vstinner@apu$ ./python -m test -v test_datetime test_datetime -m test_divide_and_round
== CPython 3.9.0a5+ (heads/pr/19122:0ac3031a80, Mar 25 2020, 02:25:19) [GCC 9.2.1 20190827 (Red Hat 9.2.1-1)]
== Linux-5.5.9-200.fc31.x86_64-x86_64-with-glibc2.30 little-endian
== cwd: /home/vstinner/python/master/build/test_python_233006
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
0:00:00 load avg: 0.82 Run tests sequentially
0:00:00 load avg: 0.82 [1/2] test_datetime
test_divide_and_round (test.datetimetester.TestModule_Pure) ... ok
test_divide_and_round (test.datetimetester.TestModule_Fast) ... skipped 'Only run for Pure Python implementation'

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

Ran 2 tests in 0.002s

OK (skipped=1)
0:00:00 load avg: 0.82 [2/2] test_datetime
test_divide_and_round (test.datetimetester.TestModule_Pure) ... ERROR
test_divide_and_round (test.datetimetester.TestModule_Fast) ... skipped 'Only run for Pure Python implementation'

======================================================================
ERROR: test_divide_and_round (test.datetimetester.TestModule_Pure)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/datetimetester.py", line 87, in test_divide_and_round
    dar = datetime_module._divide_and_round
AttributeError: module 'datetime' has no attribute '_divide_and_round'

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

Ran 2 tests in 0.006s

FAILED (errors=1, skipped=1)
test test_datetime failed
test_datetime failed

== Tests result: FAILURE ==

1 test OK.

1 test failed:
    test_datetime

Total duration: 448 ms
Tests result: FAILURE

`_divide_and_round()` would be deleted in https://github.com/python/cpython/blob/master/Lib/datetime.py#L2516.

if it's removed L2516 of datetime.py, `test_name_cleanup()` would be failed.Looks like it's a planed behavior~

add a pdb point in L11 of test_datetime.py

  8     try:
  9         pure_tests = import_fresh_module(TESTS, fresh=['datetime', '_strptime'],
 10                                          blocked=['_datetime'])
 11         import pdb;pdb.set_trace()
 12  ->     fast_tests = import_fresh_module(TESTS, fresh=['datetime',
 13                                                        '_datetime', '_strptime'])

and run `./python -m test test_datetime test_datetime`:
the first iteration:
(Pdb) pure_tests.datetime_module._divide_and_round
<function _divide_and_round at 0x7f694df8e7e0>
the second iteration(`_datetime` not blocked?):
(Pdb) pure_tests.datetime_module._divide_and_round
*** AttributeError: module 'datetime' has no attribute '_divide_and_round'

This isn't exactly "working as intended", but I believe it's a known problem with either `import_fresh_module` or `datetime`, as you can see from these comments: https://github.com/python/cpython/blob/302e5a8f79514fd84bafbc44b7c97ec636302322/Lib/test/test_datetime.py#L14-L23

Based on the git blame, those TODO comments are from Georg Brandl, so I'm adding him to the nosy list in case he has some insight.

Sorry, it seems that was far too long ago for me to remember anything :)

> I believe it's a known problem with either `import_fresh_module` or `datetime`
after removed the cleanup operation of `sys.modules` in PR19213, the  testcases is successed in my local vm(I am not sure there have any other potential risks).

FYI, with the following additions in Lib/test/test_datetime.py...

   before = set(sys.modules)
   try:
       pure_tests = import_fresh_module(TESTS, fresh=['datetime', '_strptime'],
                                     blocked=['_datetime'])
       _pure = set(sys.modules)
       fast_tests = import_fresh_module(TESTS, fresh=['datetime',
                                                      '_datetime', '_strptime'])
       _fast = set(sys.modules)
       print(f'added (pure): {sorted(_pure-before)}')
       print(f'added (fast): {sorted(_fast-before)}')
       print('---')
   finally:
      ...

I get the following output running "./python -m test test_datetime test_datetime":

   0:00:00 load avg: 0.52 Run tests sequentially
   0:00:00 load avg: 0.52 [1/2] test_datetime
   added (pure): ['_decimal', '_strptime', '_testcapi', 'decimal', 'numbers', 'test.datetimetester']
   added (fast): ['_decimal', '_strptime', '_testcapi', 'decimal', 'numbers', 'test.datetimetester']
   ---
   0:00:05 load avg: 0.52 [2/2] test_datetime
   added (pure): ['_datetime', '_strptime', 'datetime']
   added (fast): ['_datetime', '_strptime', 'datetime']
   ---
   [snipped]

That definitely tells a story. :)  Unfortunately, for now I don't have any more time to investigate.  Sorry.

My PR of bpo-40799 fix this issue.

> My PR of bpo-40799 fix this issue.
Thanks victor for your work, I closed my PR ;)