Global dependencies in FastAPI - Step-by-Step Execution

Concept Flow - Global dependencies

Define dependency function

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Declare global dependency in FastAPI app

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Request comes in

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FastAPI calls global dependency

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Dependency returns value

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Route handler uses dependency value

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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.