Depends function basics in FastAPI

Introduction

The Depends function helps you share and reuse code in FastAPI by letting you declare dependencies easily.

When you want to reuse common logic like getting a database connection in many places.

When you need to check user authentication before running an endpoint.

When you want to separate concerns by moving code out of your main route functions.

When you want FastAPI to automatically handle calling helper functions and passing their results.

When you want cleaner and more organized code by using dependency injection.

Syntax

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def common_logic():
    # some reusable code
    return 'result'

@app.get("/items/")
async def read_items(data=Depends(common_logic)):
    return {"data": data}

Depends wraps a function that FastAPI will call automatically.

The result of the dependency function is passed as an argument to your route function.

Examples

This example shows a simple dependency that returns a token string.

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_token():
    return "token123"

@app.get("/secure-data")
async def secure_data(token: str = Depends(get_token)):
    return {"token": token}

This example uses a dependency with a parameter and returns it to the route.

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def common_parameters(q: str = None):
    return {"q": q}

@app.get("/search")
async def search(params: dict = Depends(common_parameters)):
    return params

Sample Program

This program defines a dependency function get_message that returns a greeting string. The route /greet uses Depends to call this function automatically and returns its result in JSON.

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_message():
    return "Hello from dependency!"

@app.get("/greet")
async def greet(message: str = Depends(get_message)):
    return {"message": message}

Important Notes

Dependencies can be functions or classes.

FastAPI handles calling dependencies and passing their results automatically.

Use dependencies to keep your code clean and reusable.

Summary

Depends helps reuse code by injecting dependencies into routes.

It makes your code cleaner and easier to maintain.

FastAPI calls dependency functions automatically and passes their results.

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