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Pyjection is a lightweight python dependency injection library

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Pyjection is a lightweight python dependency injection library

Basic dependency injection

The most import class is DependencyInjector which lets us register classes and retrieve instances.

from pyjection.dependency_injector import DependencyInjector

class OuterClass(object):

    def __init__(self, inner_class):
        self.inner_class = inner_class

class InnerClass(object):

    def __init__(self):
        self.foo = "bar"

container = DependencyInjector()
container.register(InnerClass)
container.register(OuterClass)

outer = container.get("outer_class")
print(outer.inner_class.foo) # Will print "bar"

Class bindings

Implicit class bindings

When no id is specified in the register method Pyjection creates implicit bindings for classes. The implicit bindings assume your code follows PEP8 conventions: your classes are named in CamelCase, and your args are named in lower_with_underscores. Pinject transforms class names to injectable arg names by lowercasing words and connecting them with underscores.

Class name Arg name
Foo foo
FooBar foo_bar

Explicit class bindings

It is also possible to manually set the id of a class when during its registration by specifying it as a second arguments.

container.register(FooClass, "inner_class")

With the example above, FooClass will later be injected to arguments named inner_class

Instance retrieval

To retrieve an instance of a class from the dependency injector 2 options are available in the get method:

  • Specify the lower_with_underscores name of the class as a string
  • Give the class as parameter
from pyjection.dependency_injector import DependencyInjector

class FooClass(object):
    pass

container = DependencyInjector()
container.register(FooClass)

container.get("foo_class")
# Same as
container.get(FooClass)

Singleton injection

The dependency injector lets us register a singleton. To register a singleton the method register_singleton may be used. It takes the same arguments as register.

from pyjection.dependency_injector import DependencyInjector

class SingletonClass(object):
    pass

container = DependencyInjector()
container.register_singleton(SingletonClass)
# Or we could specify an id
container.register_singleton(SingletonClass, "other_id")

class_1 = container.get("other_id")
class_2 = container.get("other_id")
print(class_1 is class_2) # True

Explicit argument specification

Simple argument specification

Sometimes the argument we need to inject is not an instance of a class. The register and register_singleton methods return a service object that lets us specify what we want to bind to a given argument by using the add_argument method.

from pyjection.dependency_injector import DependencyInjector

class FooClass(object):

    def __init__(self, foo):
        self.foo = foo

container = DependencyInjector()
service = container.register(FooClass)
service.add_argument("foo", "bar")

foo_class = container.get("foo_class")
print(foo_class.foo) # Will print bar

Reference argument specification

A service argument can also reference another dependency injector service. It is useful when we want to inject a class not matching the argument name.

from pyjection.dependency_injector import DependencyInjector
from pyjection.reference import Reference

class OuterClass(object):

    def __init__(self, inner_class):
        self.inner_class = inner_class

class FooClass(object):

    def __init__(self):
        self.foo = "bar"

container = DependencyInjector()
container.register(FooClass)
container.register(OuterClass).add_argument("inner_class", Reference('foo_class'))

instance = container.get(OuterClass)
print(instance.inner_class.foo) # Will print bar

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