lib2to3 fails on default convert functionality

[isidentical]

BPO	40101
Nosy	@isidentical
PRs	bpo-40101: Fix parser's assumption about the return value of convert #19214

^{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 2020-03-29.15:24:48.719>
labels = ['library', '3.9']
title = 'lib2to3 fails on default convert functionality'
updated_at = <Date 2020-03-29.15:30:56.173>
user = 'https://github.com/isidentical'

bugs.python.org fields:

activity = <Date 2020-03-29.15:30:56.173>
actor = 'BTaskaya'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-03-29.15:24:48.719>
creator = 'BTaskaya'
dependencies = []
files = []
hgrepos = []
issue_num = 40101
keywords = ['patch']
message_count = 1.0
messages = ['365260']
nosy_count = 1.0
nosy_names = ['BTaskaya']
pr_nums = ['19214']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40101'
versions = ['Python 3.9']

[isidentical]

When using driver/parser without a custom convert function (like pyconvert.convert), it tries to assign used_names to the root node, which can be anything depending on the given convert function. If none given, it will be a tuple.

>>> from lib2to3.pygram import python_grammar
>>> from lib2to3.pgen2.driver import Driver
>>> 
>>> d = Driver(grammar=python_grammar)
>>> d.parse_string("test\n")
Traceback (most recent call la

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/lib2to3/pgen2/driver.py", line 103, in parse_string
    return self.parse_tokens(tokens, debug)
  File "/usr/local/lib/python3.9/lib2to3/pgen2/driver.py", line 71, in parse_tokens
    if p.addtoken(type, value, (prefix, start)):
  File "/usr/local/lib/python3.9/lib2to3/pgen2/parse.py", line 136, in addtoken
    self.pop()
  File "/usr/local/lib/python3.9/lib2to3/pgen2/parse.py", line 204, in pop
    self.rootnode.used_names = self.used_names
AttributeError: 'tuple' object has no attribute 'used_names'

[iritkatriel]

lib2to3 is deprecated. Is this still relevant?