Simple Dependency Injection in Python

May 8, 2021

Dependency injection is an software design pattern that basically amounts to parameterizing every object by the services that it uses, and forcing the instantiator of an object to supply objects representing the services needed by the created object.

I'm somewhat torn on dependency injection. On the positive side, decoupling an object from the services it uses is almost a requirement for any form of sane unit testing, since then you can test an object by supplying proxy services that can be controlled by the testing environment. On the negative side, it adds an extra layer of indirection that makes code difficult to read, difficult to maintain, and often adds visual noise that distracts from what an object does. (Dependency injection often comes with an attendant architectural infrastructure, of the kind best examplified by the Java Spring Framework.)

Is it possible to get some of the advantages of dependency injection for unit testing without straying too much from how you would normally code?

Here's a positive answer to the question in the context of Python. It is far from an ideal solution, but it does provide a lightweight form of dependency injection that should suffice for unit testing.

Consider the following simple Example class that depends on a simple Service class:

class Example:
    def __init__(self, arg):
        self._arg = arg

    def run(self):
        obj = Service(self._arg)

class Service:
    def __init__(self, arg):
        self._arg = arg + 1

    def print(self):
        print(f'In default service = {self._arg}')

We want the ability to instantiate Example with an alternate service Service2 in place of Service in some situations (such as testing):

class Service2:

    def __init__(self, arg):
        self._arg = arg + 100

    def print(self):
        print(f'In service 2 = {self._arg}')

To do that, we introduce a class decorator injectable that takes as arguments all the services used in the class that we want to be "injectable" at instantiation time. That decorator intuitively adds an instance variable for every service listed and initialized with the given service. The initialization can be overridden using a keyword argument on the class constructor with the same name as the service being injected.. To take advantage of this injection, we simply need to adjust every reference to an "injectable" service within the class so that it uses the instance variable named after the service. For example:

class Example:
    def __init__(self, arg):
        self._arg = arg

    def run(self):
        obj = self.Service(self._arg)

Instantiating the class with the default service does not require any extra work:

>>> e1 = Example(10)
In default service = 11

And instantiating the class with an alternative service simply requires passing the alternative service during instantiation as a keyword argument for the keyword corresponding to the service to be injected:

>>> e2 = Example(10, Service=Service2)
In service 2 = 110

The main downside of this approach is the need to qualify every "injectable" service reference with the self argument within the class. adding visual noise and a potential failure point if one such reference is not qualified while others are.

For completeness, here's a reference implementation of the injectable decorator:

def injectable(*services):

    def inject(cls):
        original_init = cls.__init__

        names = [ service.__name__ for service in services ]

        def new_init (self, *args, **kwargs):
            kwargs_ = { k: kwargs[k] for k in kwargs if k not in names }
            self._services =  { service.__name__: service for service in services}
            for n in names:
                if n in kwargs:
                    self._services[n] = kwargs[n]
            original_init(self, *args, **kwargs_)

        def get_attribute(self, attr):
            if attr in self._services:
                return self._services[attr]
                raise Exception('no such attribute')

        cls.__init__ = new_init
        cls.__getattr__ = get_attribute
        return cls

    return inject

You can download the full code.