First FastAPI application - Step-by-Step Execution

Concept Flow - First FastAPI application

Start FastAPI app

↓

Define route with @app.get('/')

↓

Client sends GET request to '/'

↓

Route function runs

↓

Return response (JSON)

↓

Client receives JSON response

This flow shows how a FastAPI app starts, defines a route, handles a GET request, and returns a JSON response.

Execution Sample

FastAPI

from fastapi import FastAPI
app = FastAPI()

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

This code creates a FastAPI app with one GET route at '/' that returns a JSON message.

Execution Table

Step	Action	Evaluation	Result
1	Start FastAPI app	app = FastAPI()	app instance created
2	Define GET route '/'	@app.get('/')	Route registered
3	Client sends GET request to '/'	Request received	Route function triggered
4	Run route function read_root()	return {"message": "Hello, FastAPI!"}	JSON response prepared
5	Send response to client	Response sent	{"message": "Hello, FastAPI!"} received by client

💡 Request handled and response sent, cycle complete

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
app	undefined	FastAPI instance	Route '/' registered	No change	No change
read_root	undefined	undefined	Function defined	Function returns JSON	No change

Key Moments - 3 Insights

Why do we use @app.get('/') above the function?

The @app.get('/') decorator tells FastAPI which URL path the function handles. See execution_table step 2 where the route is registered.

Why is the route function defined as async?

FastAPI supports async functions for better performance. The function runs asynchronously when the request arrives, as shown in step 4.

What does the function return and how is it sent to the client?

The function returns a Python dictionary. FastAPI converts it to JSON automatically and sends it as the response, shown in steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 3?

A Route function is triggered

B App instance is created

C Response sent to client

D Route '/' registered

💡 Hint

Check the 'Result' column in row for step 3 in execution_table

At which step does the JSON response get prepared?

A Step 2

B Step 1

C Step 4

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for step 4

If we change the route path from '/' to '/hello', which step changes?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Route registration is in step 2, see execution_table

Concept Snapshot

FastAPI app basics:
- Create app: app = FastAPI()
- Define route: @app.get("/")
- Async function returns dict
- FastAPI auto converts dict to JSON
- Client GET request triggers route function
- Response sent as JSON automatically

Full Transcript

This visual trace shows how a simple FastAPI application works. First, we create an app instance with FastAPI(). Then, we define a route using the @app.get decorator for the root path '/'. When a client sends a GET request to '/', FastAPI runs the async function read_root. This function returns a Python dictionary. FastAPI converts this dictionary into JSON and sends it back as the response. The client receives the JSON message. This flow helps beginners see how FastAPI handles requests and responses step-by-step.

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