`ast.dump( show_empty = True)` does not show most `None` values.

[hunterhogan]

Bug report

Bug description:

The minimal, reproducible example is to click on a link to view the ast documentation.

Desired output

The example, if it were to include None values, would read

ast.Module(
    body=[
        ast.AsyncFunctionDef(
            name='f',
            args=ast.arguments(
                posonlyargs=[],
                args=[],
                vararg=None,
                kwonlyargs=[],
                kw_defaults=[],
                kwarg=None,
                defaults=[]),
            body=[
                ast.Expr(
                    value=ast.Await(
                        value=ast.Call(
                            func=ast.Name(id='other_func', ctx=ast.Load()),
                            args=[],
                            keywords=[])))],
            decorator_list=[],
            returns=None,
            type_comment=None,
            type_params=[])],
    type_ignores=[])

Solution

Change

cpython/Lib/ast.py

Lines 147 to 149 in 7291eab

	if value is None and getattr(cls, name, ...) is None:
	keywords = True
	continue

To

                if value is None and getattr(cls, name, ...) is None:
                    if show_empty:
                        args.append('%s=%s' % (name, value))
                    keywords = True
                    continue

CPython versions tested on:

3.13, CPython main branch

Operating systems tested on:

Windows, Other

Linked PRs

• gh-134733: fix: ast.dump( show_empty=True ) now displays all None values #134743
• gh-134733: Fix documentation for the show_empty option of ast.dump() #134925
• [3.14] gh-134733: Fix documentation for the show_empty option of ast.dump() (GH-134925) #134940
• [3.13] gh-134733: Fix documentation for the show_empty option of ast.dump() (GH-134925) #134941

[hunterhogan]

See also:

• ast.dump() elision of empty values should use field types #134718
• ast.MatchStar.name incorrectly has class-level default value #134674
• gh-134674: remove ast.MatchStar.name class-level default value #134683

[sharktide]

Interesting! Do you want to make a PR or should I?

[hunterhogan]

Umm, I'll try.

[serhiy-storchaka]

The show_empty parameter was added in PR #116037 (issue #116023).

This issue can be fixed in two ways:

• Make ast.dump() with show_empty=True always showing None. This would change a lot of output, for example each Constant will have kind=None.
• Change the documentation -- document that show_empty=True doesn't affect None values. The code also may need a change, because currently it looks trying to do something with None.

This is a different issue than #134718, because this is clearly a bug, but #134718 may be qualified as a new feature.

cc @JelleZijlstra, @sobolevn

[terryjreedy]

This issue is the apparent discrepancy between the 3.13 ast.dump doc line "If show_empty is False (the default), empty lists and fields that are None will be omitted from the output." and the following example output with dump argument show_empty=True and continued omission . However, the doc line does not say what is supposed to happen if show_empty is True.

Arg show_empty and the dump example for True were added in 3.13 in #116023. The intention of the issue was to not "output optional fields that are set to None or an empty string". In other words, remove 'noise'. Instead of always doing this, the new optional parameter was added to make the existing noisy output available to code that requires it. Indeed, the 3.13 dump with show_empty True is identical the 3.12 example for section Async and await. This should is intentional and should not be changed.

The original patch had show_empty=True. The default was changed to False (with no deprecation period) a month later after input and discussion involving multiple people. This means that old code must add the option (but can make this simple change) for 3.13+. It was felt that the primary intended use for dump is for interactive use. The doc could and maybe should say "If show_empty is True, optional fields with empty lists are shown as in 3.12 and before".

I omit None from this suggestion because it seems that optional None fields were already omitted in 3.12 and so were properly not added for show_empty=True in 3.13. Ast.py has dumps for every node type. The 3.13 patch removed perhaps a hundred lines of form 'val=[]' but none of form 'val=None'. There are a couple 'val=None's left, and there are tests that these are retained, so I presume these are required fields. So it seems that the existing show_empty=False line should be changed to 'empty lists for optional fields are omitted from the output." Maybe it should be also be said that optional None fields are always omitted.

@JelleZijlstra @sobolevn Please review my understanding of what you did and consequent doc change suggestions.