Make ast.dump() not output optional default fields

[serhiy-storchaka]

BPO	36287
Nosy	@benjaminp, @serhiy-storchaka, @1st1, @ilevkivskyi, @eamanu, @isidentical
PRs	[WIP] bpo-36287: Ignoring optional field. #12328 bpo-36287: Make ast.dump() not output optional fields and attributes. #18843
Dependencies	bpo-39981: Default values for AST Nodes
Files	ast.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-14.09:48:05.485>
labels = ['type-feature', 'library', '3.9']
title = 'Make ast.dump() not output optional default fields'
updated_at = <Date 2020-04-08.14:28:27.407>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-04-08.14:28:27.407>
actor = 'BTaskaya'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-14.09:48:05.485>
creator = 'serhiy.storchaka'
dependencies = ['39981']
files = ['48975']
hgrepos = []
issue_num = 36287
keywords = ['patch']
message_count = 16.0
messages = ['337907', '337922', '337955', '338030', '338033', '338055', '356925', '356935', '357012', '363640', '363641', '363642', '363775', '364305', '364326', '365987']
nosy_count = 6.0
nosy_names = ['benjamin.peterson', 'serhiy.storchaka', 'yselivanov', 'levkivskyi', 'eamanu', 'BTaskaya']
pr_nums = ['12328', '18843']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36287'
versions = ['Python 3.9']

[serhiy-storchaka]

Currently ast.dump() outputs values for optional fields even if they are equal to defaults. This makes the output unnecessary verbose.

For example (kind and type_comment are optional):

>>> ast.dump(ast.parse('x = 1'))
"Module(body=[Assign(targets=[Name(id='x', ctx=Store())], value=Constant(value=1, kind=None), type_comment=None)], type_ignores=[])"

[eamanu]

Hi @serhiy.storchaka,

I send a patch to Github to review. Let me know if is necessary unittest.

Regards

[brettcannon]

@eamanu tests are basically always necessary. :)

[ilevkivskyi]

We can probably also skip type_ignores list if it is empty (which will be the case in 99% situations).

[eamanu]

Maybe we can ignore None and [] ?

[serhiy-storchaka]

None can not be ignored in Constant(value=None). [] can not be ignored in Tuple(elts=[]).

There is also a problem with using ast.dump() with annotate_fields=False:

>>> from ast import *
>>> dump(Raise(cause=Name(id='B', ctx=Load())), annotate_fields=False)
"Raise(Name('B', Load()))"
>>> dump(Raise(Name('B', Load())))
"Raise(exc=Name(id='B', ctx=Load()))"

For Raise(cause=X) it outputs a string which is evaluated to Raise(exc=X).

[isidentical]

@eamanu are you still interested in this issue? (bumped to 3.9)

[eamanu]

Hi,

Sorry sincerely I forgot this issue, if there are not any objection I can continue it.

[ilevkivskyi]

No objections from me assuming you are going forward along the way proposed by Serhiy (i.e. only skip values for certain fields if value is the default, not a blanket skip for all Nones and empty lists).

[serhiy-storchaka]

PR 18843 solves this issue by setting None as class attributes for optional fields and attributes (like "kind" or "end_col_offset").

It is not so easy for fields like "type_ignores". They are mutable lists, so this approach cannot be applied to them.

I want to get rid also from "ctx=Load()", but it is more complex change.