bpo-37116: Use PEP 570 syntax for positional-only parameters.

[serhiy-storchaka]

Based on #12620 but contains only changes that can be applied in 3.8. E.g. only getting rid of *args and name tricks and making self and cls positional-only.

https://bugs.python.org/issue37116

[gvanrossum]

All LGTM. Very nice cleanups!

It looks like we technically have to add / whenever there's a **kwds (assuming the kwds is passed through and could contain any key)...

[gvanrossum]

Why not use object()? IIUC all the code below checks for its identity first and then substitutes another dict.

[serhiy-storchaka]

Because semantically the default value for the mapping parameter is an empty dict. The check mapping is _sentinel_dict is merely an optimization (we can avoid to create a ChainMap in this case).

[pablogsal]

Thank you very much @serhiy-storchaka for working on this 🎉

[ilevkivskyi]

typing/abc/collections code LGTM.

[serhiy-storchaka]

Yes, we can add / in most cases whenever there's a **kwds.

With bpo-36518 we should not even add it if all positional arguments are required (this covers the majority of cases).

[serhiy-storchaka]

@pablogsal The two last bugs related to positional-only arguments I found when worked on this issue.

[tirkarthi]

mock changes LGTM. There are few places where _mock_self is used in the async helpers later assigning to self following older pattern. I will try to change them in another PR so that this is good for beta 1. Thanks Serhiy.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Windows7 3.x has failed when building commit 2085bd0.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/58/builds/2538) and take a look at the build logs.
4. Check if the failure is related to this commit (2085bd0) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/58/builds/2538

Click to see traceback logs

Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows7\build\lib\test\test_venv.py", line 327, in test_multiprocessing
    out, err = check_output([envpy, '-c',
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows7\build\lib\test\test_venv.py", line 42, in check_output
    raise subprocess.CalledProcessError(
subprocess.CalledProcessError: Command '['d:\\temp\\tmpzz1z4c8u\\Scripts\\python_d.exe', '-c', 'from multiprocessing import Pool; print(Pool(1).apply_async("Python".lower).get(3))']' returned non-zero exit status 3221225477.

----------------------------------------------------------------------

Ran 16 tests in 144.483s

FAILED (errors=1, skipped=2)


Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows7\build\lib\test\test_venv.py", line 327, in test_multiprocessing
    out, err = check_output([envpy, '-c',
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows7\build\lib\test\test_venv.py", line 42, in check_output
    raise subprocess.CalledProcessError(
subprocess.CalledProcessError: Command '['d:\\temp\\tmp_qt0ywa6\\Scripts\\python_d.exe', '-c', 'from multiprocessing import Pool; print(Pool(1).apply_async("Python".lower).get(3))']' returned non-zero exit status 3221225477.

----------------------------------------------------------------------

Ran 16 tests in 65.325s

FAILED (errors=1, skipped=2)