Omit optional fields from ast.dump()

[JelleZijlstra]

Feature or enhancement

Proposal:

>>> ast.dump(ast.arguments())
'arguments(posonlyargs=[], args=[], kwonlyargs=[], kw_defaults=[], defaults=[])'

After #105858, it would be nice if ast.dump() also didn't output optional fields that are set to None or an empty string, so that its output is more concise. This should make it easier to understand the structure of large ASTs where most fields are missing.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-116023: Add show_empty=False to ast.dump #116037

[JelleZijlstra]

I noticed that the representation on an empty ast.Tuple changed from Tuple(elts=[], ctx=Load()) to Tuple(ctx=Load()), and same for lists and dicts:

>>> ast.dump(ast.parse('[]'))
'Module(body=[Expr(value=List(ctx=Load()))])'
>>> ast.dump(ast.parse('{}'))
'Module(body=[Expr(value=Dict())])'
>>> ast.dump(ast.parse('()'))
'Module(body=[Expr(value=Tuple(ctx=Load()))])'

I think this is fine; it matches the constructor and it is analogous to how list(), dict(), and tuple() work. However, I thought I'd flag it in case others think these nodes should be special-cased.

[AlexWaygood]

>>> ast.dump(ast.parse('[]'))
'Module(body=[Expr(value=List(ctx=Load()))])'
>>> ast.dump(ast.parse('{}'))
'Module(body=[Expr(value=Dict())])'
>>> ast.dump(ast.parse('()'))
'Module(body=[Expr(value=Tuple(ctx=Load()))])'

Why does the repr for ast.Dict() not include ctx=Load() in it, when the reprs for Tuple and List do?

[AlexWaygood]

Why does the repr for ast.Dict() not include ctx=Load() in it, when the reprs for Tuple and List do?

I see that the same thing happens on Python 3.12, so this definitely isn't a regression... but I'd still be curious if anybody knows why this is different for the different nodes!

[JelleZijlstra]

It's because ast.Dict can't appear on the left-hand side of an assignment.

>>> [x] = (3,)
>>> (x,) = (3,)
>>> {x: x} = {3: 3}
  File "<stdin>", line 1
    {x: x} = {3: 3}
    ^^^^^^
SyntaxError: cannot assign to dict literal here. Maybe you meant '==' instead of '='?

I was thinking of whether we'd also want to default the ctx parameter of these and other nodes to Load(), but that's for another day.