@eryksun @serhiy-storchaka: Here is a simpler implementation of os.unsetenv() for Windows.

• Reuse os.putenv() code to check name
• Add many tests for invalid names
• Add tests for os.putenv()

In practice,os.unsetenv() rejects "=" anywhere in the name on all platforms: it's now validated by unit tests. I chose to not change raised exceptions (ValueError vs OSError) for invalid variable name in this PR. I plan to propose a separated PR to unify the raised exception on all platforms.

I also chose to not remove the Windows maximum variable length limit in this PR, again to make it easier to review. I also plan to propose a separated PR to limit this outdated limit.

I also chose to not remove the Windows maximum variable length limit in this PR, again to make it easier to review. I also plan to propose a separated PR to limit this outdated limit.

_MAX_ENV (32767) itself isn't outdated. However, the limit of less than 32767 characters (i.e. 32766 or less) is enforced individually on the name and value, not on the "name=value" entry. It either has to check them individually and raise ValueError or suppress the invalid parameter handler around the call via _Py_BEGIN_SUPPRESS_IPH / _Py_END_SUPPRESS_IPH and fail with a generic EINVAL error.