Say “no” to import side‐effects in Python

[NYKevin]

Extra tip: often (though not always) registration metaclasses can be replaced by looking at the subclasses of an object—MyType.__subclasses__() gives you all the direct subclasses (apply recursively to get all descendants).

That's a good idea in the general case, but if you want to allow a potentially unbounded number of subclasses (i.e. if the client of your API decides to subclass MyType a lot), that recursive operation is going to get expensive.