Describe the bug
coverage json -o - outputs to stdout. But if there are multiple coverage files, it will first output a message in stdout meant for stderr (Combined 1 file) before ti outputs the json, which makes json parsing of stdout break.
To Reproduce
https://github.com/ewjoachim/test_cov
Execute the commands specified in the readme
Answer the questions below:
- What version of Python are you using?
py3.14
- What version of coverage.py shows the problem? The output of
coverage debug sys is helpful.1. What
-- sys -------------------------------------------------------
coverage_version: 7.14.1
coverage_module: /private/tmp/test_cov/.venv/lib/python3.14/site-packages/coverage/__init__.py
core: -none-
CTracer: available from /private/tmp/test_cov/.venv/lib/python3.14/site-packages/coverage/tracer.cpython-314-darwin.so
plugins.file_tracers: -none-
plugins.configurers: -none-
plugins.context_switchers: -none-
configs_attempted: /private/tmp/test_cov/.coveragerc
/private/tmp/test_cov/.coveragerc.toml
/private/tmp/test_cov/setup.cfg
/private/tmp/test_cov/tox.ini
/private/tmp/test_cov/pyproject.toml
configs_read: /private/tmp/test_cov/pyproject.toml
config_file: None
config_contents: -none-
data_file: -none-
python: 3.14.2 (main, Jan 13 2026, 19:27:04) [Clang 21.1.4 ]
platform: macOS-26.2-arm64-arm-64bit-Mach-O
implementation: CPython
build: ('main', 'Jan 13 2026 19:27:04')
gil_enabled: True
executable: /private/tmp/test_cov/.venv/bin/python
pth_files: /private/tmp/test_cov/.venv/lib/python3.14/site-packages/a1_coverage.pth
def_encoding: utf-8
fs_encoding: utf-8
pid: 68734
cwd: /private/tmp/test_cov
path: /private/tmp/test_cov/.venv/bin
/Users/joachim/.local/share/uv/python/cpython-3.14.2-macos-aarch64-none/lib/python314.zip
/Users/joachim/.local/share/uv/python/cpython-3.14.2-macos-aarch64-none/lib/python3.14
/Users/joachim/.local/share/uv/python/cpython-3.14.2-macos-aarch64-none/lib/python3.14/lib-dynload
/private/tmp/test_cov/.venv/lib/python3.14/site-packages
environment: HOME = /Users/joachim
PYTHONDONTWRITEBYTECODE = 1
command_line: /private/tmp/test_cov/.venv/bin/coverage debug sys
time: 2026-06-19 02:46:48
-
versions of what packages do you have installed? The output of pip freeze is helpful.
Nothing but coverage
-
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.
https://github.com/ewjoachim/test_cov/commit/50774c34530788c696491a2d3c6aa17159812871
- 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/ewjoachim/test_cov.git
$ uv run coverage run hello.py
$ uv run coverage run --data-file=.coverage.2 hello.py
$ uv run coverage json -o -
Expected behavior
Only the json should be output by the last command.
Additional context
It's causing py-cov-action/python-coverage-comment-action#676
Describe the bug
coverage json -o -outputs to stdout. But if there are multiple coverage files, it will first output a message instdoutmeant forstderr(Combined 1 file) before ti outputs the json, which makes json parsing of stdout break.To Reproduce
https://github.com/ewjoachim/test_cov
Execute the commands specified in the readme
Answer the questions below:
py3.14
coverage debug sysis helpful.1. Whatversions of what packages do you have installed? The output of
pip freezeis helpful.Nothing but coverage
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.
https://github.com/ewjoachim/test_cov/commit/50774c34530788c696491a2d3c6aa17159812871
git clone,pip install, and so on. Explain like we're five!Expected behavior
Only the json should be output by the last command.
Additional context
It's causing py-cov-action/python-coverage-comment-action#676