Use starred unpacking for RUF017 in Python 3.15+

crates/ruff_linter/src/rules/ruff/mod.rs

@@ -245,6 +245,19 @@ mod tests {
         assert_diagnostics!("PY313_RUF036_runtime_evaluated", diagnostics);
     }
 
+    #[test]
+    fn quadratic_list_summation_py315() -> Result<()> {
+        let diagnostics = test_path(
+            Path::new("ruff/RUF017_0.py"),
+            &settings::LinterSettings {
+                unresolved_target_version: PythonVersion::PY315.into(),
+                ..settings::LinterSettings::for_rule(Rule::QuadraticListSummation)
+            },
+        )?;
+        assert_diagnostics!("PY315_RUF017_RUF017_0.py", diagnostics);
+        Ok(())
+    }
+
     #[test]
     fn access_annotations_from_class_dict_py310() -> Result<()> {
         let diagnostics = test_path(

crates/ruff_linter/src/rules/ruff/rules/quadratic_list_summation.rs

@@ -3,7 +3,7 @@ use itertools::Itertools;
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast::token::parenthesized_range;
-use ruff_python_ast::{self as ast, Arguments, Expr};
+use ruff_python_ast::{self as ast, Arguments, Expr, PythonVersion};
 use ruff_python_semantic::SemanticModel;
 use ruff_text_size::Ranged;
 
@@ -23,12 +23,15 @@ use crate::{AlwaysFixableViolation, Edit, Fix};
 /// quadratic complexity. The following methods are all linear in the number of
 /// lists:
 ///
+/// - `[*sublist for sublist in lists]` (Python 3.15+)
 /// - `functools.reduce(operator.iadd, lists, [])`
 /// - `list(itertools.chain.from_iterable(lists))`
 /// - `[item for sublist in lists for item in sublist]`
 ///
-/// When fixing relevant violations, Ruff defaults to the `functools.reduce`
-/// form, which outperforms the other methods in [microbenchmarks].
+/// When fixing relevant violations, Ruff uses the starred-list-comprehension
+/// form on Python 3.15 and later. On older Python versions, Ruff falls back to
+/// the `functools.reduce` form, which outperforms the other pre-3.15
+/// alternatives in [microbenchmarks].
 ///
 /// ## Example
 /// ```python
@@ -38,21 +41,18 @@ use crate::{AlwaysFixableViolation, Edit, Fix};
 ///
 /// Use instead:
 /// ```python
-/// import functools
-/// import operator
-///
-///
 /// lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
-/// functools.reduce(operator.iadd, lists, [])
+/// joined = [*sublist for sublist in lists]
 /// ```
 ///
 /// ## Fix safety
 ///
-/// This fix is always marked as unsafe because `sum` uses the `__add__` magic method while
-/// `operator.iadd` uses the `__iadd__` magic method, and these behave differently on lists.
-/// The former requires the right summand to be a list, whereas the latter allows for any iterable.
-/// Therefore, the fix could inadvertently cause code that previously raised an error to silently
-/// succeed. Moreover, the fix could remove comments from the original code.
+/// This fix is always marked as unsafe because the replacement may accept any
+/// iterable where `sum` previously required lists. On Python 3.15 and later,
+/// Ruff uses iterable unpacking within a list comprehension; on older Python
+/// versions, Ruff uses `operator.iadd`. In both cases, code that previously
+/// raised an error may silently succeed. Moreover, the fix could remove
+/// comments from the original code.
 ///
 /// ## References
 /// - [_How Not to Flatten a List of Lists in Python_](https://mathieularose.com/how-not-to-flatten-a-list-of-lists-in-python)
@@ -61,7 +61,25 @@ use crate::{AlwaysFixableViolation, Edit, Fix};
 /// [microbenchmarks]: https://github.com/astral-sh/ruff/issues/5073#issuecomment-1591836349
 #[derive(ViolationMetadata)]
 #[violation_metadata(stable_since = "v0.0.285")]
-pub(crate) struct QuadraticListSummation;
+pub(crate) struct QuadraticListSummation {
+    fix_style: QuadraticListSummationFixStyle,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum QuadraticListSummationFixStyle {
+    FunctoolsReduce,
+    StarredListComprehension,
+}
+
+impl QuadraticListSummationFixStyle {
+    fn from_target_version(target_version: PythonVersion) -> Self {
+        if target_version >= PythonVersion::PY315 {
+            Self::StarredListComprehension
+        } else {
+            Self::FunctoolsReduce
+        }
+    }
+}
 
 impl AlwaysFixableViolation for QuadraticListSummation {
     #[derive_message_formats]
@@ -70,7 +88,14 @@ impl AlwaysFixableViolation for QuadraticListSummation {
     }
 
     fn fix_title(&self) -> String {
-        "Replace with `functools.reduce`".to_string()
+        match self.fix_style {
+            QuadraticListSummationFixStyle::FunctoolsReduce => {
+                "Replace with `functools.reduce`".to_string()
+            }
+            QuadraticListSummationFixStyle::StarredListComprehension => {
+                "Replace with a starred list comprehension".to_string()
+            }
+        }
     }
 }
 
@@ -97,8 +122,41 @@ pub(crate) fn quadratic_list_summation(checker: &Checker, call: &ast::ExprCall)
         return;
     }
 
-    let mut diagnostic = checker.report_diagnostic(QuadraticListSummation, *range);
-    diagnostic.try_set_fix(|| convert_to_reduce(iterable, call, checker));
+    let fix_style = QuadraticListSummationFixStyle::from_target_version(checker.target_version());
+    let mut diagnostic = checker.report_diagnostic(QuadraticListSummation { fix_style }, *range);
+    diagnostic.try_set_fix(|| convert_to_fix(iterable, call, checker, fix_style));
+}
+
+fn convert_to_fix(
+    iterable: &Expr,
+    call: &ast::ExprCall,
+    checker: &Checker,
+    fix_style: QuadraticListSummationFixStyle,
+) -> Result<Fix> {
+    match fix_style {
+        QuadraticListSummationFixStyle::FunctoolsReduce => {
+            convert_to_reduce(iterable, call, checker)
+        }
+        QuadraticListSummationFixStyle::StarredListComprehension => Ok(
+            convert_to_starred_list_comprehension(iterable, call, checker),
+        ),
+    }
+}
+
+fn convert_to_starred_list_comprehension(
+    iterable: &Expr,
+    call: &ast::ExprCall,
+    checker: &Checker,
+) -> Fix {
+    let iterable = checker.locator().slice(
+        parenthesized_range(iterable.into(), (&call.arguments).into(), checker.tokens())
+            .unwrap_or(iterable.range()),
+    );
+
+    Fix::unsafe_edit(Edit::range_replacement(
+        format!("[*sublist for sublist in {iterable}]"),
+        call.range(),
+    ))
 }
 
 /// Generate a [`Fix`] to convert a `sum()` call to a `functools.reduce()` call.

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__PY315_RUF017_RUF017_0.py.snap

@@ -0,0 +1,143 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF017 [*] Avoid quadratic list summation
+ --> RUF017_0.py:5:1
+  |
+4 | # RUF017
+5 | sum([x, y], start=[])
+  | ^^^^^^^^^^^^^^^^^^^^^
+6 | sum([x, y], [])
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+  |
+help: Replace with a starred list comprehension
+2 | y = [4, 5, 6]
+3 | 
+4 | # RUF017
+  - sum([x, y], start=[])
+5 + [*sublist for sublist in [x, y]]
+6 | sum([x, y], [])
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+8 | sum([[1, 2, 3], [4, 5, 6]], [])
+note: This is an unsafe fix and may change runtime behavior
+
+RUF017 [*] Avoid quadratic list summation
+ --> RUF017_0.py:6:1
+  |
+4 | # RUF017
+5 | sum([x, y], start=[])
+6 | sum([x, y], [])
+  | ^^^^^^^^^^^^^^^
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+8 | sum([[1, 2, 3], [4, 5, 6]], [])
+  |
+help: Replace with a starred list comprehension
+3 | 
+4 | # RUF017
+5 | sum([x, y], start=[])
+  - sum([x, y], [])
+6 + [*sublist for sublist in [x, y]]
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+8 | sum([[1, 2, 3], [4, 5, 6]], [])
+9 | sum([[1, 2, 3], [4, 5, 6]],
+note: This is an unsafe fix and may change runtime behavior
+
+RUF017 [*] Avoid quadratic list summation
+ --> RUF017_0.py:7:1
+  |
+5 | sum([x, y], start=[])
+6 | sum([x, y], [])
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+8 | sum([[1, 2, 3], [4, 5, 6]], [])
+9 | sum([[1, 2, 3], [4, 5, 6]],
+  |
+help: Replace with a starred list comprehension
+4  | # RUF017
+5  | sum([x, y], start=[])
+6  | sum([x, y], [])
+   - sum([[1, 2, 3], [4, 5, 6]], start=[])
+7  + [*sublist for sublist in [[1, 2, 3], [4, 5, 6]]]
+8  | sum([[1, 2, 3], [4, 5, 6]], [])
+9  | sum([[1, 2, 3], [4, 5, 6]],
+10 |     [])
+note: This is an unsafe fix and may change runtime behavior
+
+RUF017 [*] Avoid quadratic list summation
+  --> RUF017_0.py:8:1
+   |
+ 6 | sum([x, y], [])
+ 7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+ 8 | sum([[1, 2, 3], [4, 5, 6]], [])
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ 9 | sum([[1, 2, 3], [4, 5, 6]],
+10 |     [])
+   |
+help: Replace with a starred list comprehension
+5  | sum([x, y], start=[])
+6  | sum([x, y], [])
+7  | sum([[1, 2, 3], [4, 5, 6]], start=[])
+   - sum([[1, 2, 3], [4, 5, 6]], [])
+8  + [*sublist for sublist in [[1, 2, 3], [4, 5, 6]]]
+9  | sum([[1, 2, 3], [4, 5, 6]],
+10 |     [])
+11 | 
+note: This is an unsafe fix and may change runtime behavior
+
+RUF017 [*] Avoid quadratic list summation
+  --> RUF017_0.py:9:1
+   |
+ 7 |   sum([[1, 2, 3], [4, 5, 6]], start=[])
+ 8 |   sum([[1, 2, 3], [4, 5, 6]], [])
+ 9 | / sum([[1, 2, 3], [4, 5, 6]],
+10 | |     [])
+   | |_______^
+11 |
+12 |   # OK
+   |
+help: Replace with a starred list comprehension
+6  | sum([x, y], [])
+7  | sum([[1, 2, 3], [4, 5, 6]], start=[])
+8  | sum([[1, 2, 3], [4, 5, 6]], [])
+   - sum([[1, 2, 3], [4, 5, 6]],
+   -     [])
+9  + [*sublist for sublist in [[1, 2, 3], [4, 5, 6]]]
+10 | 
+11 | # OK
+12 | sum([x, y])
+note: This is an unsafe fix and may change runtime behavior
+
+RUF017 [*] Avoid quadratic list summation
+  --> RUF017_0.py:21:5
+   |
+19 |     import functools, operator
+20 |
+21 |     sum([x, y], [])
+   |     ^^^^^^^^^^^^^^^
+   |
+help: Replace with a starred list comprehension
+18 | def func():
+19 |     import functools, operator
+20 | 
+   -     sum([x, y], [])
+21 +     [*sublist for sublist in [x, y]]
+22 | 
+23 | 
+24 | # Regression test for: https://github.com/astral-sh/ruff/issues/7718
+note: This is an unsafe fix and may change runtime behavior
+
+RUF017 [*] Avoid quadratic list summation
+  --> RUF017_0.py:26:5
+   |
+24 | # Regression test for: https://github.com/astral-sh/ruff/issues/7718
+25 | def func():
+26 |     sum((factor.dims for factor in bases), [])
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: Replace with a starred list comprehension
+23 | 
+24 | # Regression test for: https://github.com/astral-sh/ruff/issues/7718
+25 | def func():
+   -     sum((factor.dims for factor in bases), [])
+26 +     [*sublist for sublist in (factor.dims for factor in bases)]
+note: This is an unsafe fix and may change runtime behavior

scripts/check_docs_formatted.py

@@ -118,6 +118,7 @@
     "mixed-spaces-and-tabs",
     "no-indented-block",
     "non-pep695-type-alias",  # requires Python 3.12
+    "quadratic-list-summation",  # requires Python 3.15
     "syntax-error",
     "tab-after-comma",
     "tab-after-keyword",