`patch = subprocess` seems not to work with `--branch`

[jepler]

Describe the bug
I want to use [run]patch = subprocess together with --branch but I'm getting unexpected results.

To Reproduce

1. Python 3.13 on Debian Trixie x86_64
2. Coverage.py, version 7.10.2 with C extension
3. No other non-standard packages required
4. Script content (subfunctions.py):

import subprocess
import argparse

def f1():
    print("function 1")
def f2():
    print("function 2")
def f3():
    print("function 3")

functions = [f1, f2, f3]

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("cases", nargs='+', type=int)
    args = parser.parse_args()

    if len(args.cases) > 1:
        for c in args.cases:
            subprocess.call(["python", __file__, f"{c}"])
    else:
        case = args.cases[0]
        functions[case]()

if __name__ == '__main__':
    main()

5. .coveragerc content:

[run]
patch = subprocess

6. coverage run --branch subfunctions.py 0 1 2 && coverage combine && coverage report

Expected behavior
The coverage combine step finishes and coverage report probably shows 100% coverage including branch coverage.

Instead, the step of coverage combine errors with a message similar to:

Combined data file .coverage.localhost.233908.XKcIJtPx
Can't combine statement coverage data with branch data

Additional context

This is a minimized reproducer. I ran into a problem in the context of an existing project when I tried to adapt it to use patch=subprocess.

My guess is that the --branch flag is not being propagated to the subprocesses. When the --branch flag is not specified, things seem to work properly.

My reproducer does appear to work properly if [run] branch=true is added to the applicable coveragerc instead of being specified on the commandline.

[nedbat]

This is something I should make clearer in the docs: when using patch = subprocess, all of the options have to be specified in the configuration file. Options on the command line won't be communicated down to the subprocess.

[nedbat]

Docs updated in commit b4edc26

[jepler]

Thank you!

[nedbat]

I changed my mind. As of commit 697d4bb, the entire configuration is inherited by the subprocesses, so --branch on the command line will work.

[jepler]

Thanks again! This would have made it even easier to switch my project over to "the new way".

[nedbat]

This is now released as part of coverage 7.10.3.