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.
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.
Each TestClient instance establishes its own HTTP connection and initializes dependencies. Utilizing a single TestClient minimizes overhead, resulting in faster test execution.
FastAPI applications often initialize resources, including database connections or background tasks, during startup. Multiple instances can cause redundant initializations or resource conflicts.
<code class="language-python">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"]}</code>
<code class="language-python"># 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</code>
Create your FastAPI app in a separate file (e.g., app.py
) and import it where needed.
<code class="language-python"># app.py from fastapi import FastAPI app = FastAPI() # Add your routes here</code>
<code class="language-python"># main.py from fastapi.testclient import TestClient from app import app client = TestClient(app)</code>
pytest fixtures effectively manage shared resources, such as the TestClient:
<code class="language-python">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</code>
<code class="language-python">def test_example(test_client): response = test_client.get("/items/") assert response.status_code == 200</code>
By adhering to these guidelines, your FastAPI project will be more consistent, efficient, and easier to maintain.
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