Shared dependencies in FastAPI - Step-by-Step Execution

Concept Flow - Shared dependencies

Define dependency function

↓

Declare dependency in endpoint

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FastAPI calls dependency

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Dependency returns value

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Endpoint uses returned value

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Response sent to client

FastAPI calls shared dependency functions before endpoints, passing their results to endpoints for reuse.

Execution Sample

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def common_dep():
    return "shared value"

@app.get("/items")
async def read_items(value: str = Depends(common_dep)):
    return {"value": value}

This code defines a shared dependency function and uses it in an endpoint to provide a value.

Execution Table

Step	Action	Dependency Called	Dependency Result	Endpoint Parameter	Response
1	Request to /items received	No	N/A	N/A	N/A
2	FastAPI calls common_dep()	Yes	"shared value"	N/A	N/A
3	common_dep() returns	Yes	"shared value"	N/A	N/A
4	Endpoint read_items called with value="shared value"	No	N/A	"shared value"	N/A
5	Endpoint returns response	No	N/A	N/A	{"value": "shared value"}

💡 Request handled after dependency provides value and endpoint returns response

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
value	N/A	"shared value"	"shared value"	"shared value"

Key Moments - 3 Insights

Why does FastAPI call the dependency function before the endpoint?

FastAPI calls the dependency first to get the value it needs to pass into the endpoint parameter, as shown in execution_table step 2 and 4.

Can the same dependency be used in multiple endpoints?

Yes, shared dependencies can be reused across endpoints to avoid repeating code, as the dependency function is independent and called each time.

What happens if the dependency returns a different value each time?

The endpoint receives the new value each time because FastAPI calls the dependency fresh on every request, shown by the dependency call in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'value' when the endpoint read_items is called?

A None

B N/A

C "shared value"

D "common_dep"

💡 Hint

Check the 'Endpoint Parameter' column at step 4 in the execution_table.

At which step does FastAPI call the dependency function?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Dependency Called' column in the execution_table.

If the dependency returned "new value" instead, what would change in the execution_table?

A Dependency Result and Endpoint Parameter would show "new value"

B Response would be unchanged

C Dependency Called would be No

D Endpoint would not be called

💡 Hint

Focus on the 'Dependency Result' and 'Endpoint Parameter' columns in the execution_table.

Concept Snapshot

FastAPI Shared Dependencies:
- Define a function to provide shared data
- Use Depends() in endpoint parameters
- FastAPI calls dependency before endpoint
- Dependency return value passed to endpoint
- Enables code reuse and cleaner endpoints

Full Transcript

In FastAPI, shared dependencies are functions that provide values or logic reused by multiple endpoints. When a request comes in, FastAPI first calls the dependency function to get its result. Then it passes this result as an argument to the endpoint function. This way, the endpoint can use shared data or logic without repeating code. The execution table shows the request flow: receiving the request, calling the dependency, returning its value, calling the endpoint with that value, and finally sending the response. Variables like 'value' hold the dependency's output and are tracked through the steps. Understanding this flow helps beginners see how FastAPI manages dependencies behind the scenes.