Test with Swagger
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Test with Swagger

Start the server and test your POST endpoint in the browser

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Writing code and entering commands is only available on desktop. Open this page on a larger screen to complete this chapter.

Start the server with uvicorn

Your Python file defines the API, but it cannot listen for requests on its own. Uvicorn is the server that bridges the network to your FastAPI application.

The command uvicorn main:app --reload does three things:

  • main:app: tells uvicorn to find the app object in main.py.
  • --reload: restarts the server automatically when you save changes to your code.
  • Starts listening on http://127.0.0.1:8000.

Interactive documentation at /docs

FastAPI generates a Swagger UI page at http://127.0.0.1:8000/docs. This page lists every endpoint in your API with a "Try it out" button for each one.

Open /docs in your browser after starting the server. You will see your POST /expenses endpoint listed.

Test the POST endpoint

Click POST /expenses, then click Try it out. Paste this JSON into the request body:

{
  "description": "Lunch",
  "amount": 12.50,
  "category": "food"
}

Click Execute. You should see a 201 response with the expense data plus an id field.

Test validation

Try sending invalid data to see your validation rules in action:

  • Empty description: {"description": "", "amount": 10, "category": "food"} returns a 422 error.
  • Negative amount: {"description": "Test", "amount": -5, "category": "food"} returns a 422 error.
  • Invalid category: {"description": "Test", "amount": 10, "category": "xyz"} returns a 422 error.

Every rule you defined in your Expense model is enforced automatically. You wrote zero validation logic in your endpoint.

Instructions

Start the development server.

  1. Run uvicorn main:app --reload.

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