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os.unsetenv() is documented to be available on Windows, but it's not.
In Python 3.8, "del os.environ[key]" is implemented as:
 os.putenv(key.upper(), "")
Attached PR implements it using SetEnvironmentVariableW(name, NULL).

New changeset 56cd3710a1ea3ba872d345ea1bebc86ed08bc8b8 by Victor Stinner in branch 'master':
bpo-39413: Implement os.unsetenv() on Windows (GH-18104)
https://github.com/python/cpython/commit/56cd3710a1ea3ba872d345ea1bebc86ed08bc8b8

This bypasses the CRT's copy of the environment. If any C code calls [_w]getenv, it will still see the variable defined. The only way it won't is if setting the value also bypassed the CRT environment by directly calling SetEnvironmentVariableW.

> This bypasses the CRT's copy of the environment. If any C code calls [_w]getenv, it will still see the variable defined. The only way it won't is if setting the value also bypassed the CRT environment by directly calling SetEnvironmentVariableW.
Oh. That's a valid concern. I reopen the issue.

I checked the doc: _putenv("VAR=") removes the environment variable "VAR". It's well documented:
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/putenv-wputenv?view=vs-2019
"You can remove a variable from the environment by specifying an empty value_string, or in other words, by specifying only varname=."
So Python 3.8 os._unsetenv() is correct, but this function was not exposed, and so this issue remains relevant: Python 3.8 documents that os.unsetenv() is available on Windows, but it's not.

PR 18107 still fix this issue (add os.unsetenv() to Python 3.9 on Windows), but fix the implementation (use _wputenv() rather than SetEnvironmentVariableW(), to update the CRT).

New changeset f84f65be5602e561fef04b66bb487fbc4e560db5 by Victor Stinner in branch '3.8':
bpo-39413: os.unsetenv() is not available on Windows (GH-18108)
https://github.com/python/cpython/commit/f84f65be5602e561fef04b66bb487fbc4e560db5

New changeset 570724667d37fcb160cdef2b34190dc36e28155c by Miss Islington (bot) in branch '3.7':
bpo-39413: os.unsetenv() is not available on Windows (GH-18108)
https://github.com/python/cpython/commit/570724667d37fcb160cdef2b34190dc36e28155c

New changeset b73dd02ea744288831f71363a7467552c09875ea by Victor Stinner in branch 'master':
Revert "bpo-39413: Implement os.unsetenv() on Windows (GH-18104)" (GH-18124)
https://github.com/python/cpython/commit/b73dd02ea744288831f71363a7467552c09875ea

New changeset 161e7b36b1ea871a1352ccfc1d4f4c1eda76830f by Victor Stinner in branch 'master':
bpo-39413: Implement os.unsetenv() on Windows (GH-18163)
https://github.com/python/cpython/commit/161e7b36b1ea871a1352ccfc1d4f4c1eda76830f