Dependencies with parameters in FastAPI - Step-by-Step Execution

Concept Flow - Dependencies with parameters

Define dependency function with parameter

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Use Depends() in path operation with argument

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FastAPI calls dependency with parameter

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Dependency returns value

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Path operation receives dependency result

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Return response using dependency result

This flow shows how FastAPI calls a dependency function with parameters and passes its result to the path operation.

Execution Sample

FastAPI

from fastapi import FastAPI, Depends

app = FastAPI()

def query_extractor(q: str):
    return q

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

This code defines a dependency function that takes a parameter and uses it in a path operation.

Execution Table

Step	Action	Parameter q	Dependency Return	Path Operation Input	Response
1	Request to /items/?q=hello	hello	hello	hello	{"q": "hello"}
2	Request to /items/ without q	missing	Error (422)	N/A	Error response (422 Unprocessable Entity)
3	Request to /items/?q=fastapi	fastapi	fastapi	fastapi	{"q": "fastapi"}

💡 Execution stops after returning response or error for each request.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
q	N/A	hello	missing (error)	fastapi
Dependency Return	N/A	hello	Error (no return)	fastapi
Path Operation Input	N/A	hello	N/A	fastapi

Key Moments - 2 Insights

Why does the request without 'q' cause an error?

Because the dependency function requires 'q' as a parameter, and FastAPI cannot provide it if missing, causing a validation error as shown in step 2 of the execution_table.

How does FastAPI pass the query parameter to the dependency?

FastAPI extracts the query parameter 'q' from the request URL and passes it to the dependency function as shown in the 'Parameter q' column in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'q' passed to the dependency in step 3?

A "fastapi"

B "hello"

C missing

D None

💡 Hint

Check the 'Parameter q' column in step 3 of the execution_table.

At which step does the dependency cause an error due to missing parameter?

A Step 3

B Step 1

C Step 2

D No error occurs

💡 Hint

Look at the 'Dependency Return' and 'Response' columns in the execution_table.

If the dependency function had a default value for 'q', how would step 2 change?

A The path operation would not run

B It would return the default value instead of error

C It would still cause an error

D The server would crash

💡 Hint

Think about how default parameters affect function calls in Python and FastAPI.

Concept Snapshot

FastAPI dependencies can accept parameters.
Define a function with parameters.
Use Depends() in path operation with that function.
FastAPI injects parameters from request.
Dependency returns value used in path operation.
Missing parameters cause validation errors.

Full Transcript

This example shows how FastAPI handles dependencies with parameters. A dependency function named query_extractor takes a parameter 'q'. The path operation read_items uses Depends(query_extractor) to get 'q' from the request query parameters. When a request includes 'q', FastAPI passes it to the dependency, which returns it. The path operation then returns it in the response. If 'q' is missing, FastAPI raises a validation error (422). This flow helps reuse logic and access request data cleanly.