resolving.md

Resolving Dependencies

Dependencies can be resolved in two ways:

1. By provider reference - Using container.resolve_provider(provider)
2. By type - Using container.resolve(SomeType)

When resolving by type, the container looks for a provider whose bound_type matches the requested type. By default, the bound_type is automatically inferred from the creator's return type annotation.

Resolving of Sub Dependencies

One of the key features of Modern-DI since 2.x version is the automatic resolution of sub dependencies through type annotations. Modern-DI automatically analyzes the creator function or class constructor to identify its parameter types and attempts to resolve them from registered providers.

How It Works

When a factory is created:

1. Modern-DI parses the creator's signature to identify parameter names and types
2. For each parameter with a type annotation, it searches for a registered provider that matches by parameter type.
3. If a matching provider is found, it's automatically injected when the factory is resolved
4. If no matching provider is found and no default value is provided, an error is raised

For union-typed parameters (e.g. dep: A | B), the first type in the union that has a registered provider is used. If you need a specific type injected, use a concrete type annotation or supply the value explicitly via kwargs.

Example:

import dataclasses
from modern_di import Group, Container, 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  # Automatically resolved by type
    timeout: int = 30  # Uses default value

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

container = Container(groups=[Dependencies])
connection = container.resolve(DatabaseConnection)
assert isinstance(connection.config, DatabaseConfig)
assert connection.config.host == "localhost"
assert connection.timeout == 30