Resolving dependencies¶

modern-di exposes two ways to resolve a dependency:

By type — container.resolve(SomeType). The resolver finds the provider whose bound_type matches SomeType. This is what handlers and creator signatures normally use.
By provider reference — container.resolve_provider(Dependencies.some_provider). Resolves a specific provider directly, skipping the type lookup. Useful in tests and when two providers produce the same type.

In practice, prefer resolution by type — it lets the same code work whether you swap implementations via subclassing, Alias, or override. Reach for resolve_provider only when type-based resolution would be ambiguous.

Automatic sub-dependency resolution¶

A Factory's creator function or class constructor is introspected at declaration time. For each parameter with a type annotation, the resolver looks for a provider whose bound_type matches and injects the resolved value. Parameters with default values fall back to those defaults if no provider matches.

import dataclasses
from modern_di import Container, Group, Scope, providers


@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
class DatabaseConfig:
    host: str
    port: int


@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
class DatabaseConnection:
    config: DatabaseConfig    # auto-resolved by type
    timeout: int = 30         # uses default if unresolvable


class Dependencies(Group):
    db_config = providers.Factory(
        DatabaseConfig,
        scope=Scope.APP,
        kwargs={"host": "localhost", "port": 5432},
    )
    db_connection = providers.Factory(DatabaseConnection, scope=Scope.APP)


container = Container(groups=[Dependencies], validate=True)
container.open()

connection = container.resolve(DatabaseConnection)
assert connection.config.host == "localhost"
assert connection.timeout == 30

For union-typed parameters (dep: A | B), the resolver picks the first type in the union that has a registered provider. If you need a specific one, use a concrete annotation or pass the value explicitly via kwargs. A parameter typed X | None with no matching provider and no default value receives None rather than raising (see Factories: Optional parameters).

Resolving dependencies¶

Automatic sub-dependency resolution¶

See also¶