wirinj 0.6.1

A new dependency injection library for Python 3 with an original and clean design.

Wirinj

A new dependency injection library for Python 3 with an original and clean design.

Why another dependency injection library?

Working on a large project I tried out python-dependency-injector first. Then, I decided to switch to pinject. As none of them had the features I wanted, I finally decided to write my own library to meet my needs:

• Minimal boiler plate code.
• Easy refactorings.
• Friendly with IDE's code completion (e.g. with PyCharm).
• Using type annotations.
• Easy to define and use of factories.
• Autowiring option.
• Easy to fix dependency problems.
• Detailed injection reports.
• Powerful but simple wiring configuration.
• Avoiding name conventions.
• Avoiding dependencies to the injection library itself.
• Private injection.
• Open and extendable architecture.

Installation

python >= 3.5

$ pip install wirinj

Minimal example

minimal.py:

from wirinj import Injector, Autowiring

class Cat:
    pass

inj = Injector(Autowiring())

cat = inj.get(Cat)
print(cat)

Returns:

<__main__.Cat object at 0x7fbe537f52b0>

Basic usage

Example (basic_usage.py):

from wirinj import Injector, Definitions, Instance, Singleton, Factory

class Cat:
    pass

class Dog:
    pass

# Wiring definitions
defs = {
    'sound': 'Meow',
    Cat: Instance(),
    Dog: Singleton(),
    'cat_factory': Factory(Cat),
}

# Create injector
inj = Injector(Definitions(defs))

Get a 'sound':

sound = inj.get('sound')
print(sound)

Meow

Get a Cat:

cat1 = inj.get(Cat)
cat2 = inj.get(Cat)

print(cat1)
print(cat2)
print(cat1 is cat2)

<__main__.Cat object at 0x7fab3ddc2208>
<__main__.Cat object at 0x7fab3c1fa438>
False

Cat returns different instances each time because it is defined as an Instance().

Get the Dog:

dog = inj.get(Dog)
dog2 = inj.get(Dog)

print(dog)
print(dog2)
print(dog is dog2)

<__main__.Dog object at 0x7fab3c1fa748>
<__main__.Dog object at 0x7fab3c1fa748>
True

Dog always returns the same object because it is defined as a Singleton().

Get a cat factory and use it to create a new Cat:

cat_factory = inj.get('cat_factory')
cat3 = cat_factory()

print(cat3)

<__main__.Cat object at 0x7fc6a98119b0>

cat_factory returns a factory because it is defined as a Factory(Cat).

The @inject decorator

If you are using an IDE such as PyCharm, you will notice that code completion do not work with the previous example. The IDE cannot know the type of, for example, cat1 or dog.

You can be IDE-friendly by decorating a function with @inject:

Example (inject_function.py):

from wirinj import inject

defs = {
    'sound': 'Meow',
    Cat: Instance(),
    Dog: Singleton(),
    'cat_factory': Factory(Cat),
}

@inject(Definitions(defs))
def fn(cat1: Cat, cat2: Cat, dog: Dog, sound, cat_factory):

    print('sound:', sound)
    print('cat1:, cat1)
    print('cat2:', cat2)
    print('cat1 = cat2: ', cat1 is cat2)
    print('dog:', dog)

    cat3 = cat_factory()
    print('cat3:', cat3)

fn()

Returns:

sound:  Meow
cat1:  <__main__.Cat object at 0x7f675ce5db38>
cat2:  <__main__.Cat object at 0x7f675ce5df28>
cat1 = cat2:  False
dog:  <__main__.Dog object at 0x7f675ce5dac8>
cat3:  <__main__.Cat object at 0x7f675ce72400>

@inject inspects the arguments of the fn function signature and inject the required dependencies. You get all the needed dependencies through the function arguments. You type less and it is clearer and IDE friendly.

As cat1 and cat2 arguments matches Cat: Instance() configuration, two new Cat instances are injected.

Simillary, dog matches Dog: Singleton(). Therefore, any Dog argument will be injected with the same single Dog.

In contrast, for sound and cat_factory, the configuration entry is matched by name and not by class. This is because, in the wiring configuration dict, the key of 'sound': 'Meow' and 'cat_factory': Factory(Cat) are of type string:

• When the key is a class, the injector will look at the type annotation of the argument.
• When the key is a string the injector will look at the name of the argument.

Factories

You can object that, in the previos example, the code completion will not work with cat_factory. That's true; fortunately, this can be easily fixed:

IDE friendly factories

Just add a type annotation of type Type[Cat] to the factory argument:

Example (ide_friendly_factory.py):

from typing import Type

class Cat:
    def __init__(self, sound):
        pass

...

@inject(...)
def fn(cat_factory: Type[Cat]):
    cat = cat_factory('Meow')
    print('cat:', cat)

fn()

Now, it is recognized by the IDE:

Type[Cat], as explained in the typing module docs, represents the class Cat or a subclass of it:

from typing import Type

class Cat:
    pass

class BlackCat(Cat):
    pass

def func(cat_class: Type[Cat]):
    return cat_class()

print('Cat: ', func(Cat))
print('BlackCat: ', func(BlackCat))

Returns:

Cat:  <__main__.Cat object at 0x7fc458bc7588>
BlackCat:  <__main__.BlackCat object at 0x7fc458bc7588>

Defining the factory by type

To not be limited to a particular name, such as cat_factory, we can define the factory dependency by type.

defs = {
    Type[Cat]: Factory(),
}

This way you can name the argument as you like, as long as its type annotation is Type[Cat]. For example:

@inject(...)
def fn(cats: Type[Cat]):
    cat = cats()

Dependency definitions

Definitions class is a convenient way to configure the wiring of your classes according to a definition dict. Most of the previous examples have made use of a Definitions object with a dict as an argument. Now I'm going to explain it in detail.

Definition format

One or several dict define your wiring configuration. Each key represents the argument to be injected. The value represents how is it injected:

defs = {
    Cat: Instance(),
    'dog': Singleton(Dog),
}

inj = Injector(Definitions(defs))

dict keys

• If the key is a class it will match the argument's type annotation. E.g.: the first key in the example above causes any argument of type Cat, no matter its name, to be injected with a new instance of Cat.

• If the key is a string it will match the argument name. E.g.: the second key causes any argument whose name is 'dog' to be injected with a unique Dog instance. Here, as the class can't be inferred from the key, you need to explicitly provide the class as an argument: 'dog': Singleton(Dog).

dict values

Each value in the dict can be:

• A literal value you want to be injected. E.g. 'db_name': 'my-db'.
• Instance: inject a new instance each time.
• Singleton: inject the same single instance every time.
• Factory: inject a factory object that can be called to create new objects dynamically.
• CustomInstance: similar to Instance but you provide a custom function which returns new instances.
• CustomSingleton: similar to Singleton but you provide a custom function which returns the singleton.
• Other user defined subclasses of DependencyBuilder or Dependency.

Example (definition_types.py):

def dog_creator():
    """ Custom instantiation """
    ...
    return dog

defs = {
    House: Singleton(),
    Cat: Instance(),
    Type[Cat]: Factory(),
    Dog: CustomInstance(dog_creator),
    Type[Dog]: Factory(),
}

@inject(Definitions(defs))
def fn(house: House, cat_factory: Type[Cat], dog_factory: Type[Dog]):
    cat = cat_factory()
    dog = dog_factory()
    ...    

fn()

Providing a class

Instance, Singleton and Factory accept an optional class argument to specify the class of the object to be created. You pass the class in two use cases:

• The key is a string and therefore the dependency class is undefined.
• The argument is annotated with a base class but you want to provide a subclass of it.

Example of both cases (explicit_type.py):

class Pet:
    pass

class Cat(Pet):
    pass

class Dog(Pet):
    pass

defs = {
    'cat': Singleton(Cat),
    Pet: Singleton(Dog),
}

@inject(Definitions(defs))
def fn(cat, pet: Pet):
    print('cat is a', cat.__class__.__name__)
    print('pet is a', pet.__class__.__name__)    
fn()

returns:

cat is a Cat
pet is a Dog

Creation path

class Nail:
    pass

class Leg:
    def __init__(self, nail: Nail):
        pass

class Cat:
    def __init__(self, leg: Leg):
        pass

Imagine you ask for a Cat which requires a Leg which requires a Nail. The injector will gather:

• First, the Nail that has no dependencies.
• Then, the Leg with the Nail as an argument.
• Finally, the Cat with the Leg as an argument.

We can think of this process as a path: Cat -> nail:Nail -> leg:Leg. I call this the creation path.

You can explicitly specify a creation path constraint in the definition dict.

Example (creation_path.py):

class Animal:
    def __init__(self, sound):
        self.sound = sound

class Dog(Animal):
    pass

class Cat(Animal):
    pass

class Cow(Animal):
    pass

defs = {
    Dog: Instance(),
    Cat: Instance(),
    Cow: Instance(),

    (Dog, 'sound'): 'woof',
    (Cat, 'sound'): 'meow',
    'sound': '?',
}

@inject(Definitions(defs))
def fn(cat: Cat, dog: Dog, cow: Cow):
    print('Cat:', cat.sound)
    print('Dog:', dog.sound)
    print('Cow:', cow.sound)

fn()

Returns:

Cat: meow
Dog: woof
Cow: ?

To restrict a definition entry to a particular creation path we use a tuple in the key part. This tuple must match the last entries in the creation path.

For each tuple entry, a string refers to the argument name and a class refers to the argument type annotation.

If two entries match the required dependency, the more specific one will be chosen.

Custom dependencies

Instance and Singleton are used for simple class instantiation. When a custom process is required to create or locate the dependency, use CustomInstance or CustomSingleton. Both take a function as an argument.

Example (custom_dependencies.py):

from random import randint

class Cat:
    def __init__(self, color, weight):
        self.color = color
        self.weight = weight

    def __str__(self):
        return 'A {1} pounds {0} cat.'.format(self.color, self.weight) 

def create_cat(color):
    return Cat(color, randint(4, 20)) 

defs = {
    'color': 'blue',
    Cat: CustomInstance(create_cat),
}

@inject(Definitions(defs))
def fn(cat1: Cat, cat2: Cat, cat3: Cat):
    print(cat1)
    print(cat2)
    print(cat3)

fn()

returns:

A 11 pounds blue cat.
A 5 pounds blue cat.
A 14 pounds blue cat.

Custom dependencies with arguments

In the previous example, the object is instantiated without arguments, so all of __init__'s arguments are injected from dependencies.

If your class requires some arguments to be passed to be created (explicit arguments) in addition to the automatically injected dependencies (injection arguments), I recommend to follow these rules:

0. In the __init__ method, place the explicit arguments in the first place and then, the injection arguments. This allow you to use positional arguments when you create the object.

1. Set the default value of the injection arguments to None. This way the IDE code completion will not complain about missing arguments. Also, this is the only way you can have defaults in your explicit arguments if they are followed by injection arguments.

2. About the creation function that you pass to CustomInstance, use the same name and position for the explicit arguments as you use in the __init__ method. The rest of the arguments don't have to be related at all to the __init__ arguments. Indeed, you can specify as many dependency arguments as you need to create the object. The injection process will inspect the function signature and will provide them.

Example (custom_dependencies_with_args.py):

class Cat:
    def __init__(self, name, color=None, weight=None):
        self.name = name
        self.color = color
        self.weight = weight

    def __str__(self):
        return '{0} is a {2} pounds {1} cat.'.format(self.name, self.color, self.weight)

def create_cat(name, color):
    return Cat(name, color, randint(4, 20))

defs = {
    'color': 'black',
    Cat: CustomInstance(create_cat),
    Type[Cat]: Factory(),
}

@inject(Definitions(defs))
def fn(factory: Type[Cat]):
    cat = factory('Tom')
    print(cat)
    cat2 = factory('Sam')
    print(cat2)

fn()

returns:

Tom is a 8 pounds black cat.
Sam is a 14 pounds black cat.

About the 3 arguments of Cat.__init__:

• One comes from calling the factory.
• Another one from the dependency configuration.
• The third is generated by the custom creation function.

Setting default to Injected

Instead of setting the default value of the injection arguments to None, you can use the special value Injected:

• Pros: if one argument is missing, a specific MissingDependenciesError is raised. Otherwise, the execution would continue with None default and fail later in an unpredictable way.
• Cons: you add a dependency to wirinj.

from wirinj import Injected

class Cat:
    def __init__(self, name, color=Injected, weight=Injected):
        ...

Splitting definitions

You can split the dependency configuration in several dict definitions.

Example (splitted_definitions.py):

defs1 = {
    Cat: Instance(),
    Type[Cat]: Factory()
}

defs2 = {
    Engine: Instance(),
    Type[Engine]: Factory()
}

@inject(Definitions(defs1, defs2))
def fn(...):
    ...

fn()

Definitions accepts any number of definition dicts.

Autowiring

Dependency injection comes at the cost of having to maintain it. The Autowiring class makes this task lighter.

You add Autowiring at the end of the wiring configurations as a last resort to provide a dependency on the fly when it is undefined. The dependency type, Instance, Singleton or Factory, is chosen by heuristic rules.

Let's view an example:

Example (autowiring.py):

    class Cat:
        pass

    class Dog:
        pass

    class Horse:
        pass

    defs = {
        Cat: Instance(),
    }

    @inject(Definitions(defs), Autowiring())
    def fn(cat: Cat, dog: Dog, horse_factory: Type[Horse]):
        print(cat.__class__.__name__)
        print(dog.__class__.__name__)
        horse = horse_factory()
        print(horse.__class__.__name__)

    fn()

Returns:

Cat
Dog
Horse

3 dependencies are automatically generated by Autowiring:

• A Dog singleton.
• A Type[Horse] factory.
• A Horse instance.

Heuristic rules

Autowiring works only for arguments that have a type annotation:

• If the type of the annotation is a class, as with dog: Dog in the previous example, a singleton will be generated.
• If the type is a Type[class], as with horse_factory: Type[Horse], a factory will be provided.
• If the injection occurs in a factory, as when horse_factory() is called, an instance will be created.

Autowiring for production

In my opinion, this kind of magic should not be used in production environments; you should not take the risk to leave such important wiring decisions in the hands of a heuristic algorithm.

Fortunately, you can use AutowiringReport class to easily convert the autowiring configuration into a regular dependency definition:

Autowiring report

It's quite simple to use; just pass an AutowiringReport instance to Autowiring:

Example (autowiring_report.py):

report = AutowiringReport()

@inject(Definitions(deps), Autowiring(report))
def fn(cat: Cat, dog: Dog, horse_factory: Type[Horse]):
    ...    

fn()

print(report.get())

Returns:

...
--------------- wirinj ---------------
Autowiring report:

Definitions({
    Dog: Singleton(),
    Type[Horse]: Factory(),
    Horse: Instance(),
}),
--------------------------------------

Call report.get() to get the report. Review and copy the definitions to your configuration file, remove Autowiring, and you will be production ready.

No singletons option

You may set use_singletons to False to force all dependencies to be injected as an Instance.

Autowiring(use_singletons=False)

Injection reports

During each injection process, a dependency tree is built with all the dependencies that are being gathered.

As you change your code, your dependency configuration can get out of sync. wirinj include reporting features that can help you to solve this dependency issues:

Debugging the injection

The injection process can be debugged to expose the creation order and the dependency tree.

Take this composition of classes (cat_example_classes.py) :

class Nail:
    pass

class Leg:
    def __init__(self, nail1: Nail, nail2: Nail, nail3: Nail, nail4: Nail, nail5: Nail):
        pass

class Mouth:
    pass

class Ear:
    pass

class Eye:
    pass

class Head:
    def __init__(self, mouth: Mouth, ear1: Ear, ear2: Ear, eye1: Eye, eye2: Eye):
        pass

class Body:
    pass

class Tail:
    pass

class Cat:
    def __init__(self, head: Head, body: Body, tail: Tail, leg1: Leg, leg2: Leg, leg3: Leg, leg4: Leg):
        pass

Now, we can debug the injection process just by setting the logging level to DEBUG and then, requesting a Cat from the Injector:

Example (injection_debug_report.py):

import logging

logging.basicConfig(level=logging.DEBUG, format='%(message)s')

inj = Injector(Autowiring(use_singletons=False))

cat = inj.get(Cat)

Note that we are replacing all the dependency definitions by a simple Autowiring(). We pass the argument use_singletons=False to force all dependencies to be injected as an Instance. By default Autowiring generates Singleton dependencies and, in this case, we don't want all the legs of the Cat to be the same.

The code above returns this:

--------------- wirinj ---------------
        mouth:Mouth
        ear1:Ear
        ear2:Ear
        eye1:Eye
        eye2:Eye
    head:Head
    body:Body
    tail:Tail
        nail1:Nail
        nail2:Nail
        nail3:Nail
        nail4:Nail
        nail5:Nail
    leg1:Leg
        nail1:Nail
        nail2:Nail
        nail3:Nail
        nail4:Nail
        nail5:Nail
    leg2:Leg
        nail1:Nail
        nail2:Nail
        nail3:Nail
        nail4:Nail
        nail5:Nail
    leg3:Leg
        nail1:Nail
        nail2:Nail
        nail3:Nail
        nail4:Nail
        nail5:Nail
    leg4:Leg
:Cat
--------------------------------------

You can see how all the dependencies are gathered, and in which order. The final object is the requested Cat object.

Missing dependencies

Injection doesn't stop when a dependency is missing. It continues building the dependency tree as far as it can. This makes it possible to fix several dependency issues in one shot.

If one ore more dependencies are missing, an ERROR level report will be logged. Therefore, you don't need to change the logging level to get it; just look above the error traceback after a dependency exception.

Example (missing_dependencies_report.py):

    from examples.report.cat_example_classes import Cat, Head
    from wirinj import Injector, Definitions, Instance

    inj = Injector(Definitions({
        Cat: Instance(),
        Head: Instance(),
    }))
    cat2 = inj.get(Cat)

In the example above, we only define the wiring for Cat and Head. All the other dependencies, such as Mouth, Ear, Eye, etc, are undefined.

After running the example, we get:

--------------- wirinj ---------------
Missing dependencies:
        mouth:Mouth *** NotFound ***
        ear1:Ear *** NotFound ***
        ear2:Ear *** NotFound ***
        eye1:Eye *** NotFound ***
        eye2:Eye *** NotFound ***
    head:Head
    body:Body *** NotFound ***
    tail:Tail *** NotFound ***
    leg1:Leg *** NotFound ***
    leg2:Leg *** NotFound ***
    leg3:Leg *** NotFound ***
    leg4:Leg *** NotFound ***
:Cat
--------------------------------------
Traceback (most recent call last):
...
wirinj.errors.MissingDependenciesError: Missing dependencies.

Notice that, although the first dependency, Mouth, failed to be satisfied, the injection process continues in order to gather as much information as possible about the missing dependencies.

With this report, it becomes clear which classes are undefined, and what needs to be added in the injection configuration.

Instance error

If an exception is raised during the instantiation of any of the dependencies, you will not get the dependency tree logs as it happens when a dependency is missing.

You'll need to track the stack trace to fix the problem. However, there is a task planned in the TO-DO list to log the dependency tree in these cases too.

Private injection

You use dependency injection to decouple your class from its dependencies. A factory is a specific type of dependency which allows you to create objects dynamically.

Some factories require arguments to create the new object. These arguments are passed to the __init__ method along with other required dependencies.

I call private injection to a special form of injection that separates the __init__ arguments in two groups:

• Public arguments: those you pass to the factory.
• Private arguments: other implementation dependencies.

Let's compare the two modes:

Regular injection

Some of the __init__ arguments come from the factory call; the remaining ones are injected automatically.

Example (regular_injection.py):

class Cat:
    def __init__(self, color, weight, feeder: Feeder = None):
        ...

@inject(...)
def fn(factory: Type[Cat]):
    cat = factory('blue', 12)
...

While color and weight are passed to the factory, the feederargument is injected.

Private injection

This is the equivalent private form for the previous code:

Example (private_injection.py):

class Cat:        
    def __deps__(self, feeder: Feeder):
        pass

    @deps
    def __init__(self, color, weight):
        ...

@inject(...)
def fn(factory: Type[Cat]):
    cat = factory('blue', 12)

The @deps decorator works this way:

0. The @deps decorator on __init__ enables private injection.
1. The special method __deps__ define which dependencies are required.
2. All the dependencies are located and injected before the object is initialized.
3. Finally, the real __init__ method is called.

Notice that __deps__ is a mock method, it is not ever called. Its sole purpose is to define the dependencies through its argument signature.

Why using private injection

You split the initialization arguments in two parts:

• __init__ takes only the public API arguments.
• Private arguments are hidden and defined by the __deps__ method.

pros:

• You keep your implementation dependencies apart of the public __init__ interface.
• Calling a factory is beautifully identical to instantiate a real class.
• You keep the same __init__ signature on all subclasses.
• Each subclass can have different dependencies.
• You don't have to code a new factory wrapper for each class to hide the implementation dependencies.
• You can use positional arguments to call the factory.
• You don't need to set the default values to Null for the dependency arguments.
• You can freely use default values in __init__.

cons:

• Your class will be dependent on the @deps decorator.

@deps decorator

In the previous sections, I have explained how this decorator is used. What I'm going to explain here is related only to the internal functioning of the feature.

When you wrap __init__ with the decorator @deps, two simple things happen: 0. A hidden argument _dependencies is added to __init__ waiting for a dict to be passed. 0. The object is initialized with these dependencies before __init__ is called.

Notice that you can use this mechanism, even if you are not using other wirinj features, by setting the _dependencies argument manually. By the time __init__ is called, your object dependencies will already be initialized.

Enabling code completion with __deps__

Of course, the IDE (e.g. PyCharm) will not detect the dependencies defined by __deps__:

class Cat:        
    def __deps__(self, feeder: Feeder):
        pass

Ideally, an IDE plugin would detect the __deps__ signature, making the dependencies available in code completion.

As long as this plugin doesn't exist, you can achieve the same by adding some mock code to the __deps__ method:

class Cat:        
    def __deps__(self, feeder: Feeder):
        self.feeder = feeder

It will never be executed, but now, the IDE will recognize the dependencies as member variables:

A full injection example

This silly example aims to illustrate several aspects of the wirinj library.

The two main classes, Bob and Mike, extend PetDeliveryPerson. They are used to deliver pets to the client using one or more vehicles. Whenever a Vehicle is needed, it is built in advance.

While Bob uses his only vehicle by repeating the route several times, Mike builds a fleet of autonomous vehicles to deliver all the pets in one trip.

The classes (pet_delivery/classes.py):

class Pet:
    def __deps__(self, sound: str, weight):
        self.sound = sound
        self.weight = weight

    @deps
    def __init__(self, gift_wrapped):
        self.gift_wrapped = gift_wrapped

    def cry(self):
        return self.sound.lower() if self.gift_wrapped else self.sound.upper()


class Cat(Pet):
    pass


class Dog(Pet):
    pass


class Bird(Pet):
    pass


class Part:
    def __deps__(self, mount_sound):
        self.mount_sound = mount_sound

    @deps
    def __init__(self):
        pass

    def mount(self):
        return self.mount_sound


class Engine(Part):
    pass


class Plate(Part):
    pass


class Wheel(Part):
    pass


class Container(Part):
    pass


class VehicleBuilder:

    def __deps__(self,
                 engine_factory: Type[Engine],
                 plate_factory: Type[Plate],
                 wheel_factory: Type[Wheel],
                 container_factory: Type[Container],
                 ):
        self.engine_factory = engine_factory
        self.plate_factory = plate_factory
        self.wheel_factory = wheel_factory
        self.container_factory = container_factory

    @deps
    def __init__(self):
        pass

    def build(self, recipe: Dict):
        parts = []  # type: List[Part]
        parts += [self.engine_factory() for _ in range(recipe.get('engines', 0))]
        parts += [self.plate_factory() for _ in range(recipe.get('plates', 0))]
        parts += [self.wheel_factory() for _ in range(recipe.get('wheels', 0))]
        parts += [self.container_factory() for _ in range(recipe.get('containers', 0))]

        mounting = ''
        for part in parts:
            mounting += ' ' + part.mount()

        return mounting


class Vehicle:
    def __deps__(self, builder: VehicleBuilder, recipe: Dict, max_load_weight):
        self.builder = builder
        self.recipe = recipe
        self.max_load_weight = max_load_weight

    @deps
    def __init__(self):
        self.pets = []
        self.build()

    def go(self, miles):
        logger.info('{} goes {} miles'.format(self.__class__.__name__, miles))

    def come_back(self):
        logger.info('{} commes back'.format(self.__class__.__name__))

    def build(self):
        logger.info('{} is built: {}'.format(
            self.__class__.__name__,
            self.builder.build(self.recipe)
        ))

    def get_available_load(self):
        return self.max_load_weight - sum(pet.weight for pet in self.pets)


class Car(Vehicle):
    pass


class Van(Vehicle):
    pass


class Truck(Vehicle):
    pass


class PetLoader:

    def upload(self, pets: List[Pet], vehicle: Vehicle):
        info = 'Uploading to the {}:'.format(vehicle.__class__.__name__.lower())
        while pets:
            pet = pets.pop()
            if vehicle.get_available_load() >= pet.weight:
                vehicle.pets.append(pet)
                info += ' ' + pet.__class__.__name__
            else:
                pets.append(pet)
                break
        logger.info(info)

    def download(self, vehicle):
        logger.info('{} pets delivered'.format(len(vehicle.pets)))
        vehicle.pets = []


class PetPicker:

    def __deps__(self, pet_store: Type[Pet]):
        self.pet_store = pet_store

    @deps
    def __init__(self):
        # raise Exception('HORROR!!!!')
        pass

    def pick(self, qty, gift_wrapped):
        info = 'Picking pets up: '
        pets = []
        for _ in range(qty):
            pet = self.pet_store(gift_wrapped)
            info += ' ' + pet.cry()
            pets.append(pet)
        logger.info(info)
        return pets


class PetDeliveryPerson:

    @deps
    def __init__(self):
        pass

    def deliver(self, pet_qty, miles, gift_wrapped):
        pass


class Bob(PetDeliveryPerson):
    """Bob builds a car and deliver pets in his vehicle repeating the route several times."""

    def __deps__(self, vehicle: Vehicle, pet_picker: PetPicker, pet_loader: PetLoader):
        self.vehicle = vehicle
        self.pet_picker = pet_picker
        self.pet_loader = pet_loader

    def deliver(self, pet_qty, miles, gift_wrapped):
        # Pick up pets
        pets = self.pet_picker.pick(pet_qty, gift_wrapped)

        # Bob owns one vehicle only
        while pets:
            self.pet_loader.upload(pets, self.vehicle)
            self.vehicle.go(miles)
            self.pet_loader.download(self.vehicle)
            self.vehicle.come_back()


class Mike(PetDeliveryPerson):
    """Mike builds several autonomous vehicles and use them to deliver the pets all together"""

    def __deps__(self, vehicle_factory: Type[Vehicle], pet_picker: PetPicker, pet_loader: PetLoader):
        self.vehicle_factory = vehicle_factory
        self.pet_picker = pet_picker
        self.pet_loader = pet_loader

    @deps
    def __init__(self):
        super().__init__()
        self.vehicles = []  # type: List[Vehicle]

    def get_vehicle(self):
        if self.vehicles:
            return self.vehicles.pop()
        else:
            return self.vehicle_factory()

    def park_vehicles(self, vehicles):
        self.vehicles += vehicles

    def deliver(self, pet_qty, miles, gift_wrapped):

        # Pick up pets
        pets = self.pet_picker.pick(pet_qty, gift_wrapped)

        # Get vehicles and upload them
        vehicles = []
        while pets:
            vehicle = self.get_vehicle()
            vehicles.append(vehicle)
            self.pet_loader.upload(pets, vehicle)

        # Go
        for vehicle in vehicles:
            vehicle.go(miles)

        # Deliver pets
        for vehicle in vehicles:
            self.pet_loader.download(vehicle)

        # Come back
        for vehicle in vehicles:
            vehicle.come_back()

        # Park
        self.park_vehicles(vehicles)

Injection definitions (pet_delivery/defs.py):

pet_defs = {
    Dog: Instance(),
    Cat: Instance(),
    Bird: Instance(),

    (Dog, 'sound'): 'Woof',
    (Dog, 'weight'): 10,

    (Cat, 'sound'): 'Meow',
    (Cat, 'weight'): 5,

    (Bird, 'sound'): 'Chirp',
    (Bird, 'weight'): 0.1,
}


vehicle_defs = {
    Engine: Instance(),
    Plate: Instance(),
    Wheel: Instance(),
    Container: Instance(),

    Type[Engine]: Factory(),
    Type[Plate]: Factory(),
    Type[Wheel]: Factory(),
    Type[Container]: Factory(),

    VehicleBuilder: Singleton(),

    (Engine, 'mount_sound'): 'RRRRoarrr',
    (Plate, 'mount_sound'): 'plaf',
    (Wheel, 'mount_sound'): 'pffff',
    (Container, 'mount_sound'): 'BLOOOOM',

    Car: Instance(),
    (Car, 'max_load_weight'): 10,
    (Car, 'recipe'): {
        'engines': 1,
        'plates': 6,
        'wheels': 4,
    },

    Van: Instance(),
    (Van, 'max_load_weight'): 50,
    (Van, 'recipe'): {
        'engines': 1,
        'plates': 8,
        'wheels': 4,
    },

    Truck: Instance(),
    (Truck, 'max_load_weight'): 200,
    (Truck, 'recipe'): {
        'engines': 1,
        'plates': 20,
        'wheels': 12,
        'container': 1,
    },
}

common_defs = {
    PetPicker: Singleton(),
    PetLoader: Singleton(),

    Bob: Singleton(),
    Mike: Singleton(),
}

Running the app (pet_delivery/example_1.py):

world_one_defs = {
    (Bob, Vehicle): Singleton(Car),
    (Bob, PetPicker, Type[Pet]): Factory(Bird),

    (Mike, Type[Vehicle]): Factory(Van),
    (Mike, PetPicker, Type[Pet]): Factory(Cat),
}

world_one = Definitions(
    pet_defs,
    vehicle_defs,
    common_defs,
    world_one_defs,
)

logging.basicConfig(format='%(message)s', level=logging.INFO)

@inject(world_one)
def do(bob: Bob, mike: Mike):
    bob.deliver(100, 5, False)
    bob.deliver(50, 200, True)

    mike.deliver(20, 1000, True)

do()

Running the same app with another wiring configuration (pet_delivery/example_2.py):

world_two_defs = {
    (Bob, Vehicle): Singleton(Van),
    (Bob, PetPicker, Type[Pet]): Factory(Cat),

    (Mike, Type[Vehicle]): Factory(Truck),
    (Mike, PetPicker, Type[Pet]): Factory(Dog),
}

world_two = Definitions(
    pet_defs,
    vehicle_defs,
    common_defs,
    world_two_defs,
)

logging.basicConfig(format='%(message)s', level=logging.INFO)

@inject(world_two)
def do(bob: Bob, mike: Mike):
    bob.deliver(100, 5, False)
    bob.deliver(50, 200, True)

    mike.deliver(20, 1000, True)

do()