https://github.com/henryiii/flake8-errmsg
Flake8 checker for raw literals inside raises.
https://github.com/henryiii/flake8-errmsg
Last synced: about 1 month ago
JSON representation
Flake8 checker for raw literals inside raises.
- Host: GitHub
- URL: https://github.com/henryiii/flake8-errmsg
- Owner: henryiii
- License: apache-2.0
- Created: 2022-09-05T18:26:10.000Z (over 2 years ago)
- Default Branch: main
- Last Pushed: 2025-04-07T18:58:04.000Z (about 2 months ago)
- Last Synced: 2025-04-07T19:42:33.705Z (about 2 months ago)
- Language: Python
- Size: 92.8 KB
- Stars: 12
- Watchers: 3
- Forks: 2
- Open Issues: 1
-
Metadata Files:
- Readme: README.md
- Contributing: .github/CONTRIBUTING.md
- License: LICENSE
Awesome Lists containing this project
README
# flake8-errmsg
[![Actions Status][actions-badge]][actions-link]
[![PyPI version][pypi-version]][pypi-link]
[![PyPI platforms][pypi-platforms]][pypi-link]
## Intro
A checker for Flake8 that helps format nice error messages. The checks are:
- **EM101**: Check for raw usage of a string literal in Exception raising.
- **EM102**: Check for raw usage of an f-string literal in Exception raising.
- **EM103**: Check for raw usage of `.format` on a string literal in Exception
raising.
- **EM104**: Check for missing parentheses for built-in exceptions.
- **EM105**: Check for missing message for built-in exceptions.
The issue is that Python includes the line with the raise in the default
traceback (and most other formatters, like Rich and IPython to too). That means
a user gets a message like this:
```python
sub = "Some value"
raise RuntimeError(f"{sub!r} is incorrect")
```
```pytb
Traceback (most recent call last):
File "tmp.py", line 2, in
raise RuntimeError(f"{sub!r} is incorrect")
RuntimeError: 'Some value' is incorrect
```
If this is longer or more complex, the duplication can be quite confusing for a
user unaccustomed to reading tracebacks.
While if you always assign to something like `msg`, then you get:
```python
sub = "Some value"
msg = f"{sub!r} is incorrect"
raise RuntimeError(msg)
```
```pytb
Traceback (most recent call last):
File "tmp.py", line 3, in
raise RuntimeError(msg)
RuntimeError: 'Some value' is incorrect
```
Now there's a simpler traceback and no double message. If you have a long
message, this also often formats better when using Black, too.
Reminder: Libraries should produce tracebacks with custom error classes, and
applications should print nice errors, usually _without_ a traceback, unless
something _unexpected_ occurred. An app should not print a traceback for an
error that is known to be triggerable by a user.
## Options
There is one option, `--errmsg-max-string-length`, which defaults to 0 but can
be set to a larger value. The check will ignore string literals shorter than
this length. This option is supported in configuration mode as well. This will
only affect string literals and not f-strings. This option is also supported
when running directly, without Flake8.
## Usage
Just add this to your `.pre-commit-config.yaml` `flake8` check under
`additional_dependencies`. If you use `extend-select`, you should need no other
config.
You can also manually run this check (without Flake8's `noqa` filtering) via
script entry-point (`pipx run flake8-errmsg `) or module entry-point
(`python -m flake8_errmsg ` when installed).
## FAQ
Q: Why Python 3.10+ only?
A: This is a static checker and for developers.
Developers and static checks should be on 3.10 already. And I was lazy and match
statements are fantastic for this sort of thing. And the AST module changed in
3.8 anyway. Use [Ruff][] (which contains the checks from this plugin) if you
need to run on older versions.
Q: What other sorts of checks are acceptable?
A: Things that help with
nice errors. For example, maybe requiring `raise SystemExit(n)` over `sys.exit`,
`exit`, etc. Possibly adding a check for `warnings.warn` without setting
`stacklevel` to something (usually 2).
[actions-badge]: https://github.com/henryiii/flake8-errmsg/workflows/CI/badge.svg
[actions-link]: https://github.com/henryiii/flake8-errmsg/actions
[pypi-link]: https://pypi.org/project/flake8-errmsg/
[pypi-platforms]: https://img.shields.io/pypi/pyversions/flake8-errmsg
[pypi-version]: https://img.shields.io/pypi/v/flake8-errmsg
[ruff]: https://github.com/astral-sh/ruff