test_buffer adds two environment variables: 'OPENBLAS_MAIN_FREE': '1', 'GOTOBLAS_MAIN_FREE': '1'.

$ ./python -m test -v test_buffer
...
Warning -- os.environ was modified by test_buffer
  Before: (140174267697792, environ({...})
  After:  (140174267697792, environ({..., 'OPENBLAS_MAIN_FREE': '1', 'GOTOBLAS_MAIN_FREE': '1'}) 
test_buffer failed (env changed)

1 test altered the execution environment:
    test_buffer

Total duration: 8 sec
Tests result: SUCCESS

Why the env changed wasn't noticed before? Is it a recent change?

I just have installed NumPy. It temporary sets these two environment variables for importing the multiarray numpy extension module, but forgot to remove them if the import is failed.

test_buffer imports numpy.

try:
    with warnings.catch_warnings():
        from numpy import ndarray as numpy_array
except ImportError:
    numpy_array = None

So it is a numpy issue? Which numpy version is it?

It is particularly a NumPy issue (I have opened a bug https://github.com/numpy/numpy/issues/9861). It is exposed because I have installed NumPy with non-debug build of CPython and run the test with debug build of CPython. The test is passes with non-debug build of CPython. I am wondering if we can make a workaround and if we should do this.

Thanks. If it only occurs when mixing debug/non-debug I don't think we have to do anything about it.

I suggest to always save/restore env vars on numpy import, since we don't
control 3rd party code.

That's fine.

I can't figure out how to make the damn button green on GitHub,
so LGTM. :)

The issue is fixed on NumPy side (will be included in the next release). Is the workaround on our side still needed?

I'd push it so we don't have to worry about similar things in the
future.  The "warnings.catch_warnings()" thing is also a workaround
for some past numpy release.

New changeset 676db4bbf2e7c18dc7c35add17dd3bbdc2d3eeb3 by Serhiy Storchaka in branch 'master':
bpo-31792: Restore os.environ in test_buffer when import numpy. (#4007)
https://github.com/python/cpython/commit/676db4bbf2e7c18dc7c35add17dd3bbdc2d3eeb3

New changeset 06949585d292d1a9dbf2d0d0fa8fc8fe1dee3ccf by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31792: Restore os.environ in test_buffer when import numpy. (GH-4007) (#4009)
https://github.com/python/cpython/commit/06949585d292d1a9dbf2d0d0fa8fc8fe1dee3ccf

Thank you Victor and Stefan for your review.