aspyx 1.1.0

A DI and AOP library for Python

aspyx

Table of Contents

• Introduction
• Installation
• Registration
  • Class
  • Class Factory
  • Method
• Environment
  • Definition
  • Retrieval
• Instantiation logic
  • Injection Methods
  • Lifecycle Methods
  • Post Processors
• Custom scopes
• AOP
• Configuration
• Reflection
• Version History

Introduction

Aspyx is a small python libary, that adds support for both dependency injection and aop.

The following features are supported

• constructor and setter injection
• post processors
• factory classes and methods
• support for eager construction
• support for singleton and request scopes
• possibilty to add custom scopes
• lifecycle events methods
• bundling of injectable object sets by environment classes including recursive imports and inheritance
• container instances that relate to environment classes and manage the lifecylce of related objects
• hierarchical environments

The library is thread-safe!

Let's look at a simple example

from aspyx.di import injectable, on_init, on_destroy, environment, Environment

@injectable()
class Foo:
    def __init__(self):
        pass

    def hello(msg: str):
        print(f"hello {msg}")

@injectable()  # eager and singleton by default
class Bar:
    def __init__(self, foo: Foo): # will inject the Foo dependency
        self.foo = foo

    @on_init() # a lifecycle callback called after the constructor and all possible injections
    def init(self):
        ...


# this class will register all - specifically decorated - classes and factories in the own module
# In this case Foo and Bar

@environment()
class SampleEnvironment:
    def __init__(self):
        pass

# go, forrest

environment = Environment(SampleEnvironment)

bar = env.get(Bar)

bar.foo.hello("world")

The concepts should be pretty familiar as well as the names which are a combination of Spring and Angular names :-)

Let's add some aspects...

@advice
class SampleAdvice:
    def __init__(self): # could inject additional stuff
        pass

    @before(methods().named("hello").of_type(Foo))
    def call_before(self, invocation: Invocation):
        print("before Foo.hello(...)")

    @error(methods().named("hello").of_type(Foo))
    def call_error(self, invocation: Invocation):
        print("error Foo.hello(...)")
        print(invocation.exception)

    @around(methods().named("hello"))
    def call_around(self, invocation: Invocation):
        print("around Foo.hello()")

        return invocation.proceed()

The invocation parameter stores the complete context of the current execution, which are

• the method
• args
• kwargs
• the result
• the possible caught error

Let's look at the details

Installation

pip install aspyx

The library is tested with all Python version > 3.9

Ready to go...

Registration

Different mechanisms are available that make classes eligible for injection

Class

Any class annotated with @injectable is eligible for injection

Example:

@injectable()
class Foo:
    def __init__(self):
        pass

Please make sure, that the class defines a local constructor, as this is required to determine injected instances. All referenced types will be injected by the environemnt.

Only eligible types are allowed, of course!

The decorator accepts the keyword arguments

• eager : boolean
  if True, the container will create the instances automatically while booting the environment. This is the default.
• scope: str
  the name of a - registered - scope which will determine how often instances will be created.

The following scopes are implemented out of the box:

• singleton
  objects are created once inside an environment and cached. This is the default.
• request
  obejcts are created on every injection request
• thread
  objects are cerated and cached with respect to the current thread.

Other scopes - e.g. session related scopes - can be defined dynamically. Please check the corresponding chapter.

Class Factory

Classes that implement the Factory base class and are annotated with @factory will register the appropriate classes returned by the create method.

Example:

@factory()
class TestFactory(Factory[Foo]):
    def __init__(self):
        pass

    def create(self) -> Foo:
        return Foo()

As in @injectable, the same arguments are possible.

Method

Any injectable can define methods decorated with @create(), that will create appropriate instances.

Example:

@injectable()
class Foo:
    def __init__(self):
        pass

    @create(scope="request")
    def create(self) -> Baz:
        return Baz()

The same arguments as in @injectable are possible.

Environment

Definition

An Environment is the container that manages the lifecycle of objects. The set of classes and instances is determined by a constructor argument that controls the class registry.

Example:

@environment()
class SampleEnvironment:
    def __init__(self):
        pass

environment = Environment(SampleEnvironment)

The default is that all eligible classes, that are implemented in the containing module or in any submodule will be managed.

By adding an imports: list[Type] parameter, specifying other environment types, it will register the appropriate classes recursively.

Example:

@environment()
class SampleEnvironmen(imports=[OtherEnvironment])):
    def __init__(self):
        pass

Another possibility is to add a parent environment as an Environment constructor parameter

Example:

rootEnvironment = Environment(RootEnvironment)
environment = Environment(SampleEnvironment, parent=rootEnvironment)

The difference is, that in the first case, class instances of imported modules will be created in the scope of the own environment, while in the second case, it will return instances managed by the parent.

The method

shutdown()

is used when a container is not needed anymore. It will call any on_destroy() of all created instances.

Retrieval

def get(type: Type[T]) -> T

is used to retrieve object instances. Depending on the respective scope it will return either cached instances or newly instantiated objects.

The container knows about class hierarchies and is able to get base classes, as long as there is only one implementation.

In case of ambiguities, it will throw an exception.

Please be aware, that a base class are not required to be annotated with @injectable, as this would mean, that it could be created on its own as well. ( Which is possible as well, btw. )

Instantiation logic

Constructing a new instance involves a number of steps executed in this order

• Constructor call
  the constructor is called with the resolved parameters
• Advice injection
  All methods involving aspects are updated
• Lifecycle methods
  different decorators can mark methods that should be called during the lifecycle ( here the construction ) of an instance. These are various injection possibilities as well as an optional final on_init call
• PostProcessors
  Any custom post processors, that can add isde effects or modify the instances

Injection methods

Different decorators are implemented, that call methods with computed values

• @inject
  the method is called with all resolved parameter types ( same as the constructor call)
• @inject_environment
  the method is called with the creating environment as a single parameter
• @inject_value()
  the method is called with a resolved configuration value. Check the corresponding chapter

Example:

@injectable()
class Foo:
    def __init__(self):
        pass

    @inject_environment()
    def initEnvironment(self, env: Environment):
        ...

    @inject()
    def set(self, baz: Baz) -> None:
        ...

Lifecycle methods

It is possible to mark specific lifecyle methods.

• @on_init() called after the constructor and all other injections.
• @on_running() called an environment has initialized all eager objects.
• @on_destroy() called during shutdown of the environment

Post Processors

As part of the instantiation logic it is possible to define post processors that execute any side effect on newly created instances.

Example:

@injectable()
class SamplePostProcessor(PostProcessor):
    def process(self, instance: object, environment: Environment):
        print(f"created a {instance}")

Any implementing class of PostProcessor that is eligible for injection will be called by passing the new instance.

Please be aware, that a post processor will only handle instances after its own registration.

As injectables within a single file will be handled in the order as they are declared, a post processor will only take effect for all classes after its declaration!

Custom scopes

As explained, available scopes are "singleton" and "request".

It is easily possible to add custom scopes by inheriting the base-class Scope, decorating the class with @scope(<name>) and overriding the method get

def get(self, provider: AbstractInstanceProvider, environment: Environment, argProvider: Callable[[],list]):

Arguments are:

• provider the actual provider that will create an instance
• environmentthe requesting environment
• argPovider a function that can be called to compute the required arguments recursively

Example: The simplified code of the singleton provider ( disregarding locking logic )

@scope("singleton")
class SingletonScope(Scope):
    # constructor

    def __init__(self):
        super().__init__()

        self.value = None

    # override

    def get(self, provider: AbstractInstanceProvider, environment: Environment, argProvider: Callable[[],list]):
        if self.value is None:
            self.value = provider.create(environment, *argProvider())

        return self.value

AOP

It is possible to define different Aspects, that will be part of method calling flow. This logic fits nicely in the library, since the DI framework controls the instantiation logic and can handle aspects within a regular post processor.

Advice classes need to be part of classes that add a @advice() decorator and can define methods that add aspects.

@advice()
class SampleAdvice:
    def __init__(self):  # could inject dependencies
        pass

    @before(methods().named("hello").of_type(Foo))
    def call_before(self, invocation: Invocation):
        # arguments: invocation.args
        print("before Foo.hello(...)")

    @error(methods().named("hello").of_type(Foo))
    def call_error(self, invocation: Invocation):
        print("error Foo.hello(...)")
        print(invocation.exception)

    @around(methods().named("hello"))
    def call_around(self, invocation: Invocation):
        print("around Foo.hello()")

        return invocation.proceed()  # will leave a result in invocation.result or invocation.exception in case of an exception

Different aspects - with the appropriate decorator - are possible:

• before
  methods that will be executed prior to the original method
• around
  methods that will be executed around to the original method giving it the possibility add side effects or even change the parameters.
• after
  methods that will be executed after to the original method
• error
  methods that will be executed in case of a caught exception, which can be retrieved by invocation.exception

All methods are expected to hava single Invocation parameter, that stores, the function, args and kwargs, the return value and possible exceptions.

It is essential for around methods to call proceed() on the invocation, which will call the next around method in the chain and finally the original method. If the proceed is called with parameters, they will replace the original parameters!

The argument list to the corresponding decorators control, how aspects are associated with which methods. A fluent interface is used describe the mapping. The parameters restrict either methods or classes and are constructed by a call to either methods() or classes().

Both add the fluent methods:

• of_type(type: Type)
  defines the matching classes
• named(name: str)
  defines method or class names
• matches(re: str)
  defines regular expressions for methods or classes
• decorated_with(type: Type)
  defines decorators on methods or classes

The fluent methods named, matches and of_type can be called multiple times!

Example:

@injectable()
class TransactionAdvice:
    def __init__(self):
        pass

    @around(methods().decorated_with(transactional), classes().decorated_with(transactional))
    def establish_transaction(self, invocation: Invocation):
        ...

Configuration

It is possible to inject configuration values, by decorating methods with @inject-value(<name>) given a configuration key.

@injectable()
class Foo:
    def __init__(self):
        pass

    @value("OS")
    def inject_value(self, os: str):
        ...

This concept relies on a central object ConfigurationManager that stores the overall configuration values as provided by so called configuration sources that are defined as follows.

class ConfigurationSource(ABC):
    def __init__(self):
        pass

   ...

    @abstractmethod
    def load(self) -> dict:
        pass

The load method is able to return a tree-like structure by returning a dict.

As a default environment variables are already supported.

Other sources can be added dynamically by just registering them.

Example:

@injectable()
class SampleConfigurationSource(ConfigurationSource):
    def __init__(self):
        super().__init__()

    def load(self) -> dict:
        return {
            "a": 1, 
            "b": {
                "d": "2", 
                "e": 3, 
                "f": 4
                }
            }

Reflection

As the library heavily relies on type introspection of classes and methods, a utility class TypeDescriptor is available that covers type information on classes.

After beeing instatiated with

TypeDescriptor.for_type(<type>)

it offers the methods

• get_methods(local=False)
  return a list of either local or overall methods
• get_method(name: str, local=False)
  return a single either local or overall method
• has_decorator(decorator: Callable) -> bool
  return True, if the class is decorated with the specified decrator
• get_decorator(decorator) -> Optional[DecoratorDescriptor]
  return a descriptor covering the decorator. In addition to the callable, it also stores the supplied args in the args property

The returned method descriptors offer:

• param_types
  list of arg types
• return_type
  the retur type
• has_decorator(decorator: Callable) -> bool return True, if the method is decorated with the specified decrator
• get_decorator(decorator: Callable) -> Optional[DecoratorDescriptor]
  return a descriptor covering the decorator. In addition to the callable, it also stores the supplied args in the args property

The management of decorators in turn relies on another utility class Decorators that caches decorators.

Whenver you define a custom decorator, you will need to register it accordingly.

Example:

def transactional():
    def decorator(func):
        Decorators.add(func, transactional)
        return func

    return decorator

Version History

1.0.1

• some internal refactorings

1.1.0

• added @on_running() callback
• added thread scope