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Lightweight dependency injection for pure OOP.

Project description


Injecting class dependencies comes in 3 distinct ways :

  • using class attributes (okay)
  • using setters (better)
  • using constructor (best)

Wyre allows you to declare the dependencies of a given class using kwargs on a constructor. A single decorator @inject does the trick. This is particularly handy when your dependency tree grows large and deep. For example, this dependency chain : A < B < C < D < E, would require you to write a = A(B(C(D(E())))) in order to create an instance of your class.

Using Wyre, you keep :

  • your production code clean by writing just A() since it works recursively
  • your unit tests simple : A(b=Mock()) is all you need to mock out dependencies


class C:
    name = 'Bob'

class B:
    def __init__(self, other_dependency=C):
        self.c = other_dependency

    def say_hello(self):
        return 'Hello %s !' %

class A:
    def __init__(self, dependency=B):
        self.b = dependency

    def greetings(self):
        return self.b.say_hello()

a = A()
a.greetings() # returns 'Hello Bob !'

Since __init__ is decorated with @inject, B instance will be created and injected in A at instantiation time.


Important notes on what @inject does :

  • If an instance of a dependency is provided in kwargs, it will be preserved and not overridden by a new instance.
  • Circular dependencies are detected at instantiation time : an InjectionError will be raised.
  • You can only use it on __init__(). If you decorate any other function : an InjectionError will be raised.
  • If no dependency is found among declared kwargs : an InjectionError will be raised.
  • Wyre is strongly opinionated about dependency injection. As a matter of fact, singletons are not even considered.

Project details

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