Can you explain the principle of dependency injection in FastAPI and how it is implemented in this framework?

In FastAPI, dependency injection is implemented through a mechanism that allows for the automatic creation and passing of objects or values to request handlers (endpoints).

The principle is that you describe a function or class that provides the required resource (for example, a database connection, a service, or configuration), and FastAPI automatically calls this function and passes the result to your handler.

This is achieved using the Depends parameter from the fastapi module. For example:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_db():
    db = create_db_session()
    try:
        yield db
    finally:
        db.close()

@app.get("/items/")
def read_items(db = Depends(get_db)):
    items = db.query_items()
    return items

Here, get_db is a dependency function that creates a database session and ensures its closure after use. FastAPI automatically calls get_db and passes the result to the db parameter of the read_items function.

Thus, dependency injection in FastAPI simplifies resource management, enhances testability, and modularity of the code.