Global dependencies in FastAPI - Step-by-Step Execution

Concept Flow - Global dependencies

Define dependency function

↓

Declare global dependency in FastAPI app

↓

Request comes in

↓

FastAPI calls global dependency

↓

Dependency returns value

↓

Route handler uses dependency value

↓

Response sent back

This flow shows how FastAPI calls a global dependency for every request before running the route handler.

Execution Sample

FastAPI

from fastapi import FastAPI, Depends

def get_token():
    return "token123"

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

@app.get("/")
async def read_root(token: str = Depends(get_token)):
    return {"token": token}

This code sets a global dependency get_token that runs on every request, and the route also uses it.

Execution Table

Step	Action	Dependency Called	Returned Value	Route Handler Input	Response
1	Request received at '/'	get_token	token123	token='token123'	{"token": "token123"}
2	Request received at '/' again	get_token	token123	token='token123'	{"token": "token123"}
3	Request received at '/' third time	get_token	token123	token='token123'	{"token": "token123"}

💡 Requests end after response sent; global dependency runs each time before route handler.

Variable Tracker

Variable	Start	After 1	After 2	After 3
token	None	token123	token123	token123

Key Moments - 2 Insights

Why does get_token run even if the route handler also depends on it?

Because get_token is declared as a global dependency, FastAPI runs it before every request regardless of route-specific dependencies, as shown in execution_table rows 1-3.

Does the global dependency value get reused between requests?

No, get_token runs fresh for each request, returning 'token123' each time, as seen in variable_tracker showing 'token123' after every request.

Visual Quiz - 3 Questions

Look at the execution table, what value does get_token return at step 2?

A "token123"

B "token456"

C None

D "error"

💡 Hint

Check the 'Returned Value' column at step 2 in the execution_table.

At which step does the route handler receive the token value 'token123'?

A Step 4

B Step 1

C Step 0

D Step 3

💡 Hint

Look at the 'Route Handler Input' column in the execution_table.

If the global dependency get_token returned a different value, how would the response change?

A Route handler would not run

B Response would stay the same

C Response token value would change accordingly

D Server would crash

💡 Hint

See how the 'Response' column depends on the 'Returned Value' from get_token in the execution_table.

Concept Snapshot

Global dependencies in FastAPI run before every request.
Declare them in FastAPI() with dependencies=[Depends(your_func)].
They provide shared values or checks for all routes.
Route handlers can also depend on the same function.
Each request calls the global dependency fresh.
Useful for auth, logging, or common data.

Full Transcript

Global dependencies in FastAPI are functions that run before every request automatically. You declare them when creating the FastAPI app using the dependencies parameter with Depends. When a request comes in, FastAPI calls the global dependency function first, gets its return value, then passes that value to the route handler if it also depends on it. This happens for every request, so the dependency runs fresh each time. This is useful for things like authentication tokens or logging that you want to apply to all routes. The example code shows a get_token function returning a token string. The FastAPI app uses it as a global dependency and the route also depends on it. The execution table shows each request triggers get_token, which returns 'token123', then the route handler receives that token and returns it in the response. The variable tracker confirms the token value updates each request. This helps beginners see how global dependencies run independently of route-specific dependencies and run every time a request happens.

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