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Constructor injection for Python

Project description

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 diapyr
. venvname/bin/activate

To develop

First clone the repo using HTTP or SSH:

git clone https://github.com/combatopera/diapyr.git
git clone git@github.com:combatopera/diapyr.git

Now use pyven's pipify to create a setup.py, which pip can then use to install the project editably:

python3 -m venv pyvenvenv
pyvenvenv/bin/pip install pyven
pyvenvenv/bin/pipify diapyr

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

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