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> I would consider changing how a builtin works to be a major breaking change.

This one is unusual because it won't break old code being brought forward, only the other way around, and theoretically there are lots of things going the other direction that would break (though most of them are explicit, not silent).

In other words, it breaks backwards compatibility, but not forward compatibility.

That's not what people call backward compatibility.

Backward compatibility is the ability to run old code with new interpreters. This is not broken here.

What's broken is the ability to run new code on old interpreters. But this is already broken at every python update (new operators, methods, syntactic sugar..). We could call it reverse backward compatibility.

I suppose the meaning of the term depends on what we consider to be the subject of the compatibility.

- backwards compatible code: new code can run on an old interpreter

- backwards compatible interpreter: old code can run on a new interpreter

EDIT: After some thought, you're right. The second description is the reasonable interpretation.

"python 3.7 is backwards compatible with python 3.6"

This confusion reminds me of a scene from the 2002 film of Dave Barry's "Big Trouble".

(Driving to an airport) "Okay, we gotta pick a road. Arrivals or departures? We're arriving, but then we're departing."



No, this change is backward compatible but not forward compatible. Old code (written for old interpreter) works in new interpreter; but new code (written for new interpreter) is broken in old interpreter.

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