How to override deps with CLI or environment variable?

[jaraco]

I'd like to run a tox env with an additional constraint on the dependencies. I'd like to do that directly in the invocation, either by specifying a command-line parameter or setting an environment variable.

Based on these docs:

All tox defaults can now be changed via an environment variable: TOX_ prefix followed by the settings key, e.g. TOX_PACKAGE=wheel.

Since package is defined at the same level as deps, I'd have expected that setting TOX_DEPS would have allowed overriding the testenv.deps, but it didn't. I also tried TOX_ENV_DEPS=pydantic>=2 and TOX_TEST_ENV_DEPS and TOX_TESTENV_DEPS, but none have any effect.

I also tried tox run -e py -x 'deps=pydantic>=2' along with other variants for -x, including testenv:py.deps=pydantic>=2, but none had any effect.

The current config specifies deps=, so overriding or extending requirements functions the same in this case, though what I really want is to constrain. What I don't want to do is have to generate a file (and clean it up later) in order to test.

Is this objective achievable with tox? Why are the documented expectations not met?