Path operation dependencies in FastAPI - Mini Project: Build & Apply

Using Path Operation Dependencies in FastAPI

📖 Scenario: You are building a simple FastAPI app that manages user profiles. You want to reuse a common dependency that provides a fake current user object for multiple path operations.

🎯 Goal: Create a FastAPI app with a dependency function called get_current_user that returns a dictionary with user info. Use this dependency in two path operations: one to get the current user's profile and another to get the current user's items.

📋 What You'll Learn

Create a FastAPI app instance called app

Define a dependency function called get_current_user that returns {'username': 'alice'}

Use Depends(get_current_user) in two path operations: /users/me and /users/me/items

Each path operation should accept a parameter called current_user that receives the dependency result

💡 Why This Matters

🌍 Real World

Path operation dependencies help you reuse common logic like authentication or database sessions across many API endpoints without repeating code.

💼 Career

Understanding dependencies in FastAPI is essential for building clean, maintainable, and secure web APIs in professional backend development.

1

Create the FastAPI app and dependency function

Create a FastAPI app instance called app and define a dependency function called get_current_user that returns the dictionary {'username': 'alice'}.

FastAPI

from fastapi import FastAPI, Depends

# Create FastAPI app and dependency function here
# Your code here

Hint

Use app = FastAPI() to create the app. Define a function get_current_user that returns the dictionary {'username': 'alice'}.

2

Add the first path operation using the dependency

Add a GET path operation at /users/me that uses Depends(get_current_user) to get the current user. The function should accept a parameter called current_user and return it.

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_current_user():
    return {'username': 'alice'}

# Add GET path operation /users/me using Depends(get_current_user)
# Your code here

Hint

Use @app.get('/users/me') decorator. Define a function with parameter current_user: dict = Depends(get_current_user) and return current_user.

3

Add the second path operation using the same dependency

Add a GET path operation at /users/me/items that also uses Depends(get_current_user). The function should accept a parameter called current_user and return a dictionary with a key items and value as a list containing the string "item1".

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_current_user():
    return {'username': 'alice'}

@app.get('/users/me')
def read_current_user(current_user: dict = Depends(get_current_user)):
    return current_user

# Add GET path operation /users/me/items using Depends(get_current_user)
# Your code here

Hint

Use @app.get('/users/me/items') decorator. Define a function with parameter current_user: dict = Depends(get_current_user) and return {'items': ['item1']}.

4

Add type hints and import Depends correctly

Ensure the Depends function is imported from fastapi and that both path operation functions have the parameter current_user typed as dict with = Depends(get_current_user).

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def get_current_user():
    return {'username': 'alice'}

@app.get('/users/me')
def read_current_user(current_user: dict = Depends(get_current_user)):
    return current_user

@app.get('/users/me/items')
def read_own_items(current_user: dict = Depends(get_current_user)):
    return {'items': ['item1']}

# Verify imports and type hints
# Your code here

Hint

Check that Depends is imported from fastapi and used as a default value with type hint dict for current_user in both functions.

Practice

1. What is the main purpose of using Depends() in FastAPI path operations?

easy

A. To create a new database connection manually

B. To define the HTTP method for the route

C. To specify the response status code

D. To run shared code before handling requests

Solution

1. Step 1: Understand the role of Depends()

   Depends() is used to declare dependencies that run shared code before the main path operation function executes.

2. Step 2: Identify the purpose in FastAPI

   This helps keep code clean by reusing common logic like authentication or database access.

3. Final Answer:

   To run shared code before handling requests -> Option D

4. Quick Check:

   Depends() runs shared code before requests [OK]

Hint: Depends() runs shared code before request handling [OK]

Common Mistakes:

• Thinking Depends() sets HTTP methods
• Confusing Depends() with response status codes
• Assuming Depends() manually creates DB connections

2. Which of the following is the correct way to declare a dependency in a FastAPI path operation function?

easy

A. def read_item(item_id: int, user=Depends[get_current_user]):

B. def read_item(item_id: int, user=Depends):

C. def read_item(item_id: int, user=Depends(get_current_user)):

D. def read_item(item_id: int, user=get_current_user()):

Solution

1. Step 1: Recall the syntax for dependencies

   Dependencies are declared by assigning a parameter to Depends() with the dependency function inside.

2. Step 2: Check each option

   def read_item(item_id: int, user=Depends(get_current_user)): correctly uses user=Depends(get_current_user). Others have syntax errors or call the function directly.

3. Final Answer:

   def read_item(item_id: int, user=Depends(get_current_user)): -> Option C

4. Quick Check:

   Depends() with function inside parentheses [OK]

Hint: Use Depends(function_name) with parentheses [OK]

Common Mistakes:

• Calling the dependency function directly
• Using Depends without parentheses
• Using square brackets instead of parentheses

3. Given the code below, what will be the output when accessing /items/5?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_token():
    return "token123"

@app.get("/items/{item_id}")
def read_item(item_id: int, token: str = Depends(get_token)):
    return {"item_id": item_id, "token": token}

medium

A. {"item_id": 5, "token": "token123"}

B. {"item_id": 5, "token": null}

C. RuntimeError due to missing token parameter

D. SyntaxError in dependency declaration

Solution

1. Step 1: Understand dependency execution

   The get_token function returns "token123" and is injected into token parameter via Depends().

2. Step 2: Check the returned dictionary

   The path operation returns a dictionary with item_id and token keys, so the output includes the token string.

3. Final Answer:

   {"item_id": 5, "token": "token123"} -> Option A

4. Quick Check:

   Dependency injects token value correctly [OK]

Hint: Depends injects return value as parameter [OK]

Common Mistakes:

• Expecting token to be null without dependency
• Thinking dependency causes runtime error
• Confusing syntax with runtime errors

4. Identify the error in the following FastAPI code using dependencies:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_user():
    return "user1"

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

@app.get("/dashboard")
def dashboard(user = Depends(get_user)):
    return {"dashboard_user": user}

medium

A. Missing type annotation for 'user' in dashboard function

B. Depends() used incorrectly without parentheses

C. get_user function missing return statement

D. Path operation decorator missing on dashboard function

Solution

1. Step 1: Compare both path operation functions

   The profile function declares user: str = Depends(get_user) with a type annotation.

2. Step 2: Identify the issue in dashboard

   The dashboard function uses user = Depends(get_user) but lacks a type annotation, which FastAPI requires for dependencies.

3. Final Answer:

   Missing type annotation for 'user' in dashboard function -> Option A

4. Quick Check:

   Dependency parameters need type annotations [OK]

Hint: Always add type annotations for Depends parameters [OK]

Common Mistakes:

• Omitting type annotations on dependency parameters
• Forgetting parentheses on Depends()
• Assuming missing decorator causes error

5. You want to reuse a dependency that extracts a user from a token and also check if the user is active before allowing access to multiple routes. How should you combine these checks using FastAPI dependencies?

hard

A. Create two separate dependency functions and use Depends() on both in the path operation

B. Call the user extraction function inside the active check function and use Depends() only on the active check

C. Use a single dependency function that returns user without any checks

D. Use Depends() only on the user extraction function and check active status inside each path operation

Solution

1. Step 1: Understand dependency chaining

   You can call one dependency inside another to reuse logic and combine checks.

2. Step 2: Apply chaining for user extraction and active check

   By calling the user extraction inside the active check dependency, you only need to use Depends() on the active check in routes.

3. Final Answer:

   Call the user extraction function inside the active check function and use Depends() only on the active check -> Option B

4. Quick Check:

   Chain dependencies by calling one inside another [OK]

Hint: Chain dependencies by calling one inside another [OK]

Common Mistakes:

• Using multiple Depends() separately causing repeated calls
• Not chaining dependencies and duplicating code
• Checking user active status outside dependencies