Dependencies with parameters in FastAPI - Mini Project: Build & Apply

FastAPI Dependencies with Parameters

📖 Scenario: You are building a simple FastAPI app that greets users differently based on a greeting style parameter.This is like choosing how you say hello to a friend: formally, casually, or excitedly.

🎯 Goal: Create a FastAPI app that uses a dependency with a parameter to customize the greeting message.The app will have one endpoint /greet/ that returns a greeting message based on the style passed as a query parameter.

📋 What You'll Learn

Create a FastAPI app instance called app.

Define a dependency function get_greeting_style that accepts a parameter style from the query string.

Use the dependency in the /greet/ endpoint to return a greeting message based on the style.

Support greeting styles: 'formal', 'casual', and 'excited'.

💡 Why This Matters

🌍 Real World

Using dependencies with parameters in FastAPI helps you write clean, reusable code that can customize behavior based on user input, like greeting styles or user roles.

💼 Career

Understanding FastAPI dependencies with parameters is essential for backend developers building scalable and maintainable APIs that adapt to different client needs.

1

Create the FastAPI app instance

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

FastAPI

# Import FastAPI and create app instance
# Your code here

Hint

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

2

Define the dependency function with a parameter

Define a function called get_greeting_style that takes a parameter style of type str with a default value of "casual". Import Query from fastapi and use it to get style from the query parameters.

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

# Define get_greeting_style dependency with style parameter
# Your code here

Hint

Use def get_greeting_style(style: str = Query("casual")) and return style.

3

Create the /greet/ endpoint using the dependency

Create a GET endpoint /greet/ using @app.get("/greet/"). Define a function greet that takes a parameter style which uses the dependency get_greeting_style with Depends. Import Depends from fastapi. Inside the function, return a dictionary with key message and value based on style: if style is 'formal', return 'Good day to you.'; if 'casual', return 'Hey there!'; if 'excited', return 'Hello!!!'.

FastAPI

from fastapi import FastAPI, Query, Depends

app = FastAPI()

def get_greeting_style(style: str = Query("casual")) -> str:
    return style

# Create /greet/ endpoint using get_greeting_style dependency
# Your code here

Hint

Use @app.get("/greet/") and def greet(style: str = Depends(get_greeting_style)). Return a dictionary with the correct message based on style.

4

Add type hints and import statements for completeness

Add the import statement for Depends from fastapi if missing. Ensure all functions have proper type hints: get_greeting_style returns str, greet returns a dictionary with str keys and values. This completes the FastAPI app with dependency injection using parameters.

FastAPI

from fastapi import FastAPI, Query, Depends

app = FastAPI()

def get_greeting_style(style: str = Query("casual")) -> str:
    return style

@app.get("/greet/")
def greet(style: str = Depends(get_greeting_style)) -> dict[str, str]:
    # Your code here

Hint

Make sure to import Depends and add return type hints for both functions.

Practice

1. What is the main purpose of using dependencies with parameters in FastAPI?

easy

A. To automatically generate HTML templates

B. To create global variables accessible everywhere

C. To replace route functions with classes

D. To customize shared code by passing arguments to dependencies

Solution

1. Step 1: Understand dependency role

   Dependencies in FastAPI are reusable pieces of code that can be shared across routes.

2. Step 2: Recognize parameter use

   Adding parameters to dependencies allows customizing their behavior for different routes or situations.

3. Final Answer:

   To customize shared code by passing arguments to dependencies -> Option D

4. Quick Check:

   Dependencies with parameters = customize shared code [OK]

Hint: Dependencies with parameters customize shared logic easily [OK]

Common Mistakes:

• Thinking dependencies create global variables
• Confusing dependencies with route handlers
• Assuming dependencies generate HTML

2. Which of the following is the correct way to pass a parameter to a dependency in FastAPI?

easy

A. Depends(get_user(user_id=5))

B. Depends(get_user, user_id=5)

C. Depends(get_user)(user_id=5)

D. Depends(get_user)[user_id=5]

Solution

1. Step 1: Recall Depends usage

   Depends expects a callable or a call to a callable that returns a dependency.

2. Step 2: Passing parameters

   To pass parameters, you call the dependency function inside Depends, like Depends(get_user(user_id=5)).

3. Final Answer:

   Depends(get_user(user_id=5)) -> Option A

4. Quick Check:

   Call dependency inside Depends to pass parameters [OK]

Hint: Call dependency inside Depends() to pass parameters [OK]

Common Mistakes:

• Passing parameters directly to Depends without calling
• Using brackets [] instead of parentheses ()
• Trying to call Depends as a function with parameters

3. Given this code snippet, what will be the output when accessing the endpoint?

from fastapi import FastAPI, Depends

app = FastAPI()

def get_multiplier(factor: int):
    def multiplier(value: int):
        return value * factor
    return multiplier

@app.get("/multiply")
async def multiply(value: int, multiply_func = Depends(get_multiplier(3))):
    return {"result": multiply_func(value)}

medium

A. Error because Depends cannot take parameters

B. {"result": 6} when value=3

C. {"result": 9} when value=3

D. {"result": 3} when value=3

Solution

1. Step 1: Understand get_multiplier

   get_multiplier(3) returns a function that multiplies input by 3.

2. Step 2: Analyze endpoint call

   When calling /multiply with value=3, multiply_func(3) returns 3 * 3 = 9.

3. Final Answer:

   {"result": 9} when value=3 -> Option C

4. Quick Check:

   Dependency returns multiplier with factor 3, output 9 [OK]

Hint: Multiply value by factor passed in dependency [OK]

Common Mistakes:

• Assuming Depends cannot take parameters
• Confusing returned function with direct value
• Mixing up multiplication factor

4. Identify the error in this FastAPI dependency usage:

def get_limit(limit: int = 10):
    return limit

@app.get("/items")
async def read_items(limit = Depends(get_limit(limit=20))):
    return {"limit": limit}

medium

A. Default value in get_limit conflicts with parameter passed

B. Cannot pass parameters directly inside Depends like get_limit(limit=20)

C. Missing type annotation for limit in read_items

D. Depends should be imported from fastapi.dependencies

Solution

1. Step 1: Check Depends usage

   Depends expects a callable or a call to a callable without parameters directly inside Depends.

2. Step 2: Correct way to pass parameters

   To pass parameters, wrap get_limit in another function or use a lambda to supply parameters.

3. Final Answer:

   Cannot pass parameters directly inside Depends like get_limit(limit=20) -> Option B

4. Quick Check:

   Depends() must wrap callable, not call with parameters directly [OK]

Hint: Wrap parameterized dependency call outside Depends() [OK]

Common Mistakes:

• Calling dependency with parameters inside Depends directly
• Ignoring need for wrapper function
• Wrong import path for Depends

5. How can you create a reusable dependency with a parameter that changes per route in FastAPI? Choose the best approach.

def common_dep(param: str):
    def dependency():
        return f"Value is {param}"
    return dependency

@app.get("/route1")
async def route1(dep = Depends(common_dep("A"))):
    return {"msg": dep}

@app.get("/route2")
async def route2(dep = Depends(common_dep("B"))):
    return {"msg": dep}

hard

A. Define a function returning a dependency function with parameter, then call it inside Depends

B. Use global variables to store param values for each route

C. Pass parameters directly to Depends without wrapping

D. Create separate dependency functions for each parameter value

Solution

1. Step 1: Understand reusable dependency pattern

   Define a function that returns a dependency function customized by parameters.

2. Step 2: Apply pattern per route

   Call this function with different parameters inside Depends for each route to customize behavior.

3. Final Answer:

   Define a function returning a dependency function with parameter, then call it inside Depends -> Option A

4. Quick Check:

   Wrap dependency with parameter function, call inside Depends [OK]

Hint: Wrap parameterized dependency in function, call inside Depends [OK]

Common Mistakes:

• Using global variables instead of parameters
• Passing parameters directly to Depends without wrapping
• Duplicating dependency functions unnecessarily