type-ioc

A lightweight IoC/DI container for Python that wires dependencies automatically using type hints — no decorators, no config files, no magic strings.

from ioc import Resolver

class Database:
    def __init__(self, url: str = "sqlite://"):
        self.url = url

class UserRepository:
    def __init__(self, db: Database):
        self.db = db

class UserService:
    def __init__(self, repo: UserRepository):
        self.repo = repo

resolver = Resolver()
service = resolver(UserService)
# UserService → UserRepository → Database, all wired automatically

Installation

pip install type-ioc

uv add type-ioc

Basic resolution

Call the resolver with any class and it recursively resolves the full dependency graph via __init__ type hints.

resolver = Resolver()
service = resolver(UserService)

Each call creates a new instance unless the type is registered as a singleton. The resolver itself is always available:

resolver(Resolver) is resolver  # True

Singletons

Register a class as a singleton — the resolver creates the instance and caches it:

db = resolver.singleton(Database)
resolver(Database) is db  # True

Register an existing instance:

db = Database(url="postgresql://localhost/mydb")
resolver.singleton(Database, db)

Infer the class from the instance:

resolver.singleton(db)  # same as resolver.singleton(Database, db)

Singleton ABC

Inherit from Singleton to make a class register itself automatically on first resolution:

from ioc import Singleton

class AppConfig(Singleton):
    def __init__(self):
        self.debug = False

first = resolver(AppConfig)
second = resolver(AppConfig)
first is second  # True — auto-registered after first call

Factory bindings

bind() registers a factory that is called fresh on every resolution:

resolver.bind(Database)  # creates a new Database each time
resolver(Database) is resolver(Database)  # False

Supply a custom factory:

resolver.bind(Database, lambda: Database(url=os.environ["DB_URL"]))

Bind an abstract class or interface to a concrete implementation:

import abc

class Storage(abc.ABC):
    @abc.abstractmethod
    def save(self, data: bytes) -> None: ...

class DiskStorage(Storage):
    def save(self, data: bytes) -> None: ...

resolver.bind(Storage, DiskStorage)
result = resolver(Storage)  # returns a DiskStorage instance

Annotated / qualified bindings

Use Annotated to register multiple bindings for the same base type:

from typing import Annotated

class _ReadKey: pass
class _WriteKey: pass

ReadDB = Annotated[Database, _ReadKey]
WriteDB = Annotated[Database, _WriteKey]

resolver.singleton(ReadDB, Database(url="postgresql://read-replica/mydb"))
resolver.singleton(WriteDB, Database(url="postgresql://primary/mydb"))

Any class whose __init__ uses ReadDB or WriteDB as type hints will receive the correct instance automatically:

class ReportingService:
    def __init__(self, db: ReadDB):
        self.db = db

class WriteService:
    def __init__(self, db: WriteDB):
        self.db = db

resolver(ReportingService).db is resolver(ReadDB)  # True
resolver(WriteService).db is resolver(WriteDB)     # True

bind() and singleton() both accept Annotated keys.

Optional dependencies

Parameters typed as T | None or Optional[T] resolve to None when the type isn't registered, rather than raising an error:

class Service:
    def __init__(self, cache: Cache | None = None):
        self.cache = cache

resolver(Service).cache  # None — Cache not registered, no error
resolver.singleton(Cache)
resolver(Service).cache  # Cache instance — now it's registered

Passing arguments explicitly

Override any parameter by passing positional or keyword arguments directly to the resolver call. Positional arguments are matched by type:

db = Database(url="postgresql://localhost/test")
repo = resolver(UserRepository, db)         # positional, matched by type
repo = resolver(UserRepository, db=db)      # keyword

Primitive parameters that have no default must be provided explicitly:

class Paginator:
    def __init__(self, page: int, page_size: int = 20):
        ...

resolver(Paginator, page=1)
resolver(Paginator, 1)        # also works — matched by type

Contextual bindings

Use when/needs/give to control how a specific consumer resolves one of its dependencies, without affecting any other consumer.

Override with a factory or class

class PhotoController:
    def __init__(self, storage: Storage):
        self.storage = storage

class LocalStorage(Storage):
    def save(self, data: bytes) -> None: ...

# Give PhotoController a LocalStorage specifically
resolver.when(PhotoController).needs(Storage).give(LocalStorage)

# Or supply a factory
resolver.when(PhotoController).needs(Storage).give(lambda: LocalStorage())

Pass specific kwargs to a dependency's constructor

class Cache:
    def __init__(self, ttl: int = 3600, max_entries: int = 100):
        ...

class SessionService:
    def __init__(self, cache: Cache):
        self.cache = cache

class ReportService:
    def __init__(self, cache: Cache):
        self.cache = cache

# Each service gets a Cache with different settings
resolver.when(SessionService).needs(Cache).give(ttl=300, max_entries=500)
resolver.when(ReportService).needs(Cache).give(ttl=7200)

Multiple .give() calls on the same needs() accumulate; last write wins on conflicts:

resolver.when(SessionService).needs(Cache).give(ttl=300)
resolver.when(SessionService).needs(Cache).give(max_entries=500)
# SessionService gets Cache(ttl=300, max_entries=500)

Override class and pass kwargs together

class LruCache(Cache):
    def __init__(self, ttl: int = 3600, max_entries: int = 100, policy: str = "lru"):
        ...

resolver.when(ReportService).needs(Cache).give(LruCache, ttl=7200, max_entries=50)

Global defaults

when(X).give(**kwargs) with no needs() sets default constructor arguments for every resolution of X, regardless of which consumer triggers it:

resolver.when(Cache).give(ttl=600)

resolver(Cache).ttl                       # 600
resolver(SessionService).cache.ttl       # 600
resolver(ReportService).cache.ttl        # 600

Contextual needs rules take priority over global defaults:

resolver.when(Cache).give(ttl=600)
resolver.when(ReportService).needs(Cache).give(ttl=7200)

resolver(SessionService).cache.ttl   # 600 — global default
resolver(ReportService).cache.ttl    # 7200 — contextual override

Annotated types as resolution profiles

You can use an Annotated type as a named resolution profile. When a consumer's dependency is redirected to an Annotated type, any when(AnnotatedType) rules apply during that resolution — enabling nested contextual configuration:

_ReportCacheKey = object()
ReportCache = Annotated[Cache, _ReportCacheKey]

# Set defaults for the ReportCache profile
resolver.when(ReportCache).give(ttl=7200, max_entries=1000)

# Consumers can type-hint ReportCache directly
class ReportService:
    def __init__(self, cache: ReportCache):
        ...

# Or redirect another consumer to use the ReportCache profile
resolver.when(AnalyticsService).needs(Cache).give(ReportCache)

Global resolver

For applications that want a process-wide default resolver:

resolver = Resolver.get()   # creates on first call, returns same instance thereafter
Resolver.reset()            # discards it; next .get() creates a fresh one

Cloning

Create a child resolver that inherits specific singleton bindings from a parent. Pass as many types as you want to keep:

parent = Resolver()
parent.singleton(Database, prod_db)
parent.singleton(AppConfig, config)
parent.singleton(UserRepository, repo)

# Inherit any subset of singletons; anything not listed must be re-registered or re-resolved
child = parent.clone(Database, UserRepository)
child.singleton(AppConfig, test_config)

clone() with no arguments copies all current singleton bindings:

child = parent.clone()

Factory bindings are never copied — they belong to the resolver that registered them.

Clearing bindings

Remove a specific binding:

resolver.clear(Database)

Reset everything (singletons and factories) back to a blank state:

resolver.clear()

The resolver always keeps itself registered after a clear.

Circular dependency detection

Circular dependencies are detected at resolution time and raise CircularDependency:

from ioc import CircularDependency

try:
    resolver(ServiceA)
except CircularDependency as e:
    print(e.chain)   # [ServiceA, ServiceB, ServiceA]
    print(e.type)    # ServiceA

The resolver remains fully usable after a CircularDependency is raised.

Exception reference

All exceptions inherit from ResolutionFailure.

Exception	Raised when
CircularDependency	A dependency cycle is detected. Has .type and .chain.
DuplicateArgOfSameType	Two positional arguments of the same type are passed. Has .duplicate_type and .arguments.
InvalidBinding	An instance or class fails its type check during singleton() or bind(). Has .expected_type and .instance.
UnboundTypeRequested	An Annotated type is used as a dependency but has no registered binding. Has .type.
UnknownArgument	A positional argument can't be matched to any constructor parameter. Has .argument_type and .argument.
UnknownKeywordArgument	A keyword argument doesn't match any constructor parameter. Has .argument_name, .argument_type, and .argument.
UnresolvablePrimitive	A builtin type (int, str, etc.) is encountered with no default and no explicit value provided. Has .type.

License

MIT