Depends function basics in FastAPI - Mini Project: Build & Apply

Depends function basics

📖 Scenario: You are building a simple FastAPI web app that greets users. You want to use FastAPI's Depends function to inject a dependency that provides a greeting message.

🎯 Goal: Create a FastAPI app with a dependency function that returns a greeting message. Use Depends to inject this greeting into an endpoint that returns the message to the user.

📋 What You'll Learn

Create a dependency function called get_greeting that returns the string 'Hello, FastAPI!'.

Create a FastAPI app instance called app.

Create a GET endpoint at /greet that uses Depends(get_greeting) to receive the greeting message.

Return the greeting message as a JSON response with key message.

💡 Why This Matters

🌍 Real World

Using Depends in FastAPI helps organize code by separating concerns like authentication, database access, or configuration.

💼 Career

Understanding Depends is essential for building scalable and maintainable FastAPI applications in professional backend development.

1

Create the dependency function

Create a function called get_greeting that returns the string 'Hello, FastAPI!'.

FastAPI

# Your code here

Hint

Define a simple function with no parameters that returns the greeting string.

2

Create the FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

def get_greeting():
    return 'Hello, FastAPI!'

# Your code here

Hint

Use from fastapi import FastAPI and then app = FastAPI().

3

Create the GET endpoint using Depends

Import Depends from fastapi. Create a GET endpoint at /greet using @app.get("/greet"). Define a function greet that takes a parameter message which uses Depends(get_greeting).

FastAPI

from fastapi import FastAPI
from fastapi import Depends

def get_greeting():
    return 'Hello, FastAPI!'

app = FastAPI()

# Your code here

Hint

Use @app.get("/greet") and a function with a parameter using Depends(get_greeting).

4

Return the greeting message as JSON

In the greet function, return a dictionary with key message and value message parameter to send JSON response.

FastAPI

from fastapi import FastAPI
from fastapi import Depends

def get_greeting():
    return 'Hello, FastAPI!'

app = FastAPI()

@app.get("/greet")
def greet(message: str = Depends(get_greeting)):
    # Your code here

Hint

Return a dictionary with the greeting message to send JSON.

Practice

1. What is the main purpose of the Depends function in FastAPI?

easy

A. To create HTML templates

B. To define database models

C. To inject dependencies automatically into path operation functions

D. To handle HTTP status codes

Solution

1. Step 1: Understand what Depends does

   Depends is used to declare dependencies that FastAPI will automatically provide to your route functions.

2. Step 2: Identify the main use case

   It helps inject reusable code like authentication, database sessions, or other shared logic into routes.

3. Final Answer:

   To inject dependencies automatically into path operation functions -> Option C

4. Quick Check:

   Depends injects dependencies = C [OK]

Hint: Depends injects reusable code into routes automatically [OK]

Common Mistakes:

• Confusing Depends with database or template functions
• Thinking Depends handles HTTP status codes
• Assuming Depends creates models

2. Which of the following is the correct way to declare a dependency in a FastAPI route using Depends?

easy

A. def read_items(db=Depends(get_db)): pass

B. def read_items(db: Depends(get_db)): pass

C. def read_items(db: Depends = get_db): pass

D. def read_items(db=Depends): pass

Solution

1. Step 1: Recall Depends syntax

   The correct syntax is to assign the parameter a default value of Depends with the dependency function inside.

2. Step 2: Match the correct option

   def read_items(db=Depends(get_db)): pass uses db=Depends(get_db), which is the proper way to declare a dependency.

3. Final Answer:

   def read_items(db=Depends(get_db)): pass -> Option A

4. Quick Check:

   Depends usage = parameter=Depends(function) [OK]

Hint: Use parameter=Depends(function) to declare dependencies [OK]

Common Mistakes:

• Using type annotation instead of default value for Depends
• Passing Depends without parentheses
• Assigning Depends without a function

3. Given the code below, what will be the output when accessing the /items/ endpoint?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_number():
    return 42

@app.get('/items/')
def read_items(number: int = Depends(get_number)):
    return {"number": number}

medium

A. {"number": "get_number"}

B. {"number": 42}

C. Error: missing required parameter

D. {"number": null}

Solution

1. Step 1: Understand dependency injection

   The get_number function returns 42, and FastAPI injects this value into the number parameter.

2. Step 2: Check the returned response

   The route returns a dictionary with key "number" and value 42, so the output is {"number": 42}.

3. Final Answer:

   {"number": 42} -> Option B

4. Quick Check:

   Depends injects 42 = {"number": 42} [OK]

Hint: Depends calls function and injects return value [OK]

Common Mistakes:

• Expecting the function name instead of its return value
• Thinking parameter is missing if not passed explicitly
• Assuming null is returned if no argument given

4. What is wrong with the following FastAPI code using Depends? How to fix it?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_user():
    return "Alice"

@app.get('/user/')
def read_user(user: str = Depends):
    return {"user": user}

medium

A. Depends is missing the dependency function; fix by using Depends(get_user)

B. The route path is invalid; fix by changing '/user/' to '/users/'

C. The return type is wrong; fix by returning a list instead of dict

D. The function get_user should accept parameters; fix by adding parameters

Solution

1. Step 1: Identify the Depends usage error

   The parameter uses Depends without specifying the dependency function, which is incorrect.

2. Step 2: Correct the Depends syntax

   It should be Depends(get_user) to tell FastAPI which function to call for the dependency.

3. Final Answer:

   Depends is missing the dependency function; fix by using Depends(get_user) -> Option A

4. Quick Check:

   Depends needs function argument = Depends(get_user) [OK]

Hint: Always pass the dependency function inside Depends() [OK]

Common Mistakes:

• Using Depends without parentheses or function
• Changing route path unnecessarily
• Changing return type without reason

5. How can you use Depends to share a database session across multiple routes without repeating code? Choose the best approach.

hard

A. Use Depends without any function to automatically get the session

B. Pass the session as a global variable to all routes

C. Manually create a session inside each route function

D. Create a function that returns the session and use Depends on it in each route

Solution

1. Step 1: Understand code reuse with Depends

   Depends allows you to write a function that creates or yields a database session once and injects it wherever needed.

2. Step 2: Identify the best practice

   Creating a session function and using Depends on it in routes avoids repetition and manages session lifecycle cleanly.

3. Final Answer:

   Create a function that returns the session and use Depends on it in each route -> Option D

4. Quick Check:

   Use Depends with session function for reuse = D [OK]

Hint: Use Depends with a session function to share DB session [OK]

Common Mistakes:

• Using global variables for sessions (not safe)
• Creating sessions manually in every route (repetitive)
• Using Depends without specifying a function