[pylint] - add unnecessary-list-index-lookup (PLR1736) + autofix

crates/ruff_linter/resources/test/fixtures/pylint/unnecessary_list_index_lookup.py

@@ -0,0 +1,30 @@
+letters = ["a", "b", "c"]
+
+
+def fix_these():
+    [letters[index] for index, letter in enumerate(letters)]  # PLR1736
+    {letters[index] for index, letter in enumerate(letters)}  # PLR1736
+    {letter: letters[index] for index, letter in enumerate(letters)}  # PLR1736
+
+    for index, letter in enumerate(letters):
+        print(letters[index])  # PLR1736
+        blah = letters[index]  # PLR1736
+        assert letters[index]  == "d"  # PLR1736
+
+
+def dont_fix_these():
+    # once there is an assignment to the sequence[index], we stop emitting diagnostics
+    for index, letter in enumerate(letters):
+        letters[index] = "d"  # Ok
+        assert letters[index] == "d"  # Ok
+
+
+def value_intentionally_unused():
+    [letters[index] for index, _ in enumerate(letters)]  # PLR1736
+    {letters[index] for index, _ in enumerate(letters)}  # PLR1736
+    {index: letters[index] for index, _ in enumerate(letters)}  # PLR1736
+
+    for index, _ in enumerate(letters):
+        print(letters[index])  # Ok
+        blah = letters[index]  # Ok
+        letters[index] = "d"  # Ok

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

@@ -1317,6 +1317,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 range: _,
             },
         ) => {
+            if checker.enabled(Rule::UnnecessaryListIndexLookup) {
+                pylint::rules::unnecessary_list_index_lookup_comprehension(checker, expr);
+            }
             if checker.enabled(Rule::UnnecessaryComprehension) {
                 flake8_comprehensions::rules::unnecessary_list_set_comprehension(
                     checker, expr, elt, generators,
@@ -1341,6 +1344,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 range: _,
             },
         ) => {
+            if checker.enabled(Rule::UnnecessaryListIndexLookup) {
+                pylint::rules::unnecessary_list_index_lookup_comprehension(checker, expr);
+            }
             if checker.enabled(Rule::UnnecessaryComprehension) {
                 flake8_comprehensions::rules::unnecessary_list_set_comprehension(
                     checker, expr, elt, generators,
@@ -1364,6 +1370,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             generators,
             range: _,
         }) => {
+            if checker.enabled(Rule::UnnecessaryListIndexLookup) {
+                pylint::rules::unnecessary_list_index_lookup_comprehension(checker, expr);
+            }
             if checker.enabled(Rule::UnnecessaryComprehension) {
                 flake8_comprehensions::rules::unnecessary_dict_comprehension(
                     checker, expr, key, value, generators,
@@ -1388,6 +1397,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 range: _,
             },
         ) => {
+            if checker.enabled(Rule::UnnecessaryListIndexLookup) {
+                pylint::rules::unnecessary_list_index_lookup_comprehension(checker, expr);
+            }
             if checker.enabled(Rule::FunctionUsesLoopVariable) {
                 flake8_bugbear::rules::function_uses_loop_variable(checker, &Node::Expr(expr));
             }

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -1271,6 +1271,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::UnnecessaryListCast) {
                 perflint::rules::unnecessary_list_cast(checker, iter, body);
             }
+            if checker.enabled(Rule::UnnecessaryListIndexLookup) {
+                pylint::rules::unnecessary_list_index_lookup(checker, for_stmt);
+            }
             if !is_async {
                 if checker.enabled(Rule::ReimplementedBuiltin) {
                     flake8_simplify::rules::convert_for_loop_to_any_all(checker, stmt);

crates/ruff_linter/src/codes.rs

@@ -258,6 +258,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "R1714") => (RuleGroup::Stable, rules::pylint::rules::RepeatedEqualityComparison),
         (Pylint, "R1706") => (RuleGroup::Preview, rules::pylint::rules::AndOrTernary),
         (Pylint, "R1722") => (RuleGroup::Stable, rules::pylint::rules::SysExitAlias),
+        (Pylint, "R1736") => (RuleGroup::Preview, rules::pylint::rules::UnnecessaryListIndexLookup),
         (Pylint, "R2004") => (RuleGroup::Stable, rules::pylint::rules::MagicValueComparison),
         (Pylint, "R5501") => (RuleGroup::Stable, rules::pylint::rules::CollapsibleElseIf),
         (Pylint, "R6201") => (RuleGroup::Preview, rules::pylint::rules::LiteralMembership),

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

@@ -154,6 +154,10 @@ mod tests {
         Rule::RepeatedKeywordArgument,
         Path::new("repeated_keyword_argument.py")
     )]
+    #[test_case(
+        Rule::UnnecessaryListIndexLookup,
+        Path::new("unnecessary_list_index_lookup.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_linter/src/rules/pylint/rules/mod.rs

@@ -63,6 +63,7 @@ pub(crate) use type_param_name_mismatch::*;
 pub(crate) use unexpected_special_method_signature::*;
 pub(crate) use unnecessary_direct_lambda_call::*;
 pub(crate) use unnecessary_lambda::*;
+pub(crate) use unnecessary_list_index_lookup::*;
 pub(crate) use unspecified_encoding::*;
 pub(crate) use useless_else_on_loop::*;
 pub(crate) use useless_import_alias::*;
@@ -136,6 +137,7 @@ mod type_param_name_mismatch;
 mod unexpected_special_method_signature;
 mod unnecessary_direct_lambda_call;
 mod unnecessary_lambda;
+mod unnecessary_list_index_lookup;
 mod unspecified_encoding;
 mod useless_else_on_loop;
 mod useless_import_alias;

crates/ruff_linter/src/rules/pylint/rules/unnecessary_list_index_lookup.rs

@@ -0,0 +1,254 @@
+use ruff_python_ast::{self as ast, Expr, Stmt, StmtFor};
+
+use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::visitor;
+use ruff_python_ast::visitor::Visitor;
+use ruff_text_size::TextRange;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for uses of enumeration and accessing the value by index lookup.
+///
+/// ## Why is this bad?
+/// The value is already accessible by the 2nd variable from the enumeration.
+///
+/// ## Example
+/// ```python
+/// letters = ["a", "b", "c"]
+///
+/// for index, letter in enumerate(letters):
+///     print(letters[index])
+/// ```
+///
+/// Use instead:
+/// ```python
+/// letters = ["a", "b", "c"]
+///
+/// for index, letter in enumerate(letters):
+///     print(letter)
+/// ```
+#[violation]
+pub struct UnnecessaryListIndexLookup;
+
+impl AlwaysFixableViolation for UnnecessaryListIndexLookup {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("Unnecessary list index lookup")
+    }
+
+    fn fix_title(&self) -> String {
+        format!("Remove unnecessary list index lookup")
+    }
+}
+
+struct SubscriptVisitor<'a> {
+    sequence_name: &'a str,
+    index_name: &'a str,
+    diagnostic_ranges: Vec<TextRange>,
+    is_subcript_modified: bool,
+}
+
+impl<'a> SubscriptVisitor<'a> {
+    fn new(sequence_name: &'a str, index_name: &'a str) -> Self {
+        Self {
+            sequence_name,
+            index_name,
+            diagnostic_ranges: Vec::new(),
+            is_subcript_modified: false,
+        }
+    }
+}
+
+fn check_target_for_assignment(expr: &Expr, sequence_name: &str, index_name: &str) -> bool {
+    // if we see the sequence subscript being modified, we'll stop emitting diagnostics
+    match expr {
+        Expr::Subscript(ast::ExprSubscript { value, slice, .. }) => {
+            if let Expr::Name(ast::ExprName { id, .. }) = value.as_ref() {
+                if id == sequence_name {
+                    if let Expr::Name(ast::ExprName { id, .. }) = slice.as_ref() {
+                        if id == index_name {
+                            return true;
+                        }
+                    }
+                }
+            }
+            false
+        }
+        _ => false,
+    }
+}
+
+impl<'a> Visitor<'_> for SubscriptVisitor<'a> {
+    fn visit_expr(&mut self, expr: &Expr) {
+        if self.is_subcript_modified {
+            return;
+        }
+        match expr {
+            Expr::Subscript(ast::ExprSubscript {
+                value,
+                slice,
+                range,
+                ..
+            }) => {
+                if let Expr::Name(ast::ExprName { id, .. }) = value.as_ref() {
+                    if id == self.sequence_name {
+                        if let Expr::Name(ast::ExprName { id, .. }) = slice.as_ref() {
+                            if id == self.index_name {
+                                self.diagnostic_ranges.push(*range);
+                            }
+                        }
+                    }
+                }
+            }
+            _ => visitor::walk_expr(self, expr),
+        }
+    }
+
+    fn visit_stmt(&mut self, stmt: &Stmt) {
+        if self.is_subcript_modified {
+            return;
+        }
+        match stmt {
+            Stmt::Assign(ast::StmtAssign { targets, value, .. }) => {
+                self.is_subcript_modified = targets.iter().any(|target| {
+                    check_target_for_assignment(target, self.sequence_name, self.index_name)
+                });
+                self.visit_expr(value);
+            }
+            Stmt::AnnAssign(ast::StmtAnnAssign { target, value, .. }) => {
+                if let Some(value) = value {
+                    self.is_subcript_modified =
+                        check_target_for_assignment(target, self.sequence_name, self.index_name);
+                    self.visit_expr(value);
+                }
+            }
+            Stmt::AugAssign(ast::StmtAugAssign { target, value, .. }) => {
+                self.is_subcript_modified =
+                    check_target_for_assignment(target, self.sequence_name, self.index_name);
+                self.visit_expr(value);
+            }
+            _ => visitor::walk_stmt(self, stmt),
+        }
+    }
+}
+
+/// PLR1736
+pub(crate) fn unnecessary_list_index_lookup(checker: &mut Checker, stmt_for: &StmtFor) {
+    let Some((sequence, index_name, value_name)) =
+        enumerate_items(checker, &stmt_for.iter, &stmt_for.target)
+    else {
+        return;
+    };
+
+    let mut visitor = SubscriptVisitor::new(&sequence, &index_name);
+
+    visitor.visit_body(&stmt_for.body);
+    visitor.visit_body(&stmt_for.orelse);
+
+    for range in visitor.diagnostic_ranges {
+        let mut diagnostic = Diagnostic::new(UnnecessaryListIndexLookup, range);
+
+        diagnostic.set_fix(Fix::safe_edit(Edit::range_replacement(
+            value_name.clone(),
+            range,
+        )));
+
+        checker.diagnostics.push(diagnostic);
+    }
+}
+
+/// PLR1736
+pub(crate) fn unnecessary_list_index_lookup_comprehension(checker: &mut Checker, expr: &Expr) {
+    match expr {
+        Expr::GeneratorExp(ast::ExprGeneratorExp {
+            elt, generators, ..
+        })
+        | Expr::DictComp(ast::ExprDictComp {
+            value: elt,
+            generators,
+            ..
+        })
+        | Expr::SetComp(ast::ExprSetComp {
+            elt, generators, ..
+        })
+        | Expr::ListComp(ast::ExprListComp {
+            elt, generators, ..
+        }) => {
+            for comp in generators {
+                if let Some((sequence, index_name, value_name)) =
+                    enumerate_items(checker, &comp.iter, &comp.target)
+                {
+                    let mut visitor = SubscriptVisitor::new(&sequence, &index_name);
+
+                    visitor.visit_expr(elt.as_ref());
+
+                    for range in visitor.diagnostic_ranges {
+                        let mut diagnostic = Diagnostic::new(UnnecessaryListIndexLookup, range);
+
+                        diagnostic.set_fix(Fix::safe_edit(Edit::range_replacement(
+                            value_name.clone(),
+                            range,
+                        )));
+
+                        checker.diagnostics.push(diagnostic);
+                    }
+                }
+            }
+        }
+        _ => (),
+    }
+}
+
+fn enumerate_items(
+    checker: &mut Checker,
+    call_expr: &Expr,
+    tuple_expr: &Expr,
+) -> Option<(String, String, String)> {
+    let Expr::Call(ast::ExprCall {
+        func, arguments, ..
+    }) = call_expr
+    else {
+        return None;
+    };
+
+    // Check that the function is the `enumerate` builtin.
+    let Expr::Name(ast::ExprName { id, .. }) = func.as_ref() else {
+        return None;
+    };
+    if id != "enumerate" {
+        return None;
+    };
+    if !checker.semantic().is_builtin("enumerate") {
+        return None;
+    };
+
+    let Expr::Tuple(ast::ExprTuple { elts, .. }) = tuple_expr else {
+        return None;
+    };
+    let [index, value] = elts.as_slice() else {
+        return None;
+    };
+
+    // Grab the variable names
+    let Expr::Name(ast::ExprName { id: index_name, .. }) = index else {
+        return None;
+    };
+
+    let Expr::Name(ast::ExprName { id: value_name, .. }) = value else {
+        return None;
+    };
+
+    // If either of the variable names are intentionally ignored by naming them `_`, then don't emit
+    if index_name == "_" || value_name == "_" {
+        return None;
+    }
+
+    // Get the first argument of the enumerate call
+    let Some(Expr::Name(ast::ExprName { id: sequence, .. })) = arguments.args.first() else {
+        return None;
+    };
+
+    Some((sequence.clone(), index_name.clone(), value_name.clone()))
+}