diapyr 29

Constructor injection for Python

diapyr

Constructor injection for Python

Overview

• Automatic wiring between application objects
• Declare what collaborator types you want in your constructor using an @types decorator, they correspond with params
  • You can decorate a (factory) function in the same way, in which case the return type must also be declared using 'this' kwarg
  • Surround a type in square brackets if you want a list of all matching objects, normally diapyr provides a unique match
• Add such classes/factories to a DI instance
  • You can also add objects, for example an application config object
• Request a type from the DI instance and diapyr will attempt to make it for you, along with the rest of the object graph
• Instances are cached in the DI object
  • On exit from 'with' clause, dispose is called on any created instances that have it

Motivation

• Manual wiring is messy and tedious especially when an app gets big
• Constructor injection avoids spaghetti code
• It's trivial to depend on an object anywhere in the graph as long as you don't create a reference cycle e.g. most objects will depend on the same config instance
  • No need to resort to globals, which come with a big risk of leaking state between unit tests
• Unit testing is easy when an object only interacts with what was passed into its constructor

Convention

• When depending on a config object the constructor should first extract what it needs and assign those values to fields
• Collaborators should also be assigned to fields, and ideally the constructor won't do anything else

Advanced

• Parameter defaults are honoured, this can be used to depend on module log object in real life and pass in a mock in unit tests
• Decorating an instance method (without 'this' kwarg) will make it behave as an additional constructor
  • Take advantage of name mangling (start with double underscore e.g. __init) to avoid having to call super
• Decorating an instance method with 'this' kwarg will make it behave as a factory function
  • Adding a class to DI will implicity add all such methods it has as factories
• You can play fast and loose with types, diapyr doesn't care whether a factoried object satisfies the declared type

Install

These are generic installation instructions.

To use, permanently

The quickest way to get started is to install the current release from PyPI:

pip3 install --user diapyr

To use, temporarily

If you prefer to keep .local clean, install to a virtualenv:

python3 -m venv venvname
venvname/bin/pip install -U pip
venvname/bin/pip install diapyr
. venvname/bin/activate