When I try to format a code snippet with yapf, there comes a error.
It seems lib2to3 can't reconised SLASH.

echo "def foo(posonly1, posonly2, /, positional_or_keyword): pass" | yapf 

Traceback (most recent call last):
  File "d:\program files\python38\lib\site-packages\yapf\yapflib\pytree_utils.py", line 115, in ParseCodeToTree
    tree = parser_driver.parse_string(code, debug=False)
  File "d:\program files\python38\lib\lib2to3\pgen2\driver.py", line 103, in parse_string
    return self.parse_tokens(tokens, debug)
  File "d:\program files\python38\lib\lib2to3\pgen2\driver.py", line 71, in parse_tokens
    if p.addtoken(type, value, (prefix, start)):
  File "d:\program files\python38\lib\lib2to3\pgen2\parse.py", line 162, in addtoken
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=17, value='/', context=(' ', (1, 28))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "d:\program files\python38\lib\runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "d:\program files\python38\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "D:\Program Files\Python38\Scripts\yapf.exe\__main__.py", line 7, in <module>
  File "d:\program files\python38\lib\site-packages\yapf\__init__.py", line 362, in run_main
    sys.exit(main(sys.argv))
  File "d:\program files\python38\lib\site-packages\yapf\__init__.py", line 104, in main
    reformatted_source, _ = yapf_api.FormatCode(
  File "d:\program files\python38\lib\site-packages\yapf\yapflib\yapf_api.py", line 147, in FormatCode
    tree = pytree_utils.ParseCodeToTree(unformatted_source)
  File "d:\program files\python38\lib\site-packages\yapf\yapflib\pytree_utils.py", line 121, in ParseCodeToTree
    tree = parser_driver.parse_string(code, debug=False)
  File "d:\program files\python38\lib\lib2to3\pgen2\driver.py", line 103, in parse_string
    return self.parse_tokens(tokens, debug)
  File "d:\program files\python38\lib\lib2to3\pgen2\driver.py", line 71, in parse_tokens
    if p.addtoken(type, value, (prefix, start)):
  File "d:\program files\python38\lib\lib2to3\pgen2\parse.py", line 162, in addtoken
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=17, value='/', context=(' ', (1, 28))

The slash syntax was introduced in 3.8. I guess the grammar of lib2to3 was not updated to support new syntax elements.

Łukasz, could you please take a look?

https://bugs.python.org/issue36541 add support for walrus operator and positional arguments. Relevant commit : https://github.com/python/cpython/commit/06bfd033e847bedb6e123d131dcf46393a4555df

Relevant yapf issue on python 3.8 support : https://github.com/google/yapf/issues/772

Can you please add the exact python version you are using?

python 3.8.3

Thanks, the change was merged in December 14, 2020 just missing 3.8.7 . Can you please upgrade to 3.8.8 or 3.8.9 and try? If the issue still persists I guess it's then an issue with yapf since Python 3.8.8 has the code merged with tests.

Thanks, it works well after upgrade to version 3.8.9