flake8-errmsg

Intro

A checker for flake8 that helps format nice error messages. Currently there are two checks:

• EM101: Check for raw usage of string literals in Exception raising.
• EM102: Check for raw usage of f-string literals in Exception raising.

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:

sub = "Some value"
raise RuntimeError(f"{sub!r} is incorrect")

Traceback (most recent call last):
  File "tmp.py", line 2, in <module>
    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:

sub = "Some value"
msg = f"{sub!r} is incorrect"
raise RunetimeError(msg)

Traceback (most recent call last):
  File "tmp.py", line 3, in <module>
    raise RuntimeError(msg)
RuntimeError: 'Some value' is incorrect

Now there's a simpler traceback, less code, 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.

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 <files>) or module entry-point (python -m flake8_errmsg <files> when installed).

FAQ

Q: Why not look for "".format() too?
A: Tools like pyupgrade should help move to fstrings, so these should be rare. But it would likely be easy to add.

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.

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. A configuration option allowing short non f-strings might be useful. Possibly adding a check for warnings.warn without setting stacklevel to something (usually 2).