Equivalent of bash @ for tox environment variable substitution

[pharmpy-dev-123]

Is there a way to make the following work:

[testenv]
basepython = {env:TOXPYTHON:python3.9}
setenv =
    PYTHONPATH={toxinidir}/tests
    PYTHONUNBUFFERED=yes
    PYTHONASYNCIODEBUG=1
    PHARMPYNOCONFIGFILE=1
    COVERAGEARGS=""
    cover: COVERAGEARGS="--cov --cov-report=term-missing --cov-report xml:.coverage.xml"
    PROFILEARGS=""
    profile: PROFILEARGS="--profile-svg"
passenv =
    *
usedevelop = false
deps =
    -rrequirements.txt
    pytest>5.3.5
    pytest-asyncio>=0.17.0
    pytest-xdist>1.34.0
    cover: pytest-cov
    profile: pytest-profiling
    profile: gprof2dot==2019.11.30
    debug: pdbpp

[testenv:{py38,py39,py310,py38-unit,py39-unit,py310-unit,unit}{-cover,}{-profile,}{-debug,}]
commands =
    pytest -n auto {env:PROFILEARGS} {env:COVERAGEARGS} -vv {posargs:tests}

Currently, this example lacks spreading of the $COVERAGEARGS environment variable, '--cov --cov-report=term-missing --cov-report xml:.coverage.xml' is passed as a single argument. Furthermore, when one of these variables is empty it adds an empty string argument to the command.

Ideally, each space-separated string would be passed as a single argument, and if the environment variable is empty, it should not add any argument.

[pharmpy-dev-123]

I somewhat fixed my problem by using the following:

[base]
deps =
    pytest>5.3.5
    pytest-asyncio>=0.17.0
    pytest-xdist>1.34.0
    cover: pytest-cov
    profile: pytest-profiling
    profile: gprof2dot==2019.11.30
    debug: pdbpp

[flags]
cover = --cov --cov-report=term-missing --cov-report xml:.coverage.xml
profile = --profile-svg
debug = --pdb

[testenv:{py38,py39,py310,py38-unit,py39-unit,py310-unit,unit}{-cover,}{-profile,}{-debug,}]
skip_install = false
deps =
    -rrequirements.txt
    {[base]deps}
commands = pytest -n auto -vv \
    profile: {[flags]profile} \
    cover: {[flags]cover} \
    debug: {[flags]debug} \
    {posargs:tests}

One sad thing is that factor-filtered substitution does not seem to be recursive in commands contrary to deps. So I cannot do something like {[base]flags}.