First FastAPI application

Introduction

FastAPI helps you quickly create web apps that can handle requests and send responses. It makes building APIs simple and fast.

You want to create a simple web service that returns data.

You need a quick way to build an API for your app or project.

You want to learn how web servers work with Python.

You want to test ideas by making a small web app.

You want to build a backend that talks to a frontend app.

Syntax

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello World"}

FastAPI() creates the app instance.

@app.get("/") means this function runs when someone visits the home URL with a GET request.

Examples

This example responds to GET requests at /hello with a greeting message.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/hello")
async def say_hello():
    return {"greeting": "Hello FastAPI!"}

This example shows how to get a path parameter from the URL and return it.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

Sample Program

This is the simplest FastAPI app. When you run it and visit the home page, it sends back a JSON message.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello World"}

Important Notes

Run this app with: uvicorn filename:app --reload replacing filename with your Python file name.

Use async def for your route functions to allow FastAPI to handle many requests efficiently.

FastAPI automatically converts your return values to JSON responses.

Summary

FastAPI lets you create web apps with just a few lines of code.

Use decorators like @app.get() to define routes.

Your functions return data that FastAPI sends as JSON to users.

Practice

1. What does the @app.get("/") decorator do in a FastAPI application?

easy

A. It starts the FastAPI server automatically.

B. It imports the FastAPI library into the project.

C. It defines a route that responds to HTTP GET requests at the root URL.

D. It creates a database connection for the app.

Solution

1. Step 1: Understand the decorator role

   The @app.get("/") decorator tells FastAPI to listen for GET requests at the root URL path.

2. Step 2: Connect decorator to route handling

   When a user visits the root URL with a GET request, the function below this decorator runs and returns a response.

3. Final Answer:

   It defines a route that responds to HTTP GET requests at the root URL. -> Option C

4. Quick Check:

   Route decorator = defines GET route [OK]

Hint: Decorators like @app.get define routes for HTTP GET requests [OK]

Common Mistakes:

• Thinking @app.get starts the server
• Confusing route decorators with imports
• Assuming it creates database connections

2. Which of the following is the correct way to create a FastAPI app instance?

easy

A. app = FastAPI.create()

B. app = FastAPI()

C. app = FastAPI

D. app = fastapi()

Solution

1. Step 1: Recall FastAPI app creation syntax

   The FastAPI app is created by calling the FastAPI class with parentheses: FastAPI().

2. Step 2: Check each option for correctness

   app = FastAPI() correctly calls FastAPI(). app = fastapi() uses lowercase which is incorrect. app = FastAPI misses parentheses, so it is a class reference, not an instance. app = FastAPI.create() uses a non-existent method.

3. Final Answer:

   app = FastAPI() -> Option B

4. Quick Check:

   FastAPI instance = FastAPI() [OK]

Hint: Create app with FastAPI() including parentheses [OK]

Common Mistakes:

• Omitting parentheses when creating app
• Using lowercase 'fastapi' instead of 'FastAPI'
• Trying to call a non-existent create() method

3. Given this FastAPI code snippet, what will be the JSON response when accessing /hello?

from fastapi import FastAPI
app = FastAPI()

@app.get("/hello")
def say_hello():
    return {"message": "Hello, FastAPI!"}

medium

A. {"message": "Hello, FastAPI!"}

B. "Hello, FastAPI!"

C. {"say_hello": "Hello, FastAPI!"}

D. 404 Not Found error

Solution

1. Step 1: Identify route and return value

   The route /hello is defined with a GET method. The function returns a dictionary with key "message" and value "Hello, FastAPI!".

2. Step 2: Understand FastAPI response format

   FastAPI automatically converts returned dictionaries to JSON responses. So the response will be JSON with the same key-value pair.

3. Final Answer:

   {"message": "Hello, FastAPI!"} -> Option A

4. Quick Check:

   Returned dict = JSON response [OK]

Hint: Returned dict becomes JSON response automatically [OK]

Common Mistakes:

• Expecting plain string instead of JSON
• Confusing function name as JSON key
• Assuming route does not exist causing 404

4. What is wrong with this FastAPI code?

from fastapi import FastAPI
app = FastAPI()

@app.get("/items")
def read_items():
return {"items": [1, 2, 3]}

medium

A. The import statement is incorrect.

B. The route path should be '/item' not '/items'.

C. FastAPI does not support returning lists in JSON.

D. The function read_items is missing an indentation for the return statement.

Solution

1. Step 1: Check Python syntax indentation

   Python requires the return statement inside the function to be indented. Here, return is not indented, causing a syntax error.

2. Step 2: Verify other parts

   The route path '/items' is valid, returning lists is supported, and the import is correct.

3. Final Answer:

   The function read_items is missing an indentation for the return statement. -> Option D

4. Quick Check:

   Python indentation error = missing indent [OK]

Hint: Indent return inside function to avoid syntax errors [OK]

Common Mistakes:

• Ignoring Python indentation rules
• Thinking route path must be singular
• Believing FastAPI can't return lists

5. You want to create a FastAPI app that returns a greeting with a user name from the URL path, like /greet/Alice. Which code correctly implements this?

hard

A. from fastapi import FastAPI
app = FastAPI()

@app.get("/greet/{name}")
def greet(name: str):
return {"greeting": f"Hello, {name}!"}

B. from fastapi import FastAPI
app = FastAPI()

@app.get("/greet")
def greet(name: str):
return {"greeting": f"Hello, {name}!"}

C. from fastapi import FastAPI
app = FastAPI()

@app.get("/greet/{name}")
def greet():
return {"greeting": "Hello!"}

D. from fastapi import FastAPI
app = FastAPI()

@app.get("/greet/{name}")
def greet(name):
return {"greeting": "Hello, name!"}

Solution

1. Step 1: Use path parameter in route

   The route must include {name} in the path to capture the user name from the URL.

2. Step 2: Define function with typed parameter and return greeting

   The function should accept name: str and return a dictionary with a greeting using an f-string to insert the name.

3. Step 3: Check other options for errors

   from fastapi import FastAPI app = FastAPI() @app.get("/greet") def greet(name: str): return {"greeting": f"Hello, {name}!"} misses the path parameter in the route. from fastapi import FastAPI app = FastAPI() @app.get("/greet/{name}") def greet(): return {"greeting": "Hello!"} does not accept the name parameter. from fastapi import FastAPI app = FastAPI() @app.get("/greet/{name}") def greet(name): return {"greeting": "Hello, name!"} does not use f-string, so it returns the literal text "name".

4. Final Answer:

   Uses path parameter and returns personalized greeting. -> Option A

5. Quick Check:

   Path param + f-string = personalized greeting [OK]

Hint: Use {param} in route and f-string to insert it in response [OK]

Common Mistakes:

• Not including path parameter in route string
• Forgetting to type the function parameter
• Returning literal string instead of formatted string