session.env contains None values

[pquentin]

Thank you for maintaining nox!

Current Behavior

The session.env dictionary contains the following variables set to None:

• PIP_REQUIRE_VIRTUALENV
• PIP_RESPECT_VIRTUALENV
• UV_SYSTEM_PYTHON
• __PYVENV_LAUNCHER__
• CONDA_PREFIX

Additionally, it only contains two string values:

• VIRTUAL_ENV
• NOX_CURRENT_SESSION

It does not contain eg. PATH, which is #928. But since my symptoms are different, I opened a different issue.

As a result, when I pass env=session.env in a subprocess call, I get the following exception:

    env_list.append(k + b'=' + os.fsencode(v))
                               ~~~~~~~~~~~^^^
  File "<frozen os>", line 849, in fsencode
TypeError: expected str, bytes or os.PathLike object, not NoneType

Expected Behavior

I would expect session.env to 1/ actually contain the environment 2/ not contain any None values.

Steps To Reproduce

With this noxfile:

import nox

@nox.session()
def test(session):
    for k, v in session.env.items():
        print(k, v)

$ nox -s test
__PYVENV_LAUNCHER__ None
UV_SYSTEM_PYTHON None
PIP_REQUIRE_VIRTUALENV None
PIP_RESPECT_VIRTUALENV None
VIRTUAL_ENV /private/tmp/.nox/test
CONDA_PREFIX None
NOX_CURRENT_SESSION test

Environment

- OS: macOS 15.3
- Python: 3.13.1
- Nox: 2025.02.09

Anything else?

I have bisected this, and the culprit appears to be #874.

[henryiii]

The problem is, do you expect to be able to modify it? If you want session.env[...] = ... to work, it needs to be the raw underlying dictionary, which contains None's (mask this env var), and no PATH, since that has to be computed. Usually, that's what people use this for (I think), since session.run* is the intended way to use this, not raw subprocess calls.

I might try to see if I can come up with something clever to replace dict, otherwise maybe a new method could be added for getting a dict usable in subprocess?

The root problem is that session.run has a include_outer_env=False option, and we can't put the outer environment in here and then pick it out later.

[pquentin]

I am happy to switch to the run methods, I opened this in case it was an unintended change.

[pquentin]

Closing as I was able to use session.run() in elastic/eland#754. Thank you for your help!