Default values for AST Nodes

[isidentical]

BPO	39981
Nosy	@benjaminp, @serhiy-storchaka, @pablogsal, @isidentical
PRs	bpo-39981: Default values for AST nodes #19031 bpo-39981: Introduce default values for AST node classes #21417

^{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 2020-03-16.19:17:20.926>
labels = ['type-feature', 'library', '3.9']
title = 'Default values for AST Nodes'
updated_at = <Date 2020-07-09.13:33:40.873>
user = 'https://github.com/isidentical'

bugs.python.org fields:

activity = <Date 2020-07-09.13:33:40.873>
actor = 'BTaskaya'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-03-16.19:17:20.926>
creator = 'BTaskaya'
dependencies = []
files = []
hgrepos = []
issue_num = 39981
keywords = ['patch']
message_count = 1.0
messages = ['364355']
nosy_count = 4.0
nosy_names = ['benjamin.peterson', 'serhiy.storchaka', 'pablogsal', 'BTaskaya']
pr_nums = ['19031', '21417']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39981'
versions = ['Python 3.9']

[isidentical]

For omitting some defaults, @serhiy.storchaka already added support to initialize some ast nodes with some default values (optional fields). An example;
>>> ast.Constant().kind is None
True

This isn't exactly a default value, but some kind of class attribute. I think we can push this one step further and initialize all kinds of default values (both optionals and sequences). An example;
>>> func = ast.FunctionDef("easy_func", ast.arguments(), body=[ast.Pass()])
>>> func = ast.fix_missing_locations(func)
>>> exec(compile(ast.Module(body=[func]), "<stdin>", "exec"))
>>> easy_func()

compared to this (other way around, compiler gives errors so does most of the ast based tool, including ast.unparser)
>>> func = ast.FunctionDef("easy_func", ast.arguments(posonlyargs=[], args=[], kwonlyargs=[], kw_defaults=[], defaults=[]), decorator_list=[], body=[ast.Pass()])
>>> func = ast.fix_missing_locations(func)
>>> exec(compile(ast.Module(body=[func], type_ignores=[]), "<stdin>", "exec"))
>>> easy_func()

[JelleZijlstra]

This was implemented in #105858.