Global dependencies in FastAPI

Introduction

Global dependencies let you run shared code for many parts of your app automatically. This helps keep your code clean and avoid repeating yourself.

You want to check user authentication for many routes.

You need to connect to a database for multiple endpoints.

You want to add common headers or logging for all requests.

You want to share a configuration or resource across your app.

You want to handle errors or validation globally.

Syntax

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI(dependencies=[Depends(your_dependency_function)])

The dependencies list in FastAPI() applies to all routes automatically.

Each dependency function can return values or perform actions before the route runs.

Examples

This example adds a global dependency that provides default query parameters to all routes.

FastAPI

from fastapi import FastAPI, Depends

async def common_parameters():
    return {"q": "default"}

app = FastAPI(dependencies=[Depends(common_parameters)])

This example globally checks a header token for all requests and blocks unauthorized access.

FastAPI

from fastapi import FastAPI, Depends, Header, HTTPException

async def verify_token(x_token: str = Header(...)):
    if x_token != "secret-token":
        raise HTTPException(status_code=400, detail="Invalid Token")

app = FastAPI(dependencies=[Depends(verify_token)])

Sample Program

This FastAPI app uses a global dependency to check a header token on every request. If the token is wrong, it returns an error. Otherwise, it returns a list of items.

FastAPI

from fastapi import FastAPI, Depends, Header, HTTPException

async def verify_token(x_token: str = Header(...)):
    if x_token != "secret-token":
        raise HTTPException(status_code=400, detail="Invalid Token")

app = FastAPI(dependencies=[Depends(verify_token)])

@app.get("/items/")
async def read_items():
    return {"items": ["apple", "banana"]}

Important Notes

Global dependencies run before each request handler automatically.

You can combine global dependencies with route-specific dependencies.

Use global dependencies to keep your code DRY (Don't Repeat Yourself).

Summary

Global dependencies apply shared logic to all routes in your FastAPI app.

They help with authentication, database connections, and common validations.

Defined once, they run automatically for every request.

Practice

1. What is the main purpose of global dependencies in FastAPI?

easy

A. To create database models

B. To define route-specific logic only

C. To style the API responses

D. To run shared logic automatically for every request

Solution

1. Step 1: Understand what global dependencies do

   Global dependencies are functions or classes that run for every request to apply shared logic.

2. Step 2: Identify their main use

   They help with tasks like authentication, database connections, or validations that apply to all routes.

3. Final Answer:

   To run shared logic automatically for every request -> Option D

4. Quick Check:

   Global dependencies = shared logic for all requests [OK]

Hint: Global dependencies run once per request for all routes [OK]

Common Mistakes:

• Thinking global dependencies are for styling
• Confusing global with route-specific dependencies
• Assuming they create database models

2. Which of the following is the correct way to add a global dependency in FastAPI?

easy

A. app = FastAPI(dependencies=[Depends(common_dependency)])

B. app = FastAPI(global_dep=Depends(common_dependency))

C. app = FastAPI(dependency=common_dependency())

D. app = FastAPI(use_global=common_dependency)

Solution

1. Step 1: Recall FastAPI global dependency syntax

   Global dependencies are passed as a list to the 'dependencies' parameter when creating the FastAPI app.

2. Step 2: Match the correct syntax

   app = FastAPI(dependencies=[Depends(common_dependency)]) uses 'dependencies=[Depends(common_dependency)]', which is the correct pattern.

3. Final Answer:

   app = FastAPI(dependencies=[Depends(common_dependency)]) -> Option A

4. Quick Check:

   Global dependencies use dependencies=[Depends(...)] [OK]

Hint: Use dependencies=[Depends(...)] in FastAPI() constructor [OK]

Common Mistakes:

• Using wrong parameter names like global_dep or use_global
• Calling the dependency function instead of passing Depends()
• Passing a single dependency without a list

3. Given this code snippet, what will be printed when a request is made to /items/42?

from fastapi import FastAPI, Depends

app = FastAPI()

def common_dep():
    print("Global dependency called")

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

medium

A. No output printed because common_dep is not global

B. Global dependency called printed once per request

C. Global dependency called printed twice per request

D. SyntaxError due to missing global dependency

Solution

1. Step 1: Check how common_dep is used

   common_dep is used as a route dependency only on the read_item function, not as a global dependency.

2. Step 2: Understand when common_dep runs

   It runs only when the /items/{item_id} route is called, printing the message once per request to that route.

3. Final Answer:

   Global dependency called printed once per request -> Option B

4. Quick Check:

   Route dependency prints only when route is called, not global [OK]

Hint: Route dependencies run per route, not global unless set globally [OK]

Common Mistakes:

• Assuming common_dep is global when it's route-specific
• Expecting multiple prints per request
• Confusing global and route dependencies

4. Identify the error in this FastAPI app code that tries to use a global dependency:

from fastapi import FastAPI, Depends

def common_dep():
    print("Running global dependency")

app = FastAPI(dependencies=Depends(common_dep))

medium

A. common_dep is not defined before use

B. Missing parentheses when calling FastAPI()

C. dependencies parameter expects a list, not a single Depends

D. Depends cannot be used as a global dependency

Solution

1. Step 1: Check the dependencies parameter type

   The 'dependencies' argument expects a list of Depends instances, not a single Depends object.

2. Step 2: Identify the fix

   Wrap Depends(common_dep) inside a list: dependencies=[Depends(common_dep)]

3. Final Answer:

   dependencies parameter expects a list, not a single Depends -> Option C

4. Quick Check:

   dependencies=[Depends(...)] requires a list [OK]

Hint: Always pass dependencies as a list, even if one item [OK]

Common Mistakes:

• Passing Depends(...) directly without list
• Defining dependency after app creation
• Confusing Depends usage with function call

5. You want to add a global dependency that checks user authentication and also logs request info. Which is the best way to combine these in FastAPI?

hard

A. Add both auth check and logging as separate global dependencies in the dependencies list

B. Add auth check as global dependency and logging inside each route

C. Create one dependency function that calls both auth check and logging, then add it globally

D. Use middleware for auth and global dependency for logging

Solution

1. Step 1: Understand global dependencies list

   FastAPI allows multiple global dependencies by passing a list to the dependencies parameter.

2. Step 2: Combine auth and logging as separate dependencies

   Adding both as separate Depends in the list keeps concerns separated and runs both for every request.

3. Step 3: Evaluate other options

   Create one dependency function that calls both auth check and logging, then add it globally mixes concerns in one function, B is inconsistent, D mixes middleware and dependencies unnecessarily.

4. Final Answer:

   Add both auth check and logging as separate global dependencies in the dependencies list -> Option A

5. Quick Check:

   Multiple global dependencies = dependencies=[Depends(...), Depends(...)] [OK]

Hint: Use a list of Depends for multiple global dependencies [OK]

Common Mistakes:

• Combining unrelated logic in one dependency
• Using middleware instead of dependencies for all tasks
• Adding some logic only per route, not globally