Punq

An unintrusive library for dependency injection in modern Python. Inspired by Funq, Punq is a dependency injection library you can understand.

• No global state
• No decorators
• No weird syntax applied to arguments
• Small and simple code base with 100% test coverage and developer-friendly comments.

Installation

Punq is available on the cheese shop

pip install punq

Documentation is available on Read the docs.

Quick Start

Punq avoids global state, so you must explicitly create a container in the entrypoint of your application:

import punq

container = punq.Container()

Once you have a container, you can register your application's dependencies. In the simplest case, we can register any arbitrary object with some key:

container.register("connection_string", instance="postgresql://...")

We can then request that object back from the container:

conn_str = container.resolve("connection_string")

Usually, though, we want to register some object that implements a useful service.:

class ConfigReader:
    def get_config(self):
        pass

class EnvironmentConfigReader(ConfigReader):
    def get_config(self):
        return {
        "logging": {
            "level": os.env.get("LOGGING_LEVEL", "debug")
        },
        "greeting": os.env.get("GREETING", "Hello world")
        }

container.register(ConfigReader, EnvironmentConfigReader)

Now we can resolve the ConfigReader service, and receive a concrete implementation:

config = container.resolve(ConfigReader).get_config()

If our application's dependencies have their own dependencies, Punq will inject those, too:

class Greeter:
    def greet(self):
        pass


class ConsoleGreeter(Greeter):
    def __init__(self, config_reader: ConfigReader):
        self.config = config_reader.get_config()

    def greet(self):
        print(self.config['greeting'])


container.register(Greeter, ConsoleGreeter)
container.resolve(Greeter).greet()

If you just want to resolve an object without having any base class, that's okay:

class Greeter:
    def __init__(self, config_reader: ConfigReader):
        self.config = config_reader.get_config()

    def greet(self):
        print(self.config['greeting'])

container.register(Greeter)
container.resolve(Greeter).greet()

And if you need to have a singleton object for some reason, we can tell punq to register a specific instance of an object:

class FileWritingGreeter:
    def __init__(self, path, greeting):
        self.path = path
        self.message = greeting
        self.file = open(self.path, 'w')

    def greet(self):
        self.file.write(self.message)


one_true_greeter = FileWritingGreeter("/tmp/greetings", "Hello world")
container.register(Greeter, instance=one_true_greeter)

You might not know all of your arguments at registration time, but you can provide them later:

container.register(Greeter, FileWritingGreeter)
greeter = container.resolve(Greeter, path="/tmp/foo", greeting="Hello world")

Conversely, you might want to provide arguments at registration time, without adding them to the container:

container.register(Greeter, FileWritingGreeter, path="/tmp/foo", greeting="Hello world")