Larry Hastings <[email protected]> added the comment:
> - Is it possible to create __annotations__ lazily? (IIRC in January we came
> to a conclusion about this, something like yes for modules but for classes,
> or the other way around?)
Maybe for modules, definitely not for classes. The problem is that best
practice for classes is to look in the class dict for __annotations__. That
sidesteps the inheritance problem.
Functions already lazily create a dict __annotations__ if not set. And it's
not stored in the function dict. So nobody ever looks in the function dict for
__annotations__. That all works fine.
Would you prefer that *just classes* lazily create __annotations__? It's not
hard code to write, but I was being cautious. "always set __annotations__ to
an empty dict" is the easiest code to write and get correct. And since modules
are not all that numerous even in the largest projects, it seemed like this
un-optimized approach would have a minimal contribution to the heat death of
the universe.
It's also remotely possible that someone out there does look in the module dict
for __annotations__, though I admit I've never seen it. It seems like it'd be
tempting to write your code that way though:
if isinstance(o, (type, types.ModuleType)):
ann = o.__dict__.get("__annotations__", None)
elif callable(o):
ann = o.__annotations__
else:
raise ValueError(f"{o!r} doesn't support annotations")
> - Why would __annotations__ ever be None?
Simply because it's the cheapest sensible way to indicate "this object has no
annotations". It would be nice if the world accepted __annotations__ being
None to mean "no annotations". But I don't think we're there.
I note that the function object has a special setter for __annotations__
(func_set_annotations()), since at least Python 3.1, which explicitly only
allows setting __annotations__ to either a dict or None. (I didn't have 3.0
handy.)
> Why do you allow setting it to None?
Me? I've never contributed code to Python that restricts the permissible types
of values one is allowed to set on __annotations__.
Function objects are opinionated as mentioned (either dict or None), classes
and modules have no opinion whatsoever, and allow you to set __annotations__ to
any value. That was all true long before I started working on annotations.
> Do we know of people ever write '__annotations__ = None' in their class or
> write 'cls.__annotations__ = None'?
AFAIK I've never seen anyone set __annotations__ to None.
> - And where's your PR?
Not done yet. I only started it last night, and there were a lot of test
failures--it turns out, a *lot* of regression tests do a sanity check that
__annotations__ isn't set on classes (and maybe modules too). For example, I
was surprised that test_opcodes has two such test failures.
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<https://bugs.python.org/issue43901>
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