Dependencies with parameters in FastAPI

Introduction

Dependencies with parameters let you reuse code that needs extra information to work. This helps keep your app clean and organized.

When you want to check user permissions based on a role passed as a parameter.

When you need to connect to different databases depending on a parameter.

When you want to customize behavior of a dependency based on input values.

When you want to share common logic but with small variations.

When you want to inject configuration values into your dependencies.

Syntax

FastAPI

from fastapi import Depends
from fastapi import FastAPI

app = FastAPI()

def common_dependency(param: str):
    # use param inside
    return f"Value is {param}"

@app.get("/")
async def read_root(value: str = Depends(lambda: common_dependency('example'))):
    return {"message": value}

You define a function that takes parameters and use it inside Depends().

You can pass parameters by wrapping the dependency call in a lambda or another function.

Examples

This example passes a fixed string 'hello' to the dependency and returns it uppercased.

FastAPI

from fastapi import Depends, FastAPI

app = FastAPI()

def get_query_param(q: str):
    return q.upper()

@app.get("/items/")
async def read_items(q: str = Depends(lambda: get_query_param('hello'))):
    return {"query": q}

This example checks if the role is 'admin' and raises an error if not.

FastAPI

from fastapi import Depends, FastAPI, HTTPException

app = FastAPI()

def verify_role(role: str):
    if role != "admin":
        raise HTTPException(status_code=403, detail="Not authorized")
    return True

@app.get("/admin/")
async def admin_access(authorized: bool = Depends(lambda: verify_role('admin'))):
    return {"status": "Welcome admin"}

Sample Program

This program defines a dependency that checks if the role is 'user'. The endpoint uses this dependency with the parameter 'user'. If the role is correct, it returns a success message.

FastAPI

from fastapi import FastAPI, Depends, HTTPException

app = FastAPI()

def check_permission(role: str):
    if role != "user":
        raise HTTPException(status_code=403, detail="Access denied")
    return f"Permission granted for {role}"

@app.get("/dashboard")
async def dashboard(permission: str = Depends(lambda: check_permission('user'))):
    return {"message": permission}

Important Notes

Use lambda or small wrapper functions to pass parameters to dependencies.

Dependencies with parameters help avoid repeating similar code with small changes.

Remember to handle errors inside dependencies to keep your app safe.

Summary

Dependencies with parameters let you customize shared code.

You pass parameters by wrapping the dependency call.

This keeps your code clean and easy to maintain.

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