`-c`/`--config-file` has two failure modes for invalid paths: silent no-op for unrecognized extensions, and raw `FileNotFoundError` for recognized ones

[SecondarySkyler]

Summary
-c/--config-file does not validate its input consistently. Depending on the extension of the (possibly nonexistent) path given, pytest either silently proceeds with an empty configuration, or crashes with an unhandled FileNotFoundError traceback. Neither is correct — both should be a clean UsageError.

Steps to reproduce

mkdir repro; cd repro
cat > config.ini <<'EOF'
[pytest]
addopts = -v
EOF

Then

pytest -c config.in         # real file, wrong extension
pytest -c fake_config.in    # doesn't exist, wrong extension
pytest -c fake_config.ini   # doesn't exist, correct extension

Actual Behavior
pytest -c config.in and pytest -c fake_config.in both produce identical result:

==================================================== test session starts =====================================================
platform darwin -- Python 3.12.13, pytest-9.1.1, pluggy-1.6.0
rootdir: /Users/user/repro
configfile: config.in
plugins: env-1.6.0, metadata-3.1.1, html-4.2.0
collected 0 items

=================================================== no tests ran in 0.01s ====================================================

note configfile: is reported even though the file was never checked to exist.
pytest -c fake_config.ini, by contrast, raises an unhandled FileNotFoundError and prints a raw Python traceback instead of a clean pytest error.

Root Cause
In src/_pytest/config/findpaths.py:

• load_config_dict_from_file() dispatches on Path.suffix(). Only .ini, .cfg, and .toml have handling branches, so everything else is implicitly returned as None.
• for a .ini file, _parse_ini_config() apparently checks only for malformed-content errors and not missing-file errors, so a nonexistent .ini path raises an uncaught FileNotFoundError

Environment

• pytest 9.1.1
• Python 3.12.13
• macOS (darwin)

Related Issues
Probably #13246

[molloyzak13]

reply_target: #14716
Reporter's right about the root cause. Had a look through findpaths.py and there are two failure paths.

load_config_dict_from_file() dispatches on filepath.suffix and only handles .ini, .cfg, and .toml. Anything else falls through the if-elif chain and implicitly returns None. No existence check, no suffix validation.

_parse_ini_config() calls iniconfig.IniConfig(str(path)), which opens the file inside its constructor. The function catches iniconfig.ParseError only, so a missing file bypasses the handler and propagates as a raw traceback.

Catching OSError (or adding a path.exists() guard) in _parse_ini_config() would handle missing-file and permission-denied cases, and an else branch in load_config_dict_from_file() could raise UsageError for unrecognised suffixes instead of silently returning None.

[SecondarySkyler]

If you consider the issue worthwile, I would like to work on it. Moreover if you have further suggestions on how to fix the problem, they are welcome.
My plan is to add validation in the inifile branch of determine_setup() (in src/_pytest/config/findpaths.py) that checks the path passed to -c exists and has a recognized extension before calling load_config_dict_from_file, raising a UsageError in either case — similar to how --rootdir is already validated a few lines below in the same function.
Happy to adjust the approach if you have a preference — e.g. whether both failure modes (missing extension vs. missing file) should share one error message or be distinguished, or whether this should live in findpaths.py vs. somewhere else. Let me know if there's anything I should be aware of before diving in.