prediction)):
    # The dependency injection handles calling perform_prediction
    # Note: Changed perform_prediction to return a dict directly for simplicity here
    # A more approach might involve a class-based dependency
    return result

Now, in your test file, you can override the perform_prediction dependency:

# In your test file (e.g., test_predictions.py)
from fastapi.testclient import TestClient
from my_app.main import app # Import your FastAPI app instance
from my_app.predictor import perform_prediction # Import the original dependency
from my_app.schemas import PredictionInput, PredictionOutput

# Create a mock prediction function for testing
async def mock_perform_prediction(data: PredictionInput):
    # Simulate prediction logic - return a fixed, known result
    # You can add assertions here about 'data' if needed
    print(f"Mock prediction called with: {data}")
    return {"prediction": 123.45, "probability": 0.88}

# Use FastAPI's dependency override feature
app.dependency_overrides[perform_prediction] = mock_perform_prediction

client = TestClient(app) # Create TestClient *after* overriding

def test_predict_with_mock(client):
    input_data = {"feature1": 10.5, "feature2": "categoryA"}
    response = client.post("/predict", json=input_data)

    assert response.status_code == 200
    response_data = response.json()
    # Assert against the output defined in the mock function
    assert response_data["prediction"] == 123.45
    assert response_data["probability"] == 0.88

# Remember to clear the override if other tests need the original dependency
# This is often handled better with pytest fixtures (see below)
def teardown_function(): # Example using pytest teardown
    app.dependency_overrides.clear()

Using pytest Fixtures for Cleaner Overrides:

pytest fixtures provide a cleaner way to manage setup and teardown, including dependency overrides:

# In conftest.py or your test file
import pytest
from fastapi.testclient import TestClient
from my_app.main import app
from my_app.predictor import perform_prediction
from my_app.schemas import PredictionInput

@pytest.fixture(scope="function") # Scope can be adjusted
def client_with_mock_predictor():
    # Define the mock function inside the fixture
    async def mock_perform_prediction_fixture(data: PredictionInput):
        return {"prediction": 123.45, "probability": 0.88}

    # Apply the override
    app.dependency_overrides[perform_prediction] = mock_perform_prediction_fixture
    # Yield the TestClient
    yield TestClient(app)
    # Teardown: Clear the override after the test using this fixture finishes
    app.dependency_overrides.clear()

# In your test file (e.g., test_predictions.py)
def test_predict_with_fixture(client_with_mock_predictor): # Use the fixture
    input_data = {"feature1": 10.5, "feature2": "categoryA"}
    # Use the client provided by the fixture
    response = client_with_mock_predictor.post("/predict", json=input_data)

    assert response.status_code == 200
    response_data = response.json()
    assert response_data["prediction"] == 123.45
    assert response_data["probability"] == 0.88

Pros of Mocking:

Speed: Tests run very quickly as no real model inference occurs.
Isolation: Tests focus solely on the API layer, independent of model behavior or loading issues.
Control: You precisely control the output of the mocked function for predictable assertions.

Cons of Mocking:

Doesn't test integration: It doesn't verify that the actual model loading and prediction function work correctly when called by the API.
Maintenance: If the signature or behavior of the real function changes, the mock might need updating.

Using a Dummy Model

An alternative to mocking is to use a very simple, fast "dummy" model during tests. This dummy model should mimic the interface (e.g., predict method) of your real model but perform a trivial operation.

You can achieve this using dependency injection, similar to mocking, but instead of mocking the prediction function, you might override the dependency that loads the model (get_model in our earlier example).

# In your test setup (e.g., conftest.py or test file)

class DummyModel:
    """A simple model stand-in for testing."""
    def predict(self, input_data):
        # Simple logic, e.g., return a fixed value or based on input length
        print("DummyModel predict called")
        return [sum(input_data.values())] # Example dummy prediction

    def predict_proba(self, input_data):
        # Dummy probabilities
        return [[0.1, 0.9]] # Example

def get_dummy_model():
    print("Providing DummyModel")
    return DummyModel()

# In your pytest fixture or test setup
@pytest.fixture(scope="function")
def client_with_dummy_model():
    # Assume get_model is the dependency used to load the model
    # from my_app.predictor import get_model # Import the original dependency
    # app.dependency_overrides[get_model] = get_dummy_model # Override it

    # If perform_prediction directly uses the model:
    # You might need to structure your dependency differently,
    # e.g., have perform_prediction accept the model via Depends
    # async def perform_prediction(data: Input, model = Depends(get_model)): ...

    # For demonstration, let's assume we adjust perform_prediction to use the model from get_model
    # This part requires adapting your actual app structure
    # app.dependency_overrides[get_model] = get_dummy_model # Example override

    yield TestClient(app) # Assuming override is applied correctly
    app.dependency_overrides.clear() # Cleanup

# In your test file
# def test_predict_with_dummy_model(client_with_dummy_model):
#     # ... perform test using the client ...
#     # Assertions will depend on the dummy model's behavior

Note: The exact implementation depends heavily on how your model is loaded and accessed within your endpoint logic. Ensure your dependencies are structured to allow overriding the model provider.

Pros of Dummy Model:

Tests Integration Path: Verifies more of the path, including the code that calls the model's methods.
Simpler Mock Logic: The dummy model itself contains the simple logic, potentially simplifying the test setup compared to complex mocks.

Cons of Dummy Model:

Still Not the Real Model: Doesn't test interactions with the actual ML model.
Requires Dummy Implementation: You need to create and maintain this simple model mimic.

Structuring Prediction Tests

Regardless of the strategy (mocking or dummy model), structure your tests to cover various scenarios:

Happy Path: Test with valid input data and assert the expected successful response (status code 200) and correct output structure/values based on your mock or dummy model.
Validation Errors: Test with invalid input data (missing fields, incorrect types) and assert the expected error response (status code 422). Pydantic handles this, but testing confirms your schemas are applied.
Edge Cases (if feasible): If your mock or dummy model allows, test inputs that might represent edge cases (e.g., zero values, specific categories) to ensure the surrounding API logic handles them.
Response Schema Adherence: Explicitly check that the keys and data types in the JSON response match your Pydantic response_model.

Testing prediction endpoints effectively involves isolating the API logic from the complexities of the ML model itself during unit tests. Using TestClient combined with dependency overrides to inject mocks or dummy models provides a way to ensure your API behaves correctly, handles inputs and outputs as expected, and integrates properly with the prediction mechanism, creating more reliable ML deployments. Remember to complement these unit/integration tests with separate, dedicated tests for your model's performance and accuracy within your ML workflow.

Was this section helpful?

References

Testing FastAPI Applications, Sebastián Ramírez, Current - Official documentation providing a practical guide to testing FastAPI endpoints using TestClient and managing dependencies.
Fixtures, The pytest Development Team, Current - Official documentation explaining how to use pytest fixtures for setup, teardown, and managing test resources, relevant for dependency overrides.
Designing Machine Learning Systems: An Iterative Process for Production-Ready Applications, Chip Huyen, 2022 (O'Reilly Media) - A comprehensive book covering the full lifecycle of ML systems, including strategies for testing deployed models and managing their dependencies in production environments.