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A simple, lightweight Python Dependency Injection Container (IOC), inspired by Pimple

Project description

Medley is a simple, lightweight Dependency Injection Container for Python, inspired by Pimple.

Requirements

Medley requires Python >=2.7 or Python >=3.2

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Installation

Install Medley using pip

$ pip install medley

Usage

Build your container by creating a MedleyContainer instance:

from medley import MedleyContainer

container = MedleyContainer()

Medley manages two different kind of data: services and parameters.

Defining Services

A service is an object that does something as part of a larger system. Examples of services: a database connection, a templating engine, or a mailer. Almost any global object can be a service.

Services are defined by callables (usually lambdas) that return an instance of an object:

Example using lambdas:

# define some services
container['session_storage'] = lambda c: SessionStorage('SESSION_ID')

container['session'] = lambda c: Session(c['session_storage'])

Notice that service definition functions do require the container argument. Lambdas must have access to the current container instance, allowing references to other services or parameters.

A service decorator is also available to wrap defined functions as a service

@container.service('session_storage')
def session_storage(c):
    return SessionStorage('SESSION_ID')

@container.service('session')
def session(c):
    return Session(c['session_storage'])

Objects are lazy-loaded, so the order in which you define services does not matter.

Getting a defined service is easy:

session = container['session']

# the above call is roughly equivalent to the following code:
# storage = SessionStorage('SESSION_ID')
# session = Session(storage)

Defining Factory Services

By default, each time you get a service, Medley returns the same instance of it. If you want a different instance to be returned for all calls, wrap your anonymous function with the factory() method

container['session'] = container.factory(lambda c: Session(c['session_storage']))

# you may also use a decorator

@container.create_factory('session')
def session(c):
    return Session(c['session_storage'])

Now, each call to container['session'] returns a new instance of the session.

Defining Parameters

Defining a parameter allows to ease the configuration of your container from the outside and to store global values:

# define some parameters
container['cookie_name'] = 'SESSION_ID';
container['session_storage_class'] = container.protect(SessionStorage);

If you change the session_storage service definition like below:

container['session_storage'] = lambda c: c['session_storage_class'](c['cookie_name'])

You can now easily change the cookie name by overriding the cookie_name parameter instead of redefining the service definition.

Protecting Parameters

Because Medley sees all callables as service definitions, you need to wrap callables with the protect() method to store them as parameters.

from random import random

container['random_func'] = container.protect(lambda: random())

# class types also need to be protected
container['session_storage_class'] = container.protect(SessionStorage);

Modifying Services after Definition

In some cases you may want to modify a service definition after it has been defined. You can use the extend() method to define additional code to be run on your service just after it is created:

container['session_storage'] = lambda c: c['session_storage_class'](c['cookie_name'])

container.extend('session_storage', lambda storage, c: storage.some_call()

The first argument of the lambda is the name of the service to extend, the second a function that gets access to the object instance and the container.

The available extends decorator is usually more user-friendly when extending definitions, particularly when a service needs to be modified and returned

@container.service('session_storage')
def session_storage(c):
    return c['session_storage_class'](c['cookie_name'])

@container.extends('session_storage')
def extended_session_storage(storage, c):
    storage.some_call()
    return storage

Extending a Container

You can build a set of libraries with Medley using the Providers. You might want to reuse some services from one project to the next one; package your services into a provider by implementing medley.ServiceProviderInterface:

from medley import MedleyContainer, ServiceProviderInterface

class FooProvider(ServiceProviderInterface):

    def register(container: MedleyContainer):
        # register some services and parameters on container
        container['foo'] = lambda c: return 'bar'

Then, register the provider on a MedleyContainer:

container.register(FooProvider())

Fetching the Service Creation Function

When you access an object via container['some_id'], Medley automatically calls the function that you defined, which creates the service object for you. If you want to get raw access to this function, you can use the raw() method:

container['session'] = lambda c: Session(c['session_storage'])

session_function = container.raw('session')

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