`os.environ['VAR'] = ''` is not portable (windows)

[asottile]

Bug report

import os

def test_env(x):
    print(f'setting X={x}')
    os.environ['X'] = x
    assert os.environ.get('X') == x, os.environ.get('X')
    os.reload_environ()
    assert os.environ.get('X') == x, os.environ.get('X')

test_env('1')
test_env('')

it is possible to set an empty environment variable on windows (via SetEnvironmentVariableW) -- but not from python's os.environ . there's more context here: pre-commit/pre-commit#3766 (comment) (due to _wputenv)

on linux (I only attempted with amd64 ubuntu 26.04) the script above succeeds -- on windows it fails with the following:

>python t.py
setting X=1
setting X=
Traceback (most recent call last):
  File "C:\Users\asott\workspace\t.py", line 11, in <module>
    test_env('')
    ~~~~~~~~^^^^
  File "C:\Users\asott\workspace\t.py", line 8, in test_env
    assert os.environ.get('X') == x, os.environ.get('X')
           ^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: None

note also that this bug also results in os.environ diverging from the real environment (line 8 above is an assertion failure only after os.reload_environ())

note that I've only marked "3.14" as the version I tested on -- I suspect this also affects mainline but I do not have an easy way to build on the machine I'm working on at the moment

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[aisk]

This was discussed before in gh-83594. Using SetEnvironmentVariableW would solve this, but it bypasses the CRT, so C code in the same process that calls getenv() won't see the variable. That's why _wputenv is used, which leads to the current inconsistency.

I think it might be reasonable to just treat this as platform-specific behavior and document it? It's easy to work around at the application level.

Another option is to combine the two APIs for empty values, but that's a bit messy.

So I'd like to hear what @eryksun and @zooba think.

[asottile]

seems wrong to me to care that the CRT environ is in sync over os.environ itself