PYTHONWARNDEFAULTENCODING causes unavoidable warnings

[henryiii]

Describe the bug
If you set PYTHONWARNDEFAULTENCODING, which detects missing encoding settings when opening or reading files, coverage seems to spit out warnings that can't easily be skipped. Adding an encoding= (and testing with PYTHONWARNDEFAULTENCODING set) would fix this, and if it was set to utf-8 instead of native, could possibly provide a smoother transition to the version of Python where the default changes.

To Reproduce
How can we reproduce the problem? Please be specific. Don't link to a failing CI job. Answer the questions below:

1. What version of Python are you using? 3.12 and 3.13
2. What version of coverage.py shows the problem? The output of coverage debug sys is helpful. All
3. What versions of what packages do you have installed? The output of pip freeze is helpful. Doesn't matter, checked the file in main.
4. What code shows the problem? Give us a specific commit of a specific repo that we can check out. If you've already worked around the problem, please provide a commit before that fix. Set PYTHONWARNDEFAULTENCODING
5. What commands should we run to reproduce the problem? Be specific. Include everything, even git clone, pip install, and so on. Explain like we're five!

git clone https://github.com/pypa/build
tox run -e py313

Expected behavior
No warnings.

Additional context
Add any other context about the problem here.

py313: commands[0]> pytest -ra --cov --cov-config pyproject.toml --cov-report=html:/Users/henryschreiner/git/software/build/.tox/py313/htmlcov --cov-context=test --cov-report=xml:/Users/henryschreiner/git/software/build/.tox/coverage.py313.xml -n auto
======================================================================================================================================================================== test session starts =========================================================================================================================================================================
platform darwin -- Python 3.13.3, pytest-8.3.5, pluggy-1.5.0
cachedir: .tox/py313/.pytest_cache
installed packages of interest: build==1.2.2.post1 filelock==3.18.0 packaging==25.0 pip==25.1.1 pyproject_hooks==1.2.0 setuptools==80.4.0 virtualenv==20.31.2 wheel==0.45.1
rootdir: /Users/henryschreiner/git/software/build
configfile: pyproject.toml
testpaths: tests
plugins: rerunfailures-15.1, mock-3.14.0, cov-6.1.1, xdist-3.6.1
16 workers [196 items]
..............................................s...................................................................ssssssssssssssssssssssssssssssssssssssssssss..................s...s.............../Users/henryschreiner/git/software/build/.tox/py313/lib/python3.13/site-packages/coverage/html.py:54: EncodingWarning: 'encoding' argument not specified
  with open(data_filename(fname)) as data_file:
/Users/henryschreiner/git/software/build/.tox/py313/lib/python3.13/site-packages/coverage/html.py:709: EncodingWarning: 'encoding' argument not specified
  with open(status_file) as fstatus:
/Users/henryschreiner/git/software/build/.tox/py313/lib/python3.13/site-packages/coverage/html.py:750: EncodingWarning: 'encoding' argument not specified
  with open(status_file, "w") as fout:

I believe you can simply set PYTHONWARNDEFAULTENCODING with tox, but I can't run your tox, it just reports "no module named pip" and dies on every job. (I do have tox-uv installed) If I add requires = tox pip, then I can get it working, I think, trying that., no, doesn't work. If I remove the line that injects a pip install, then it runs, though out of the box I get a lot of errors like "coverage: command not found". Ah, got it to pass, had to set up a venv with the dev deps in it. Why can't tox work out of the box? I thought that was the point of using a test runner? Ouch, I see setup.py <commands> in the run. Anyway, now I can test and see if adding the env var shows warnings.

[nedbat]

I'm surprised about the tox difficulties, sorry about those.

[nedbat]

This is fully fixed in commit 6999a4e.

[henryiii]

RE: tox, I think this can be fixed with the requires feature, I'll try to look at this (if I remember or am pinged) at some point.

[nedbat]

Can you show me the problem happening with these commands:

make install
tox

[henryiii]

The problem is that tox shouldn't require pre-setup. It's supposed to do its own setup. I found doc/contributing.rst and figured that I could use:

uv venv
. .venv/bin/activate.fish
uv pip install -r requirements/dev.pip
tox

And that worked.

But tox is supposed to create virtual environments and not require external setup beforehand (requirements/dev.pip). That's what tox (and nox, which I work on) are for; our "mission" if you will is you can just type "?ox" without any setup. I think I can fix this by using tox's requirements feature or small tweaks somewhere.

(I never run make commands, they tend to not be virtual environment aware, running an install command can will break my system. I don't go so far as removing make from my system, but it's close. ;) )

[nedbat]

If you can make it simpler I'm open to seeing changes, but frankly, I don't think it's too much to ask people to read and use the docs I've written about how to make things go. If the changes aren't difficult, and work reliably, I'm open to them.

[nedbat]

FWIW, I tried this:

git clone http://github.com/nedbat/coveragepy
cd coveragepy
uv venv
source ./.venv/bin/activate
uv pip install tox
tox

and it seemed to run all the tests successfully. I'm not sure what went wrong for you. Maybe my steps here weren't what you meant?

[nedbat]

This is now released as part of coverage 7.8.1.