At last, a nibble on the other fishing line! ;-)
On 1/11/21 1:47 PM, Brett Cannon wrote:
So the biggest potential breakages are code that:
1. Directly get the attribute from __dict__
2. The fact that it would no longer be inherited
Am I missing anything else?
Those are the big ones, the ones I expect people to actually
experience. I can name three more breakages, though these get
progressively more obscure:
* Nobody expect o.__annotations__ to ever be None (unless they
assigned None to it themselves). If the attribute is set they
expect its value to be a dict.
* "del o.__annotations__" currently works on modules and classes if
the attribute is set. "del fn.__annotations__" always works.
* On modules and classes you can set o.__annotations__ to any Python
value. (Functions already only permit you to set it to None or a dict.)
I have no idea if anybody is depending on these behaviors. The lesson
that years of Python core dev has taught me is: if Python exhibits a
behavior, somebody out there depends on it, and you'll break their code
if you change it. Or, expressed more succinctly, any change is a
breaking change for somebody. So the question is, is the improvement
this brings worth the breakage it also brings? In this case, I sure hope so!
For issue #2, if the default was `None`, then couldn't that be used as
an implicit feature marker that you can't (incorrectly) rely on
inheritance to percolate up the annotations of the superclass if the
subclass happens to not define any annotations?
Currently Python never sets o.__annotations__ to None on any object. So
yes, assuming the user doesn't set it to None themselves, this would be
new behavior. If I understand your question correctly, yes, users could
write new code that says
if o.__annotations__ is None:
# great, we're in Python 3.10+ and no annotation was set on o!
....
else:
....
Or they could just look at sys.version_info ;-)
Thanks for your feedback,
//arry/
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