8. Dependency Injection (FastAPI Superpower)

What Dependency Injection Actually Means

Dependency injection is a design pattern where components receive their dependencies from the framework rather than creating them internally. This separates business logic from infrastructure concerns such as databases, configuration, or authentication. The result is code that is easier to test, reason about, and evolve.

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Depends() Explained Properly

In FastAPI, Depends() declares that a function requires an external dependency to execute. FastAPI resolves these dependencies automatically at request time, managing their creation, execution order, and cleanup. This mechanism is explicit, type-safe, and tightly integrated with request handling.

from fastapi import Depends, FastAPI

app = FastAPI()

def get_config():
    return {"env": "dev"}

@app.get("/info")
def read_info(config: dict = Depends(get_config)):
    return config

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Reusable Logic for Authentication, Databases, and Configuration

Dependencies allow shared logic to be defined once and reused across multiple endpoints. Common use cases include authentication checks, database session management, and configuration loading. This avoids duplication and ensures consistent behavior across the application.

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Dependency Chains and Overrides

FastAPI supports dependency chains, where one dependency depends on another. This enables complex composition while maintaining clarity and control. Dependencies can also be overridden, which is especially useful for testing and environment-specific behavior.

def get_db():
    return "database_connection"

def get_user(db=Depends(get_db)):
    return {"db": db}