simplistic dependency injection container for python
This package provides a simple inversion-of-control container.
pip install pythonioc
The main idea is: services are registered to a service-registry and can be injected into users of that service (which can of course be services themselves).
You have two options:
- use a global registry (never create a registry yourself)
- use a local registry
Examples are below, more details will follow.
import pythonioc # register your service with a default name (here: 'someService', see Notes On Names) @pythonioc.Service class SomeService(object): # called when the service is auto-instantiated. def postInit(self): pass @pythonioc.NamedService('DifferentNameService') class DifferentService(object): pass # for classes which we cannot decorate: pythonioc.registerService(ExternalService) # when we don't even have the class (or don't care about lazy-initialization) pythonioc.registerServiceInstance(SomeService()) class ServiceUser(object): # inject the dependency by class service = pythonioc.Inject(SomeService) # inject the dependency by name (for cyclic dependencies) service2 = pythonioc.Inject('DifferentNameService') myUser = ServiceUser() myUser.service # --> automatically created and injected service instance. # explicitly get a service pythonioc.getService(SomeService) pythonioc.getService('DifferentNameService')
class Service(object): # this will make the service registry inject a service named "someOtherService" which # comes from class SomeOtherService _someOtherService = None def __init__(self): pass # will be called after everything is injected def postInit(self): pass # will be called right before the object is destroyed (the registry's clean # method is called) def preDestroy(self): pass class SomeOtherService(object): pass # let's register our services reg = ServiceRegistry() reg.registerService(Service) reg.registerService(SomeOtherService)
Once everything is registered, a service can be injected by
class WiredUser(object): _service=None def __init__(self, *args): pass wiredUser = reg.createWired(WiredUser, 'arg1', 'arg2')
Wired objects are not automatically part of the service registry, only if added by calling reg.registerServiceInstance.
Wired objects can inject their own service registry, so they can create wired objects on the fly:
class WiredUser(object): _service=None class UserCreator(object): _serviceRegistry=None def createUser(self): return self._serviceRegistry.createWired(WiredUser) userCreator = reg.createWired(UserCreator) # create some wired users userA = userCreator.createUser() userB = userCreator.createUser()
Notes on Names
Services added to the registry need a name. If no name is provided, the class’ name (or the instance’s class name) is used. The name’s first character is lowered by convention.
import pythonioc class MyService(object): pass pythonioc.registerService(MyService) # --> name is 'myService' pythonioc.registerServiceInstance(MyService()) # --> name is 'myService' pythonioc.registerService(MyService, serviceName='customName') # --> name is 'customName' pythonioc.registerServiceInstance(MyService(), serviceName='customName2') # --> name is 'customName2'
Notes on Dependency Cycles
Generally, dependency cycles are resolved by lazily initializing services. Critical cycles can still occur, when two services depend on each other within their postInit-methods that are executed after initialization. Those cycles are detected by keeping track of the currently running postInit methods and throwing an exception.
The service creation is thread safe, so accessing a non-initialized service with a long-running init or postInit method from two different threads will block one of the threads.
Bug reports, suggestions for improvements etc. are always welcome!
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