gh-116023: Add show_empty=False to ast.dump

[sobolevn]

My thoughts:

• This parameter should be optional and switched off by default, so we preserve the output if people rely on it: as debugging tool or somewhere in tests
• It should not touch attributes, because they already have a different flag

• Issue: Omit optional fields from ast.dump() #116023

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📚 Documentation preview 📚: https://cpython-previews--116037.org.readthedocs.build/

[JelleZijlstra]

Thank you for picking this up!

[sobolevn]

Updated! Thank you!

[sobolevn]

I will wait for some third opinion, maybe someone else has input: whether this should be a default or not.

Thanks for the review!

[sobolevn]

Maybe @carljm or @hauntsaninja ? :)

[hauntsaninja]

I think if we have the feature it should be on by default. Since it's sort of a convenience / conciseness feature, it's not as useful if it's off by default. Also seems fairly safe to keep on by default, since that's basically what 3.12's ast.dump will give you.

[carljm]

FWIW, I agree with @hauntsaninja and @JelleZijlstra. I think in the more common use cases, seeing empty values is noisy and not useful, and show_empty=True can be passed if you want to see them. I don't think there are any guarantees around backwards-compatibility for the textual representation of an AST.

that's basically what 3.12's ast.dump will give you.

That depends on whether you are talking about ast.dump(ast.arguments()) (dumping a manually-constructed AST) or ast.dump(ast.parse("def x(): pass")) (dumping a parsed AST). For the latter case, show_empty=False is a new behavior that no previous Python version has ever had.

I still think it's the better behavior, though. I would rather see

Module(body=[FunctionDef(name='x', args=arguments(), body=[Pass()])])

by default, instead of

Module(body=[FunctionDef(name='x', args=arguments(posonlyargs=[], args=[], kwonlyargs=[], kw_defaults=[], defaults=[]), body=[Pass()], decorator_list=[], type_params=[])], type_ignores=[]).

[sobolevn]

Ok, fair enough! I've changed the default from show_empty=True to show_empty=False.
I've also updated tests and docs. Please, take a look at my new changes :)

[sobolevn]

Waiting for some time for @hauntsaninja and @carljm to provide feedback :)
Jelle, thanks a lot for the review and for finding annotate_fields bug!

[carljm]

LGTM, modulo comments. Thanks!

[carljm]

LGTM, one minor nit on the docs.

[sobolevn]

(Waiting for one more day)

[sobolevn]

Thanks everyone! 🎉

[sobolevn]

Maybe we should also mention this in 3.13.rst in ast section?