epoxy 1.0

Inversion of Control Framework for Python

Epoxy

Introduction

Epoxy is an inversion of control framework for Python applications packaged with a set of adapters that can be used with a number of existing technologies. Epoxy allows you to clearly separate out the concern of mapping dependencies to components from defining those components.

Epoxy is probably overkill for small apps (like the example below) but shows its true value on larger projects where a higher degree of flexibility and lower degree of coupling is desirable.

License

This library is available under the terms of the MPL. Please see the LICENSE file for details.

An Example

Here’s an example of an epoxy component:

from epoxy.component import Component, Dependency

class PrinterComponent(Component):

    prefix = StringSetting(required=False, default="PRINTING")

    def print(self, stuff):
        print ("[%s] %%s" % self.prefix) % stuff

    def start(self):
        print self.print("Warming up printer")


class ScreenRenderer(Component):

    printer = Dependency()

    def render_screen(self):
        self.printer.print("My Stuff")

    def start(self):
        self.printer.print("Screen Renderer Initialized")

In this example, ScreenRenderer does not need to know the precise printer which will be used as a dependency at runtime, it just needs to declare the dependency and use it (there is an assumed interface, as with much in python we just use duck typing).

To wire these components up at runtime, we would write a yaml file that would have something like this:

components:
  printer:
    class: my.module:PrinterComponent
    settings:
      prefix: PREFIX

  screen_renderer:
    class: my.module:ScreenRenderer
    dependencies:
      printer: printer

entry-point:
  screen_renderer:render_screen

Finally, to load in our configuration and run the application, we would write something like the following:

from epoxy.configuration import YamlConfigurationLoader
from epoxy.core import ComponentManager

loader = YamlConfigurationLoader("myapp.yml")
config = loader.load_configuration()
component_mgr = ComponentManager()
component_mgr.launch_configuration(config)

This would run our entry point and print the following to the screen:

[PREFIX] Warming up printer
[PREFIX] Screen Renderer Initialized
[PREFIX] my stuff

Writing a Component

Any Component that is written should follow these basic rules in order to play nicely:

1. No-args constructor: Components should have a no-args constructor in order to be instantiated by the framework. To initialize a Component in your own code (maybe from a unit test) you can use the “from_dependencies” classmethod available on any component and pass in any relevant dependencies/settings as keyword arguments.

   For instance, instantiating our previous object graph would look like the following:

   printer = PrinterComponent.from_dependencies(prefix="FROM_DEPS")
   renderer = ScreenRenderer.from_dependencies(printer=printer)

   When __init__ is executed it is guaranteed that all dependencies will be present and initialized (but not started).

2. Implement start() method: After all objects in the graph are instantiated, they will be started. Like with __init__, dependencies will be present and will have had start() called. Calls to start() should not block. For long-running tasks, using an entry-point is more appropriate.

3. Avoid Dependency Cycles: If the object graph has dependencies, you will get errors when trying to build the dependency graph and you will not be able to run your application. There are many different strategies for breaking these cycles. If you run into this issue and are ticked off, just know that fixing this issue is making your application better.