bpo-37868: Improve is_dataclass for instances.

[ericvsmith]

https://bugs.python.org/issue37868

[serhiy-storchaka]

Suggested change

	__dataclass_fields = []
	__dataclass_fields__ = []

[ericvsmith]

Thanks for the review, Serhiy.

[miss-islington]

Thanks @ericvsmith for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7, 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-15339 is a backport of this pull request to the 3.8 branch.

[bedevere-bot]

GH-15340 is a backport of this pull request to the 3.7 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows7 SP1 3.7 has failed when building commit 02c1457.

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/130/builds/1050) and take a look at the build logs.
4. Check if the failure is related to this commit (02c1457) 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/130/builds/1050

Summary of the results of the build (if available):

Click to see traceback logs

From https://github.com/python/cpython
 * branch            3.7        -> FETCH_HEAD
Reset branch '3.7'

Could Not Find C:\buildbot.python.org\3.7.kloth-win64\build\Lib\*.pyc
The system cannot find the file specified.
Could Not Find C:\buildbot.python.org\3.7.kloth-win64\build\PCbuild\python*.zip

test_threadsignals skipped -- Can't test signal on win32
minkernel\crts\ucrt\src\appcrt\lowio\write.cpp(50) : Assertion failed: (_osfile(fh) & FOPEN)
test_epoll skipped -- test works only on Linux 2.6
test_wait3 skipped -- os.fork not defined
test_dbm_gnu skipped -- No module named '_gdbm'
test_xxtestfuzz skipped -- No module named '_xxtestfuzz'
test_kqueue skipped -- test works only on BSD
minkernel\crts\ucrt\src\appcrt\lowio\write.cpp(50) : Assertion failed: (_osfile(fh) & FOPEN)
minkernel\crts\ucrt\src\appcrt\lowio\close.cpp(49) : Assertion failed: (_osfile(fh) & FOPEN)
minkernel\crts\ucrt\src\appcrt\lowio\close.cpp(49) : Assertion failed: (_osfile(fh) & FOPEN)
test_ttk_guionly skipped -- gui not available (WSF_VISIBLE flag not set)
test_posix skipped -- No module named 'posix'
test_gdb skipped -- Couldn't find gdb on the path
test_ossaudiodev skipped -- No module named 'ossaudiodev'
test_grp skipped -- No module named 'grp'
test_spwd skipped -- No module named 'spwd'
test_syslog skipped -- No module named 'syslog'
test_nis skipped -- No module named 'nis'
test_ioctl skipped -- No module named 'fcntl'
test_curses skipped -- No module named '_curses'
test_poll skipped -- select.poll not defined
test_readline skipped -- No module named 'readline'
test_tix skipped -- gui not available (WSF_VISIBLE flag not set)
test_resource skipped -- No module named 'resource'
test_openpty skipped -- os.openpty() not available.
test_devpoll skipped -- test works only on Solaris OS family
test_fork1 skipped -- object <module 'os' from 'C:\\buildbot.python.org\\3.7.kloth-win64\\build\\lib\\os.py'> has no attribute 'fork'
test_fcntl skipped -- No module named 'fcntl'
test_pwd skipped -- No module named 'pwd'
test_dbm_ndbm skipped -- No module named '_dbm'
test_multiprocessing_forkserver skipped -- forkserver is not available on Windows
test_wait4 skipped -- object <module 'os' from 'C:\\buildbot.python.org\\3.7.kloth-win64\\build\\lib\\os.py'> has no attribute 'fork'
test_zipfile64 skipped -- test requires loads of disk-space bytes and a long time to run
test_pty skipped -- No module named 'termios'
test_crypt skipped -- No module named '_crypt'
test_tk skipped -- gui not available (WSF_VISIBLE flag not set)
test_multiprocessing_fork skipped -- fork is not available on Windows
test_pipes skipped -- pipes module only works on posix

Could Not Find C:\buildbot.python.org\3.7.kloth-win64\build\PCbuild\python*.zip