Dependency Injection

Execution Workflow

Choose the contract to inject first: settings object, service, repository, session, current user, or request-derived value.
Pick the narrowest scope that should own it: app, router, controller, or handler.
Implement the provider as a callable wrapped in Provide, choosing async, sync, or generator form based on lifecycle needs.
Keep the dependency key identical to the injected parameter name.
Use overrides intentionally at lower scopes and in tests.
Mark special cases with Dependency(...) when validation, defaults, or OpenAPI behavior need to be explicit.
Verify cleanup behavior for generator dependencies and startup-time failure for missing explicit dependencies.

Register dependencies at the narrowest scope that matches their reuse boundary.
Keep dependency keys stable and descriptive: settings, db_session, current_user, tenant, clock.
Keep injected annotations runtime-importable. Avoid hiding injected types behind TYPE_CHECKING unless you also provide a working runtime signature namespace.
Accept keyword arguments and self in providers, never positional-only runtime arguments.
Match the provider key and the consumer parameter name exactly.
Prefer injecting services and repositories instead of raw transport objects unless the dependency is explicitly request-derived.
Avoid hidden mutable singletons. If state must be shared, make lifecycle and mutability obvious.

Litestar resolves dependencies from outer to inner scope, with lower scopes overriding higher ones when the same key is reused.

Choose the lowest scope that still matches the reuse boundary. Reusing the same key is an intentional override contract, not a naming accident.

Use async providers for I/O-bound work that is naturally asynchronous.
Use sync providers only when the work is truly synchronous.
Set sync_to_thread=True for blocking I/O or CPU-heavy sync providers.
Set sync_to_thread=False for cheap, non-blocking sync providers to document intent and avoid warnings.
Use generator providers when setup and cleanup belong to the same logical resource.
Use callable instances when constructor-time configuration is required but invocation should still be DI-managed.

Read only the sections you need:

For scope examples, provider forms, request-derived values, yield cleanup, overrides, caching, nested dependency graphs, and service wiring patterns, read references/provider-patterns.md.
For Annotated[..., Dependency(...)] usage, validation control, OpenAPI-safe defaults, and fail-fast explicit dependency markers, read references/dependency-markers.md.

Keep validation enabled unless the provider is trusted and the performance or type-system tradeoff is justified.
Keep dependency graphs shallow enough to reason about quickly.
Compose stable layers: settings -> client/session -> repository -> service.
Use request-derived providers as translation boundaries from transport inputs to domain types.
Reuse the same dependency keys in tests to swap in fakes without touching handler code.

Registering everything at app scope because it is convenient.
Using dependencies as an opaque service locator with unclear names like dep1.
Returning mutable global singletons from providers and mutating them per request.
Performing blocking I/O in sync providers without sync_to_thread=True.
Using skip_validation=True broadly instead of fixing the type boundary.
Repeating lookup logic in handlers when a request-derived provider should own it.

Confirm every injected parameter name matches a registered dependency key.
Confirm the chosen scope matches reuse and override needs.
Confirm sync providers declare the right sync_to_thread behavior.
Confirm generator dependencies always close, even on exceptions.
Confirm nested provider graphs stay readable and testable.
Confirm Dependency(default=...) is used when fallback dependencies must stay out of OpenAPI.
Confirm explicit Dependency() markers catch missing required wiring at startup.
Confirm tests can override keys cleanly without monkeypatching internals.

Use litestar-databases for SQLAlchemy or Piccolo session provisioning.
Use litestar-testing for dependency override patterns and client-based verification.
Use litestar-security or litestar-authentication when DI is carrying auth context.
Use litestar-routing when the main issue is route/controller structure rather than service wiring.