Why You Should Use a Single FastAPI App and TestClient Instance

In FastAPI development, particularly for larger projects, employing a single FastAPI application instance and a single TestClient instance throughout your project is crucial for maintaining consistency, optimizing performance, and ensuring reliability. Let's examine the reasons behind this best practice and explore practical examples.

1. Application-Wide Consistency

Creating multiple FastAPI app instances can introduce inconsistencies. Each instance possesses its own internal state, middleware configuration, and dependency management. Sharing stateful data, such as in-memory storage or database connections, across multiple instances can lead to unpredictable behavior and errors.

2. Enhanced Performance

Each TestClient instance establishes its own HTTP connection and initializes dependencies. Utilizing a single TestClient minimizes overhead, resulting in faster test execution.

3. Preventing Initialization Problems

FastAPI applications often initialize resources, including database connections or background tasks, during startup. Multiple instances can cause redundant initializations or resource conflicts.

Hands-On Code Example

Correct Approach: Single App and TestClient

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient

# Single FastAPI app instance
app = FastAPI()

# Simple in-memory database
database = {"items": []}

# Dependency function
def get_database():
    return database

@app.post("/items/")
def create_item(item: str, db: dict = Depends(get_database)):
    db["items"].append(item)
    return {"message": f"Item '{item}' added."}

@app.get("/items/")
def list_items(db: dict = Depends(get_database)):
    return {"items": db["items"]}

# Single TestClient instance
client = TestClient(app)

# Test functions
def test_create_item():
    response = client.post("/items/", json={"item": "foo"})
    assert response.status_code == 200
    assert response.json() == {"message": "Item 'foo' added."}

def test_list_items():
    response = client.get("/items/")
    assert response.status_code == 200
    assert response.json() == {"items": ["foo"]}

Incorrect Approach: Multiple Instances

# Incorrect: Multiple app instances
app1 = FastAPI()
app2 = FastAPI()

# Incorrect: Multiple TestClient instances
client1 = TestClient(app1)
client2 = TestClient(app2)

# Problem: State changes in client1 won't affect client2

Common Problems with Multiple Instances

1. Inconsistent State: Shared state (like a database) behaves independently across different app instances.
2. Redundant Dependency Initialization: Dependencies such as database connections might be initialized multiple times, potentially leading to resource depletion.
3. Overlapping Startup/Shutdown Events: Multiple app instances trigger startup and shutdown events independently, causing unnecessary or conflicting behavior.

Best Practices

Project Structure for Reusability

Create your FastAPI app in a separate file (e.g., app.py) and import it where needed.

# app.py
from fastapi import FastAPI

app = FastAPI()
# Add your routes here

# main.py
from fastapi.testclient import TestClient
from app import app

client = TestClient(app)

Leveraging pytest Fixtures for Shared Instances

pytest fixtures effectively manage shared resources, such as the TestClient:

import pytest
from fastapi.testclient import TestClient
from app import app

@pytest.fixture(scope="module")
def test_client():
    client = TestClient(app)
    yield client  # Ensures proper cleanup

def test_example(test_client):
    response = test_client.get("/items/")
    assert response.status_code == 200

Relevant Documentation

• Starlette TestClient
• Testing with FastAPI
• pytest Fixtures

By adhering to these guidelines, your FastAPI project will be more consistent, efficient, and easier to maintain.

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