__init_subclass__ in Python

A hook that runs when a subclass is defined — a registry without a metaclass.

class Plugin:
    registry = {}

    def __init_subclass__(cls, name=None, **kwargs):
        super().__init_subclass__(**kwargs)
        cls.registry[name or cls.__name__.lower()] = cls


class Csv(Plugin, name="csv"):
    pass


class Json(Plugin):
    pass


print(sorted(Plugin.registry))
print(Plugin.registry["csv"] is Csv)

How it works

1. It is implicitly a classmethod on the parent.
2. Keyword arguments in the class statement arrive here.
3. Cheaper to read than a metaclass, and usually enough.

Keywords and builtins used here

• Csv
• Json
• Plugin
• __init_subclass__
• class
• cls
• def
• pass
• print
• sorted
• super

The run, in numbers

Lines

18

Characters to type

297

Tokens

89

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 49 seconds.

Type this snippet

Step 2 of 3 in Class machinery, step 36 of 53 in Pythonic Python.

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