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Python application component initialization system

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systemPY

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Python application component initialization system

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Full documantation is available at Read the Docs

The problem

The regular application contain many atomic components. Asyncio makes theirs initializing a little bit complicated. It's OK, when you have single entrypoint and initialize your application components via your framework. While you add new components to your application iteratively, you don't see any problem

When you create any new entrypoint, you have to think a lot, how to initialize application components again, which callbacks should be called and in which order. But it's a half of the problem! You have to implement also graceful shutdown

The most painful part is one-time scripts. It's kind of The Banana Gorilla Problem: you wanted a banana but you have to initialize a gorilla holding the banana and the entire jungle, and then graceful shutdown it

Solution

This library allows you to implement application startup and shutdown in declarative way. You have to implement a class for each your component, write the startup and shutdown code. Then combine required components as mixins into the current application Unit class. Then create an instance and pass dependencies as keyword arguments. In case it's daemon run instance.run_sync() methed

Basic principles

There are 6 most significant stages of the application lifecycle:

  • on_init executes exactly once on application startup

  • pre_startup is called before event loop startup

  • on_startup is called exactly when event loop started

  • on_shutdown is called when application is going shutdown or reload but event loop still working

  • post_shutdown is called after event loop stopped or drained. When application is going to reload, then it should be called pre_startup

  • on_exit executes exactly once when application is stopping

You may to create Unit classes for each your application component where you may put your code. Then you may combine these Unit class mixins into the current worker class, which aggregate your defined callbacks and run in the right order. Depending on application type, these callbacks may be called by primary application or by you are

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