Dependencies

Dependencies provide reusable logic to API endpoints. They are commonly used for:

• Database sessions
• Authentication
• Permissions
• Other shared functionality

FastAPI provides Depends() to declare a dependency that an endpoint needs.

Code

from fastapi import FastAPI, Depends

app = FastAPI()

def get_current_user():
    return {
        "id": 1,
        "name": "Tarun"
    }


@app.get("/profile")
def profile(
    user=Depends(get_current_user)
):
    return user

Understanding the Code

First, import Depends from FastAPI:

from fastapi import Depends

Then create a reusable dependency:

def get_current_user():
    return {
        "id": 1,
        "name": "Tarun"
    }

This function provides the current user’s information.

The dependency is used in the /profile endpoint:

@app.get("/profile")
def profile(
    user=Depends(get_current_user)
):
    return user

Here, Depends(get_current_user) tells FastAPI to execute get_current_user() and provide its result to the user parameter.

Example Request

GET /profile

Output

{
    "id": 1,
    "name": "Tarun"
}

Why Use Dependencies?

Dependencies help avoid repeating the same logic across multiple endpoints.

For example, authentication logic can be placed in one dependency:

def get_current_user():
    # Authentication logic
    return user

Then multiple endpoints can reuse it:

@app.get("/profile")
def profile(user=Depends(get_current_user)):
    return user

This makes FastAPI applications easier to organize and maintain.

Summary

FastAPI dependencies provide reusable logic to API endpoints.

The basic pattern is:

def get_current_user():
    return {
        "id": 1,
        "name": "Tarun"
    }


@app.get("/profile")
def profile(user=Depends(get_current_user)):
    return user

Dependencies are especially useful for database sessions, authentication, and permissions.