Update tests

crates/ruff/resources/test/fixtures/ruff/RUF017.py

@@ -0,0 +1,14 @@
+x = [1, 2, 3]
+y = [4, 5, 6]
+
+# RUF017
+sum([x, y], start=[])
+sum([x, y], [])
+sum([[1, 2, 3], [4, 5, 6]], start=[])
+sum([[1, 2, 3], [4, 5, 6]], [])
+sum([[1, 2, 3], [4, 5, 6]],
+    [])
+
+# OK
+sum([x, y])
+sum([[1, 2, 3], [4, 5, 6]])

crates/ruff/src/checkers/ast/analyze/expression.rs

@@ -873,6 +873,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::UnsupportedMethodCallOnAll) {
                 flake8_pyi::rules::unsupported_method_call_on_all(checker, func);
             }
+            if checker.enabled(Rule::QuadraticListSummation) {
+                ruff::rules::quadratic_list_summation(checker, call);
+            }
         }
         Expr::Dict(ast::ExprDict {
             keys,

crates/ruff/src/codes.rs

@@ -816,6 +816,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "014") => (RuleGroup::Nursery, rules::ruff::rules::UnreachableCode),
         (Ruff, "015") => (RuleGroup::Unspecified, rules::ruff::rules::UnnecessaryIterableAllocationForFirstElement),
         (Ruff, "016") => (RuleGroup::Unspecified, rules::ruff::rules::InvalidIndexType),
+        (Ruff, "017") => (RuleGroup::Nursery, rules::ruff::rules::QuadraticListSummation),
         (Ruff, "100") => (RuleGroup::Unspecified, rules::ruff::rules::UnusedNOQA),
         (Ruff, "200") => (RuleGroup::Unspecified, rules::ruff::rules::InvalidPyprojectToml),
 

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

@@ -40,6 +40,7 @@ mod tests {
         feature = "unreachable-code",
         test_case(Rule::UnreachableCode, Path::new("RUF014.py"))
     )]
+    #[test_case(Rule::QuadraticListSummation, Path::new("RUF017.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(

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

@@ -40,3 +40,6 @@ pub(crate) enum Context {
     Docstring,
     Comment,
 }
+pub(crate) use quadratic_list_summation::*;
+
+mod quadratic_list_summation;

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

@@ -0,0 +1,133 @@
+use anyhow::Result;
+
+use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Edit, Fix};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::{self as ast, Arguments, Expr, Ranged};
+use ruff_python_semantic::SemanticModel;
+
+use crate::importer::ImportRequest;
+use crate::{checkers::ast::Checker, registry::Rule};
+
+/// ## What it does
+/// Checks for the use of `sum()` to flatten lists of lists, which has
+/// quadratic complexity.
+///
+/// ## Why is this bad?
+/// The use of `sum()` to flatten lists of lists is quadratic in the number of
+/// lists, as `sum()` creates a new list for each element in the summation.
+///
+/// Instead, consider using another method of flattening lists to avoid
+/// quadratic complexity. The following methods are all linear in the number of
+/// lists:
+///
+/// - `functools.reduce(operator.iconcat, lists, [])`
+/// - `list(itertools.chain.from_iterable(lists)`
+/// - `[item for sublist in lists for item in sublist]`
+///
+/// ## Example
+/// ```python
+/// lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
+/// joined = sum(lists, [])
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import functools
+/// import operator
+///
+///
+/// lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
+/// functools.reduce(operator.iconcat, lists, [])
+/// ```
+///
+/// ## References
+/// - [_How Not to Flatten a List of Lists in Python_](https://mathieularose.com/how-not-to-flatten-a-list-of-lists-in-python)
+/// - [_How do I make a flat list out of a list of lists?_](https://stackoverflow.com/questions/952914/how-do-i-make-a-flat-list-out-of-a-list-of-lists/953097#953097)
+#[violation]
+pub struct QuadraticListSummation;
+
+impl AlwaysAutofixableViolation for QuadraticListSummation {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Avoid quadratic list summation")
+    }
+
+    fn autofix_title(&self) -> String {
+        format!("Replace with `functools.reduce`")
+    }
+}
+
+/// RUF017
+pub(crate) fn quadratic_list_summation(checker: &mut Checker, call: &ast::ExprCall) {
+    let ast::ExprCall {
+        func,
+        arguments,
+        range,
+    } = call;
+
+    if !func_is_builtin(func, "sum", checker.semantic()) {
+        return;
+    }
+
+    if !start_is_empty_list(arguments, checker.semantic()) {
+        return;
+    };
+
+    let Some(iterable) = arguments.args.first() else {
+        return;
+    };
+
+    let mut diagnostic = Diagnostic::new(QuadraticListSummation, *range);
+    if checker.patch(Rule::QuadraticListSummation) {
+        diagnostic.try_set_fix(|| convert_to_reduce(iterable, call, checker));
+    }
+    checker.diagnostics.push(diagnostic);
+}
+
+/// Generate a [`Fix`] to convert a `sum()` call to a `functools.reduce()` call.
+fn convert_to_reduce(iterable: &Expr, call: &ast::ExprCall, checker: &Checker) -> Result<Fix> {
+    let (reduce_edit, reduce_binding) = checker.importer().get_or_import_symbol(
+        &ImportRequest::import("functools", "reduce"),
+        call.start(),
+        checker.semantic(),
+    )?;
+
+    let (iadd_edit, iadd_binding) = checker.importer().get_or_import_symbol(
+        &ImportRequest::import("operator", "iadd"),
+        iterable.start(),
+        checker.semantic(),
+    )?;
+
+    let iterable = checker.locator().slice(iterable.range());
+
+    Ok(Fix::suggested_edits(
+        Edit::range_replacement(
+            format!("{reduce_binding}({iadd_binding}, {iterable}, [])"),
+            call.range(),
+        ),
+        [reduce_edit, iadd_edit],
+    ))
+}
+
+/// Check if a function is a builtin with a given name.
+fn func_is_builtin(func: &Expr, name: &str, semantic: &SemanticModel) -> bool {
+    let Expr::Name(ast::ExprName { id, .. }) = func else {
+        return false;
+    };
+    id == name && semantic.is_builtin(id)
+}
+
+/// Returns `true` if the `start` argument to a `sum()` call is an empty list.
+fn start_is_empty_list(arguments: &Arguments, semantic: &SemanticModel) -> bool {
+    let Some(keyword) = arguments.find_keyword("start") else {
+        return false;
+    };
+
+    match &keyword.value {
+        Expr::Call(ast::ExprCall {
+            func, arguments, ..
+        }) => arguments.is_empty() && func_is_builtin(func, "list", semantic),
+        Expr::List(ast::ExprList { elts, ctx, .. }) => elts.is_empty() && ctx.is_load(),
+        _ => false,
+    }
+}

crates/ruff/src/rules/ruff/snapshots/ruff__rules__ruff__tests__RUF017_RUF017.py.snap

@@ -0,0 +1,53 @@
+---
+source: crates/ruff/src/rules/ruff/mod.rs
+---
+RUF017.py:5:1: RUF017 [*] Avoid quadratic list summation
+  |
+4 | # RUF017
+5 | sum([x, y], start=[])
+  | ^^^^^^^^^^^^^^^^^^^^^ RUF017
+6 | sum([x, y], [])
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+  |
+  = help: Replace with `functools.reduce`
+
+ℹ Suggested fix
+  1 |+import functools
+  2 |+import operator
+1 3 | x = [1, 2, 3]
+2 4 | y = [4, 5, 6]
+3 5 | 
+4 6 | # RUF017
+5   |-sum([x, y], start=[])
+  7 |+functools.reduce(operator.iadd, [x, y], [])
+6 8 | sum([x, y], [])
+7 9 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+8 10 | sum([[1, 2, 3], [4, 5, 6]], [])
+
+RUF017.py:7:1: RUF017 [*] Avoid quadratic list summation
+  |
+5 | sum([x, y], start=[])
+6 | sum([x, y], [])
+7 | sum([[1, 2, 3], [4, 5, 6]], start=[])
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ RUF017
+8 | sum([[1, 2, 3], [4, 5, 6]], [])
+9 | sum([[1, 2, 3], [4, 5, 6]],
+  |
+  = help: Replace with `functools.reduce`
+
+ℹ Suggested fix
+   1  |+import functools
+   2  |+import operator
+1  3  | x = [1, 2, 3]
+2  4  | y = [4, 5, 6]
+3  5  | 
+4  6  | # RUF017
+5  7  | sum([x, y], start=[])
+6  8  | sum([x, y], [])
+7     |-sum([[1, 2, 3], [4, 5, 6]], start=[])
+   9  |+functools.reduce(operator.iadd, [[1, 2, 3], [4, 5, 6]], [])
+8  10 | sum([[1, 2, 3], [4, 5, 6]], [])
+9  11 | sum([[1, 2, 3], [4, 5, 6]],
+10 12 |     [])
+
+

ruff.schema.json

@@ -2471,6 +2471,7 @@
         "RUF013",
         "RUF015",
         "RUF016",
+        "RUF017",
         "RUF1",
         "RUF10",
         "RUF100",