> On 26 Apr 2022, at 07:32, Larry Hastings <la...@hastings.org> wrote: > >
[… snip …] > Next we have the "continue" class statement. I'm going to spell it like this: > > continue class C(BaseClass, ..., metaclass=MyMetaclass): > # class body goes here > ... > > I'll mention other possible spellings later. The first change I'll point out > here: we've moved the base classes and the metaclass from the "forward" > statement to the "continue" statement. Technically we could put them either > place if we really cared to. But moving them here seems better, for reasons > you'll see in a minute. > > Other than that, this "continue class" statement is similar to what I (we) > proposed before. For example, here C is an expression, not a name. > > Now comes the one thing that we might call a "trick". The trick: when we > allocate the ForwardClass instance C, we make it as big as a class object can > ever get. (Mark Shannon assures me this is simply "heap type", and he knows > far more about CPython internals than I ever will.) Then, when we get to the > "continue class" statement, we convince metaclass.__new__ call to reuse this > memory, and preserve the reference count, but to change the type of the > object to "type" (or what-have-you). C has now been changed from a > "ForwardClass" object into a real type. (Which almost certainly means C is > now mutable.) > A problem with this trick is that you don’t know how large a class object can get because a subclass of type might add new slots. This is currently not possible to do in Python code (non-empty ``__slots__`` in a type subclass is rejected at runtime), but you can do this in C code. Ronald — Twitter / micro.blog: @ronaldoussoren Blog: https://blog.ronaldoussoren.net/
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