AFAIK cls.__subclasses__() only returns the classes which the interpreter has already loaded.

This means there can be more subclasses in modules where not imported by the current interpreter up to now.

https://docs.python.org/3.7/library/stdtypes.html?highlight=subclasses#class.__subclasses__

I think it would be nice to add a hint to the docs that python imports are lazy.

What do you think?

Thanks for the suggestion, but I don't think so.

Python imports are not lazy.  They are ordered.  Python is an *interpreted* language, so __subclasses__ is only going to hold those subclasses whose class definitions have been executed. This is fundamental to how the language works, and does not warrant a special note here.  Note that the entry already includes something that should give a strong hint about this even if you hadn't previously thought about it: the statement that it only includes subclasses that are "still alive".  This implies they aren't in the list until they become alive.