D403 does not trigger for a single-word docstring that ends with a period

[jbmoorhouse]

Keywords searched for

D403 -> Potentially related to #2989

Ruff version

0.3.0

Issue

For a docstring with one word (admittedly not a typical docstring 😉), if the word is not capitalised and there is no fullstop, the correct errors are raised

However, when adding a full stop, D403 is suppressed/not raised

[MichaReiser]

I think this is probably a bug in the check here

ruff/crates/ruff_linter/src/rules/pydocstyle/rules/capitalized.rs

Lines 62 to 64 in 230c9ce

	let Some(first_word) = body.split(' ').next() else {
	return;
	};

It splits by whitespace to get the first word. But that means it returns early if it's a single word. We should probably change this to

let first_word = body.split(' ').next().unwrap_or(&body);

[MichaReiser]

Okay, that's not sufficient, because it then gets skipped in

    for char in first_word.chars() {
        if !char.is_ascii_alphabetic() && char != '\'' {
            dbg!("Non ascii character");
            return;
        }
    }

I need to look into the pycodestyle implementation if this is intentional.

[MichaReiser]

Okay, I was wrong. Split is used correctly. It always returns the string even if the split character isn't found. It bails because . isn't an ASCII character, and this matches pydocstyle's implementation.

I'll put up a PR but I"m not sure if we should change the current behavior because a single word isn't really a phrase...