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A dependency injection container for Python

Project description


PyPI version Python Version from PEP 621 TOML Code style: black Imports: isort License GitHub pull requests pre-commit Checked with mypy codecov


ophiDIan is a Dependency Injection (DI) container for Python. Unlike other DI containers that utilize decorators or configuration files to accomplish their goal, ophiDIan uses type hints to identify and resolve dependencies. In other words, by using type hinting to resolve dependencies, ophiDIan avoids making your components dependent upon the DI framework.

Ophidian follows the Register, Resolve, Release (RRR) pattern.


Install with pip install ophidian


Dependency definitions

For this tutorial, we'll use the following class definitions:

from abc import ABC, abstractmethod

class AbstractA(ABC):
    def do_something(self):

class AbstractB(ABC):
    def do_something_else(self):

class DependencyA(AbstractA):
    def do_something(self):
        print(f"{id(self)}: I am dependency A")

class DependencyB(AbstractB):
    def __init__(self, name: str):
        self._name = name
    def do_something_else(self):
        print(f"{id(self)} My name is {self._name}")

class TestClass1:
    def __init__(self, a: AbstractA):
        self.a = a

class TestClass2:
    __test__ = False

    def __init__(self, b: AbstractB):
        self.b = b

class TestClass3:
    def __init__(self, a: AbstractA, b: AbstractB):
        self.a = a
        self.b = b


The first step in the RRR pattern is to Register dependencies. This can be performed using either the register() or register_instance() methods. Next, objects can be built by Resolving. Finally, when the dependency is no longer needed, it can be Released.

from ophidian import DIContainer

dependency_b_instance = DependencyB("Mike")

di_container = DIContainer()
di_container.register(AbstractA, DependencyA)
di_container.register_instance(AbstractB, dependency_b_instance)

test_class_1 = di_container.resolve(TestClass1)
test_class_2 = di_container.resolve(TestClass2)
test_class_3 = di_container.resolve(TestClass3)

assert id(test_class_1.a) != id(test_class_3.a)
assert id(test_class_2.b) == id(dependency_b_instance)
assert id(test_class_2.b) == id(test_class_3.b)
assert isinstance(test_class_1.a, AbstractA)
assert isinstance(test_class_1.a, DependencyA)
assert isinstance(test_class_2.b, AbstractB)
assert isinstance(test_class_2.b, DependencyB)

# Note: All dependencies will automatically be released when the `di_container`
#       instance is cleaned up by the garbage collector.

In the above example, the DIContainer.register() method is used tell the DI container that components that depend upon AbstractA should be supplied a new instance of the concrete class DependencyA. The DiContainer.register_instance() method is used to tell the DI container that all components that depend upon AbstractB should be supplied with the same instance of DependencyB.


Conventions are mainly used to resolve primitive dependencies. For example, if a component depends upon a file path for a configuration file passed as a string, a convention can be registered to provide the dependency.

from ophidian import DIContainer

class Configuration:
    def __init__(self, a: DependencyA, config_file_path: str):

di_container = DIContainer()
di_container.register(AbstractA, DependencyA)
di_container.register_convention(str, "config_file_path", "/tmp/my_config_file")

configuration = di_container.resolve(Configuration)

di_container.release_convention(str, "config_file_path")

We wouldn't want to use register() or register_instance(), as we wouldn't want all string dependencies to be resolved with the configuration file path. Instead, the DI container is informed about an established "convention" within the code: components that depend upon the configuration file path expect a string parameter named "config_file_path". See Primitive Dependencies by Mark Seemann for more information about conventions.



class DIContainer()

A dependency injection (DI) container that uses type annotations to resolve and inject dependencies.


def __init__()


def register(interface: Type[T], concrete_type: Type[T])

Register a concrete type that satisfies a given interface.

:param interface: An interface or abstract base class that other classes depend upon
:param concrete_type: A type (class) that implements interface
:raises TypeError: If concrete_type is not a class, or not a subclass of interface


def register_instance(interface: Type[T], instance: T)

Register a concrete instance that satisfies a given interface.

:param interface: An interface or abstract base class that other classes depend upon
:param instance: An instance (object) of a type that implements interface
:raises TypeError: If instance is not an instance of interface


def register_convention(type_: Type[T], name: str, instance: T)

Register an instance as a convention

At times — particularly when dealing with primitive types — it can be useful to define a convention for how dependencies should be resolved. For example, you might want any class that specifies hostname: str in its constructor to receive the hostname of the system it's running on. Registering a convention allows you to assign an object instance to a type, name pair.


    class TestClass:
        def __init__(self, hostname: str):
            self.hostname = hostname

    di_container = DIContainer()
    di_container.register_convention(str, "hostname", "my_hostname.domain")

    test = di_container.resolve(TestClass)
    assert test.hostname == "my_hostname.domain"

:param type_: The type (class) of the dependency
:param name: The name of the dependency parameter
:param instance: An instance (object) of type_ that will be injected into constructors that specify [name]: [type_] as parameters



class UnresolvableDependencyError(ValueError):

Raised when one or more dependencies cannot be successfully resolved.

Running the tests

Running the tests is as simple as poetry install && poetry run pytest

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