Explain what is meant by dependency in FastAPI and how it is implemented.

In FastAPI, a dependency is a way to inject components or resources (such as databases, configurations, authorization) into request handlers (endpoints) without explicitly creating them inside functions.

This is implemented using a function or class that returns the required object, along with the Depends decorator. FastAPI automatically calls this function, manages its lifecycle, and passes the result to the handler function.

Example:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_db():
    db = create_db_session()  # create a database session
    try:
        yield db
    finally:
        db.close()  # close the session after use

@app.get("/items/")
async def read_items(db = Depends(get_db)):
    items = db.query(Item).all()
    return items

Here, get_db is a dependency that creates and provides a database session. FastAPI calls it automatically and passes it to read_items. This simplifies testing, reusability, and resource management.