Piye cara dependency injection (Depends) nggunan?

Question

Accepted Answer

Sistem **dependency injection** FastAPI ngudal sampeyan nyatakaken dependensi sing bisa dibalekake (fungsi/kelas) sing FastAPI tawag lan **injeksikaken** menyang operasi path sampeyan liwat `Depends()`. Dienggo kanggo logika bareng kayata sesi basis data, autentikasi, lan parameter umum — njaga kode tetep DRY lan bisa diuji.

## Dependensi dasar

```python
from fastapi import Depends

# a dependency is just a callable that returns something
def pagination(skip: int = 0, limit: int = 10):
    return {"skip": skip, "limit": limit}

@app.get("/items")
def list_items(params: dict = Depends(pagination)):   # FastAPI calls pagination() and injects the result
    return params
# the skip/limit query params are now handled by the reusable dependency
```

Nyatakaken `params = Depends(pagination)` nyawisaken FastAPI kanggo tawag `pagination` (ngluwisaken *parametere* saka sawisata uga) lan ngirim asile menyang fungsi sampeyan. Dependensi iki bisa dibalekake ing akeh endpoint.

## Dependensi karo nebus (yield) — conto: sesi basis data

```python
def get_db():
    db = SessionLocal()
    try:
        yield db                  # provide the session to the endpoint
    finally:
        db.close()                # cleanup runs AFTER the response — guaranteed

@app.get("/users")
def list_users(db = Depends(get_db)):
    return db.query(User).all()    # uses the injected, auto-closed session
```

Dependensi `yield` nyedya sumber lan ndalan nebus sawisata — pola standar kanggo sesi basis data (mbukak sadurunge, tutup sawise, otomatis).

## Dependensi bisa gumantung ing dependensi

```python
def get_current_user(token: str = Depends(get_token)):   # depends on another dependency
    return decode(token)

@app.get("/me")
def me(user: User = Depends(get_current_user)):           # auth via injection
    return user
```

Dependensi nggawé wit — FastAPI ngluwisaken kabeh rantai. Iki carane autentikasi lumrah ditindakaken (dependensi `get_current_user`).

## Dependensi kelas lan balekaken

```python
@app.get("/items", dependencies=[Depends(verify_api_key)])   # run a dependency without using its return
def items(): ...                                              # e.g. enforce auth as a side effect
```

## Nenapake kenapa

Dependency injection minangka salah sawijining fitur paling kuwat lan penting FastAPI — carane sampeyan nuduhaken lan balekaken logika kanthi rapi saindhenging endpoint (sesi basis data, autentikasi, parameter query umum, mriksa ijin) tanpa penggandaan.

Ngumangerti iki penting amarga dadi fondasi pola inti: **dependensi `yield`** minangka cara standar kanggo urus sesi basis data (karo nebus dijamin sawise saben sawisata), lan **dependensi `get_current_user`** minangka cara idiomatik kanggo nindakaken autentikasi.

DI uga ndadekaken kode kanthi dhuwur **bisa diuji** — sampeyan bisa ngganti dependensi karo mock ing tes.

Kalunan sistem (dependensi ngluwisaken parametere pribadi saka sawisata, bisa gumantung ing dependensi liya nggawé wit, lan ngrangkul karo dokumentasi) ndadekaken alat serbaguna kanggo nata aplikasi nyata, dadi nguwasai `Depends` minangka kunci kanggo mbangun aplikasi FastAPI sing terstruktur kanthi apik, bisa dikontrol, lan bisa diuji.