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Dependency injection thanks to type hints in Python 3.6+

Project description

auto-init is a dependency injection tool that works in Python 3.6+ thanks to type hints. If you write nice object oriented code and separate interfaces from implementations then you could benefit from this.


Did you know that if you provide a type hint for an attribute in a class body and do not set a value then the attribute isn’t actually initialised (neither in class, nor in instances)?

class Point:
    x: int
    y: int
    z: int = None

For p, an instance of Point, only p.z is set. This is great because we can build a dependency injection mechanism on top of this!

Simple Example

from auto_init import AutoInitContext

ctx = AutoInitContext()
p: Point = ctx.get_instance(Point)
assert p.x == 0
assert p.y == 0
assert p.z is None

Note that the Point class could also be a dataclass and it would work too.

Not So Simple Example

import logging

from auto_init import AutoInitContext

class Worker:
    enterprise: "Enterprise"
    log: logging.Logger

class Reporter:
    enterprise: "Enterprise"
    log: logging.Logger

class Enterprise:
    worker: Worker
    reporter: Reporter

ctx = AutoInitContext()

enterprise: Enterprise = ctx.get_instance(Enterprise)
assert enterprise.worker.log is enterprise.reporter.log
assert enterprise.worker.enterprise is enterprise
assert enterprise.reporter.enterprise is enterprise


pip install auto-init



Create a new auto-initialisation context.

register_singleton(instance_type: Type, factory: Callable=None)
Register a singleton type. This is different from register_instance in that here auto-init is responsible for the creation as well as initialisation of the singleton instance. This should be used when the singleton itself has dependencies that need to be injected. See the example under auto_init/examples/ . If factory is not supplied, the instance_type is used to create the instance.
register_factory(instance_type: Type, factory: Callable)
Register a callable which is called to create a new instance of the specified type when on is requested.
register_instance(instance, instance_type: Type=None)
Register an instance that should always be returned when an instance of the specified type is requested.
get_instance(instance_type: Type) -> Any
Get an instance of the specified type.
Initialise any unitialised attributes of the instance.



  • Supports initialising attributes annotated with typing.List and typing.Dict as empty lists or dictionaries. Other typing.*-annotated attributes are initialised as None.
  • Do not attempt to get type hints from typing.* classes, it raises exceptions in Python 3.7 and does not return anything useful in Python 3.6 anyway.
  • Fixes dependencies for Python 3.6, removes unnecessary contextvars and dataclasses dependencies for Python 3.7.
  • Travis CI setup.


  • Complete rewrite to deal with circular references. Intrusive approach is no good.
  • Forward references in type hints don’t really work in Python 3.7 even with futures. Let’s use Python 3.6.


  • Added AutoInitContext.explicit_only – allows marking the context as only initialising the types with specified providers and leaving all others untouched.
  • If a type hint includes a default value (declares a class attribute) then the corresponding instance attribute will be a copy by reference of the class attribute unless there is an explicit provider specified in the context. This means that x: int = None will be initialised as None, not as 0.


  • Non-intrusive auto-init: function auto_init and method AutoInitContext.auto_init that initialises instances without making any changes to user’s classes.
  • Added AutoInitContext.singleton_types – allows to specify singleton types non-intrusively.

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