D403: Require capitalizing single word sentence

crates/ruff_linter/resources/test/fixtures/pydocstyle/D403.py

@@ -25,3 +25,9 @@ def non_ascii():
 
 def all_caps():
     """th•s is not capitalized."""
+
+def single_word():
+    """singleword."""
+
+def single_word_no_dot():
+    """singleword"""

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

@@ -59,26 +59,36 @@ pub(crate) fn capitalized(checker: &mut Checker, docstring: &Docstring) {
     }
 
     let body = docstring.body();
-    let Some(first_word) = body.split(' ').next() else {
-        return;
-    };
-
-    // Like pydocstyle, we only support ASCII for now.
-    for char in first_word.chars() {
-        if !char.is_ascii_alphabetic() && char != '\'' {
-            return;
-        }
-    }
+    let first_word = body.split_once(' ').map_or_else(
+        || {
+            // If the docstring is a single word, trim the punctuation marks because
+            // it makes the ASCII test below fail.
+            body.trim_end_matches(['.', '!', '?'])
+        },
+        |(first_word, _)| first_word,
+    );
 
     let mut first_word_chars = first_word.chars();
     let Some(first_char) = first_word_chars.next() else {
         return;
     };
+
+    if !first_char.is_ascii() {
+        return;
+    }
+
     let uppercase_first_char = first_char.to_ascii_uppercase();
     if first_char == uppercase_first_char {
         return;
     }
 
+    // Like pydocstyle, we only support ASCII for now.
+    for char in first_word.chars().skip(1) {
+        if !char.is_ascii_alphabetic() && char != '\'' {
+            return;
+        }
+    }
+
     let capitalized_word = uppercase_first_char.to_string() + first_word_chars.as_str();
 
     let mut diagnostic = Diagnostic::new(

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D403_D403.py.snap

@@ -19,4 +19,37 @@ D403.py:2:5: D403 [*] First word of the first line should be capitalized: `this`
 4 4 | def good_function():
 5 5 |     """This docstring is capitalized."""
 
+D403.py:30:5: D403 [*] First word of the first line should be capitalized: `singleword` -> `Singleword`
+   |
+29 | def single_word():
+30 |     """singleword."""
+   |     ^^^^^^^^^^^^^^^^^ D403
+31 | 
+32 | def single_word_no_dot():
+   |
+   = help: Capitalize `singleword` to `Singleword`
 
+ℹ Safe fix
+27 27 |     """th•s is not capitalized."""
+28 28 | 
+29 29 | def single_word():
+30    |-    """singleword."""
+   30 |+    """Singleword."""
+31 31 | 
+32 32 | def single_word_no_dot():
+33 33 |     """singleword"""
+
+D403.py:33:5: D403 [*] First word of the first line should be capitalized: `singleword` -> `Singleword`
+   |
+32 | def single_word_no_dot():
+33 |     """singleword"""
+   |     ^^^^^^^^^^^^^^^^ D403
+   |
+   = help: Capitalize `singleword` to `Singleword`
+
+ℹ Safe fix
+30 30 |     """singleword."""
+31 31 | 
+32 32 | def single_word_no_dot():
+33    |-    """singleword"""
+   33 |+    """Singleword"""