add workaround for flake8_6 enforcing three-letter codes in config to readme. Add tests (#234)

Author: jakkdl

README.md

@@ -123,6 +123,9 @@ flake8-async **/*.py
 but we prefer using a config file. The file needs to start with a section marker `[flake8]` and the following options are then parsed using flake8's config parser, and can be used just like any other flake8 options.
 Note that it's not currently possible to use a configuration file when running `flake8-async` standalone.
 
+### `ValueError` when trying to `ignore` error codes in config file
+Error codes with more than three letters are not possible to `ignore` in config files since flake8>=6, as flake8 tries to validate correct configuration with a regex. We have decided not to conform to this, as it would be a breaking change for end-users requiring them to update `noqa`s and configurations, we think the `ASYNC` code is much more readable than e.g. `ASYxxx`, and ruff does not enforce such a limit. The easiest option for users hitting this error is to instead use the `--disable` option as documented [below](#--disable). See further discussion and other workarounds in https://github.com/python-trio/flake8-async/issues/230
+
 ### `--enable`
 Comma-separated list of error codes to enable, similar to flake8 --select but is additionally more performant as it will disable non-enabled visitors from running instead of just silencing their errors.
 

tests/test_config_and_args.py

@@ -210,6 +210,7 @@ def _test_trio200_from_config_common(tmp_path: Path) -> str:
   other -> async,
   sync_fns.* -> the_async_equivalent,
 select = ASYNC200
+extend-ignore = E
 """
     )
     assert tmp_path.joinpath("example.py").write_text(
@@ -403,3 +404,33 @@ def test_disable_noqa_ast(
         == "./example.py:1:1: ASYNC106 trio must be imported with `import trio` for the"
         " linter to work.\n"
     )
+
+
+@pytest.mark.xfail(reason="flake8>=6 enforces three-letter error codes in config")
+def test_config_ignore_error_code(tmp_path: Path) -> None:
+    assert tmp_path.joinpath(".flake8").write_text(
+        """
+[flake8]
+ignore = ASYNC100
+"""
+    )
+    res = subprocess.run(
+        ["flake8", "--help"], cwd=tmp_path, capture_output=True, check=False
+    )
+    assert not res.stderr
+    assert res.returncode == 0
+
+
+# but make sure we can disable selected codes
+def test_config_disable_error_code(tmp_path: Path) -> None:
+    # select ASYNC200 and create file that induces ASYNC200
+    _test_trio200_from_config_common(tmp_path)
+    # disable ASYNC200
+    with open(tmp_path.joinpath(".flake8"), "a", encoding="utf-8") as file:
+        file.write("disable = ASYNC200")
+
+    # it now returns no errors
+    res = subprocess.run(["flake8"], cwd=tmp_path, capture_output=True, check=True)
+    assert not res.stdout
+    assert not res.stderr
+    assert res.returncode == 0