xlevus/python-diana

A simple dependency injection framework.

Python-Diana

A simple async-friendly dependency injection framework for Python 3.

Supports:

• Async methods
• Parametrized dependencies
• Type annotations

Simple Example

import diana

class MyThing(object):
    def __init__(self, prefix):
        self.prefix = prefix

    def get(self, suffix):
        return self.prefix + suffix

class MyModule(diana.Module):
    @diana.provider
    def provide_thing(self) -> MyThing:
        return MyThing("a_prefix_")


@diana.injector
def requires_a_thing(*, thing: MyThing):
    return thing.get('suffix')

diana.injector.load_module(MyModule())

requires_a_thing()  # returns "a_prefix_suffix"

Parametrized Dependencies

import diana
import typing

Snake = typing.NewType('Snake', str)

class SnakeModule(diana.Module):
    @diana.provider
    def provide_snake(self, length: int) -> Snake:
        return Snake('-' + ('=' * length) + e)


@diana.injector(a_snake=Snake)
@diana.injector.param('a_snake', length=5)
def snake_printer(a_snake):
    print(a_snake, "Hissss")

diana.injector.load_module(SnakeModule())

snake_printer() # Prints: -=====e Hissss

Modules

Modules provide dependencies.

Note

The same dependency can be provided by both an async and sync providers as shown in the AType providers below.

class MyModule(diana.Module):
    def load(self, injector):
        # Called when the module is loaded against `injector`.
        pass

    def unload(self, injector):
        # Called when the module is unloaded against `injector`.
        pass

    @diana.provider
    def provide_atype(self) -> AType:
        return AType()

    @diana.provider
    async def provide_atype_async(self) -> AType:
        await async_stuff()
        return AType()

    @diana.provides(BType)
    def provide_btype(self):
        return BType()


@MyModule.provider
def provide_ctype(module) -> CType:
    return CType()


@MyModule.provides(DType)
def provide_dtype(module):
    return DType()

Injection Styles

There are three formats for injecting dependencies into functions

• Type Annotation w/ inspect
• Explicit
• Parametrized

The following examples are all equivalent.

# Type Annotation w/ Inspect
@diana.injector
def func_a(*, a: AType, b: BType):
    pass

# Explicit
@diana.injector.inject(a=AType, b=BType)
def func_a(*, a, b):
    pass

# Parametrized.
# Note, if the second argument to param() is omitted, the type must be
# hinted by one of the previous methods.
@diana.injector.param('a', AType, a_param=...)
@diana.injector.param('b', BType, b_param=...)
def func_a(*, a, b):
    pass

In all cases, injected arguments must be keyword-only.

Alternatively, a dependency can be manually provided, bypassing any injection.

func_a(a=AType(), b=BType())

Missing Features

Compared to other dependency injection frameworks, a few features are missing.

• Scope management - Currently there is no provision for scope to be managed by diana and remains the Module/provider's responsibility.
• Constructor/Instance Injecting - it is not possible to have Diana set attributes on instances by decorating the class definition.
• Thread safety - There have been no attempts to make Diana thread safe. In theory, if modules are only loaded once (presumably at runtime), thread safety can be managed by the Modules/providers.