init_subclass#

David Beazley on Twitter said:

I think 95% of the problems once solved by a metaclass can be solved by __init_subclass__ instead

This inspired me to finally learn how to use it! I used my asyncinject project as an experimental playground.

The __init_subclass__ class method is called when the class itself is being constructed. It gets passed the cls and can make modifications to it.

Here’s the pattern I used:

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class AsyncInject:
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    def __init_subclass__(cls, **kwargs):
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        super().__init_subclass__(**kwargs)
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        # Decorate any items that are 'async def' methods
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        cls._registry = {}
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        inject_all = getattr(cls, "_inject_all", False)
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        for name in dir(cls):
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            value = getattr(cls, name)
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            if inspect.iscoroutinefunction(value) and (
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                inject_all or getattr(value, "_inject", None)
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            ):
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                setattr(cls, name, _make_method(value))
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                cls._registry[name] = getattr(cls, name)
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        # Gather graph for later dependency resolution
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        graph = {
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            key: {
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                p
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                for p in inspect.signature(method).parameters.keys()
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                if p != "self" and not p.startswith("_")
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            }
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            for key, method in cls._registry.items()
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        }
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        cls._graph = graph

As you can see, it’s using getattr() and setattr() against the cls object to make modifications to the class - in this case it’s running a decorator against various methods and adding two new class properties, _registry and _graph.

The **kwargs thing there is interesting: you can define keyword arguments and use them when you subclass, like this:

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class MySubClass(AsyncInject, inject_all=True):
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    ...

This doesn’t work with my above example, but I could change it to start like this instead:

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class AsyncInject:
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    def __init_subclass__(cls, inject_all=False, **kwargs):
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        super().__init_subclass__(**kwargs)
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        # Decorate any items that are 'async def' methods
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        cls._registry = {}
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        for name in dir(cls):
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            value = getattr(cls, name)
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            if inspect.iscoroutinefunction(value) and (
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                inject_all or getattr(value, "_inject", None)
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            ):
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                setattr(cls, name, _make_method(value))
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                cls._registry[name] = getattr(cls, name)

init_subclass#

Further reading#