[ruff] Implement check for Decimal called with a float literal (RUF032) (#12909)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_linter/resources/test/fixtures/ruff/RUF032.py

@@ -0,0 +1,120 @@
+import decimal
+
+# Tests with fully qualified import
+decimal.Decimal(0)
+
+decimal.Decimal(0.0)  # Should error
+
+decimal.Decimal("0.0")
+
+decimal.Decimal(10)
+
+decimal.Decimal(10.0)  # Should error
+
+decimal.Decimal("10.0")
+
+decimal.Decimal(-10)
+
+decimal.Decimal(-10.0)  # Should error
+
+decimal.Decimal("-10.0")
+
+a = 10.0
+
+decimal.Decimal(a)
+
+
+# Tests with relative import
+from decimal import Decimal
+
+
+val = Decimal(0)
+
+val = Decimal(0.0)  # Should error
+
+val = Decimal("0.0")
+
+val = Decimal(10)
+
+val = Decimal(10.0)  # Should error
+
+val = Decimal("10.0")
+
+val = Decimal(-10)
+
+val = Decimal(-10.0)  # Should error
+
+val = Decimal("-10.0")
+
+a = 10.0
+
+val = Decimal(a)
+
+
+# Tests with shadowed name
+class Decimal():
+    value: float | int | str
+
+    def __init__(self, value: float | int | str) -> None:
+        self.value = value
+
+
+val = Decimal(0.0)
+
+val = Decimal("0.0")
+
+val = Decimal(10.0)
+
+val = Decimal("10.0")
+
+val = Decimal(-10.0)
+
+val = Decimal("-10.0")
+
+a = 10.0
+
+val = Decimal(a)
+
+
+# Retest with fully qualified import
+
+val = decimal.Decimal(0.0)  # Should error
+
+val = decimal.Decimal("0.0")
+
+val = decimal.Decimal(10.0)  # Should error
+
+val = decimal.Decimal("10.0")
+
+val = decimal.Decimal(-10.0)  # Should error
+
+val = decimal.Decimal("-10.0")
+
+a = 10.0
+
+val = decimal.Decimal(a)
+
+
+class decimal():
+    class Decimal():
+        value: float | int | str
+
+        def __init__(self, value: float | int | str) -> None:
+            self.value = value
+
+
+val = decimal.Decimal(0.0)
+
+val = decimal.Decimal("0.0")
+
+val = decimal.Decimal(10.0)
+
+val = decimal.Decimal("10.0")
+
+val = decimal.Decimal(-10.0)
+
+val = decimal.Decimal("-10.0")
+
+a = 10.0
+
+val = decimal.Decimal(a)

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

@@ -1011,6 +1011,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::UnnecessaryIterableAllocationForFirstElement) {
                 ruff::rules::unnecessary_iterable_allocation_for_first_element(checker, expr);
             }
+            if checker.enabled(Rule::DecimalFromFloatLiteral) {
+                ruff::rules::decimal_from_float_literal_syntax(checker, call);
+            }
             if checker.enabled(Rule::IntOnSlicedStr) {
                 refurb::rules::int_on_sliced_str(checker, call);
             }

crates/ruff_linter/src/codes.rs

@@ -959,6 +959,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "029") => (RuleGroup::Preview, rules::ruff::rules::UnusedAsync),
         (Ruff, "030") => (RuleGroup::Preview, rules::ruff::rules::AssertWithPrintMessage),
         (Ruff, "031") => (RuleGroup::Preview, rules::ruff::rules::IncorrectlyParenthesizedTupleInSubscript),
+        (Ruff, "032") => (RuleGroup::Preview, rules::ruff::rules::DecimalFromFloatLiteral),
         (Ruff, "100") => (RuleGroup::Stable, rules::ruff::rules::UnusedNOQA),
         (Ruff, "101") => (RuleGroup::Stable, rules::ruff::rules::RedirectedNOQA),
 

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

@@ -57,6 +57,7 @@ mod tests {
     #[test_case(Rule::UnusedAsync, Path::new("RUF029.py"))]
     #[test_case(Rule::AssertWithPrintMessage, Path::new("RUF030.py"))]
     #[test_case(Rule::IncorrectlyParenthesizedTupleInSubscript, Path::new("RUF031.py"))]
+    #[test_case(Rule::DecimalFromFloatLiteral, Path::new("RUF032.py"))]
     #[test_case(Rule::RedirectedNOQA, Path::new("RUF101.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());

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

@@ -0,0 +1,85 @@
+use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::{self as ast};
+use ruff_python_codegen::Stylist;
+use ruff_text_size::{Ranged, TextRange};
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for `Decimal` calls passing a float literal.
+///
+/// ## Why is this bad?
+/// Float literals have limited precision that can lead to unexpected results.
+/// The `Decimal` class is designed to handle numbers with fixed-point precision,
+/// so a string literal should be used instead.
+///
+/// ## Example
+///
+/// ```python
+/// num = Decimal(1.2345)
+/// ```
+///
+/// Use instead:
+/// ```python
+/// num = Decimal("1.2345")
+/// ```
+///
+/// ## Fix Safety
+/// This rule's fix is marked as unsafe because it changes the underlying value
+/// of the `Decimal` instance that is constructed. This can lead to unexpected
+/// behavior if your program relies on the previous value (whether deliberately or not).
+#[violation]
+pub struct DecimalFromFloatLiteral;
+
+impl AlwaysFixableViolation for DecimalFromFloatLiteral {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("`Decimal()` called with float literal argument")
+    }
+
+    fn fix_title(&self) -> String {
+        "Use a string literal instead".to_string()
+    }
+}
+
+/// RUF032: `Decimal()` called with float literal argument
+pub(crate) fn decimal_from_float_literal_syntax(checker: &mut Checker, call: &ast::ExprCall) {
+    let Some(arg) = call.arguments.args.first() else {
+        return;
+    };
+
+    if !is_arg_float_literal(arg) {
+        return;
+    }
+
+    if checker
+        .semantic()
+        .resolve_qualified_name(call.func.as_ref())
+        .is_some_and(|qualified_name| matches!(qualified_name.segments(), ["decimal", "Decimal"]))
+    {
+        let diagnostic =
+            Diagnostic::new(DecimalFromFloatLiteral, arg.range()).with_fix(fix_float_literal(
+                arg.range(),
+                &checker.generator().expr(arg),
+                checker.stylist(),
+            ));
+        checker.diagnostics.push(diagnostic);
+    }
+}
+
+fn is_arg_float_literal(arg: &ast::Expr) -> bool {
+    match arg {
+        ast::Expr::NumberLiteral(ast::ExprNumberLiteral {
+            value: ast::Number::Float(_),
+            ..
+        }) => true,
+        ast::Expr::UnaryOp(ast::ExprUnaryOp { operand, .. }) => is_arg_float_literal(operand),
+        _ => false,
+    }
+}
+
+fn fix_float_literal(range: TextRange, float_literal: &str, stylist: &Stylist) -> Fix {
+    let content = format!("{quote}{float_literal}{quote}", quote = stylist.quote());
+    Fix::unsafe_edit(Edit::range_replacement(content, range))
+}

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

@@ -3,6 +3,7 @@ pub(crate) use assert_with_print_message::*;
 pub(crate) use assignment_in_assert::*;
 pub(crate) use asyncio_dangling_task::*;
 pub(crate) use collection_literal_concatenation::*;
+pub(crate) use decimal_from_float_literal::*;
 pub(crate) use default_factory_kwarg::*;
 pub(crate) use explicit_f_string_type_conversion::*;
 pub(crate) use function_call_in_dataclass_default::*;
@@ -36,6 +37,7 @@ mod assignment_in_assert;
 mod asyncio_dangling_task;
 mod collection_literal_concatenation;
 mod confusables;
+mod decimal_from_float_literal;
 mod default_factory_kwarg;
 mod explicit_f_string_type_conversion;
 mod function_call_in_dataclass_default;

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF032_RUF032.py.snap

@@ -0,0 +1,191 @@
+---
+source: crates/ruff_linter/src/rules/ruff/mod.rs
+---
+RUF032.py:6:17: RUF032 [*] `Decimal()` called with float literal argument
+  |
+4 | decimal.Decimal(0)
+5 | 
+6 | decimal.Decimal(0.0)  # Should error
+  |                 ^^^ RUF032
+7 | 
+8 | decimal.Decimal("0.0")
+  |
+  = help: Use a string literal instead
+
+ℹ Unsafe fix
+3 3 | # Tests with fully qualified import
+4 4 | decimal.Decimal(0)
+5 5 | 
+6   |-decimal.Decimal(0.0)  # Should error
+  6 |+decimal.Decimal("0.0")  # Should error
+7 7 | 
+8 8 | decimal.Decimal("0.0")
+9 9 | 
+
+RUF032.py:12:17: RUF032 [*] `Decimal()` called with float literal argument
+   |
+10 | decimal.Decimal(10)
+11 | 
+12 | decimal.Decimal(10.0)  # Should error
+   |                 ^^^^ RUF032
+13 | 
+14 | decimal.Decimal("10.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+9  9  | 
+10 10 | decimal.Decimal(10)
+11 11 | 
+12    |-decimal.Decimal(10.0)  # Should error
+   12 |+decimal.Decimal("10.0")  # Should error
+13 13 | 
+14 14 | decimal.Decimal("10.0")
+15 15 | 
+
+RUF032.py:18:17: RUF032 [*] `Decimal()` called with float literal argument
+   |
+16 | decimal.Decimal(-10)
+17 | 
+18 | decimal.Decimal(-10.0)  # Should error
+   |                 ^^^^^ RUF032
+19 | 
+20 | decimal.Decimal("-10.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+15 15 | 
+16 16 | decimal.Decimal(-10)
+17 17 | 
+18    |-decimal.Decimal(-10.0)  # Should error
+   18 |+decimal.Decimal("-10.0")  # Should error
+19 19 | 
+20 20 | decimal.Decimal("-10.0")
+21 21 | 
+
+RUF032.py:33:15: RUF032 [*] `Decimal()` called with float literal argument
+   |
+31 | val = Decimal(0)
+32 | 
+33 | val = Decimal(0.0)  # Should error
+   |               ^^^ RUF032
+34 | 
+35 | val = Decimal("0.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+30 30 | 
+31 31 | val = Decimal(0)
+32 32 | 
+33    |-val = Decimal(0.0)  # Should error
+   33 |+val = Decimal("0.0")  # Should error
+34 34 | 
+35 35 | val = Decimal("0.0")
+36 36 | 
+
+RUF032.py:39:15: RUF032 [*] `Decimal()` called with float literal argument
+   |
+37 | val = Decimal(10)
+38 | 
+39 | val = Decimal(10.0)  # Should error
+   |               ^^^^ RUF032
+40 | 
+41 | val = Decimal("10.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+36 36 | 
+37 37 | val = Decimal(10)
+38 38 | 
+39    |-val = Decimal(10.0)  # Should error
+   39 |+val = Decimal("10.0")  # Should error
+40 40 | 
+41 41 | val = Decimal("10.0")
+42 42 | 
+
+RUF032.py:45:15: RUF032 [*] `Decimal()` called with float literal argument
+   |
+43 | val = Decimal(-10)
+44 | 
+45 | val = Decimal(-10.0)  # Should error
+   |               ^^^^^ RUF032
+46 | 
+47 | val = Decimal("-10.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+42 42 | 
+43 43 | val = Decimal(-10)
+44 44 | 
+45    |-val = Decimal(-10.0)  # Should error
+   45 |+val = Decimal("-10.0")  # Should error
+46 46 | 
+47 47 | val = Decimal("-10.0")
+48 48 | 
+
+RUF032.py:81:23: RUF032 [*] `Decimal()` called with float literal argument
+   |
+79 | # Retest with fully qualified import
+80 | 
+81 | val = decimal.Decimal(0.0)  # Should error
+   |                       ^^^ RUF032
+82 | 
+83 | val = decimal.Decimal("0.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+78 78 | 
+79 79 | # Retest with fully qualified import
+80 80 | 
+81    |-val = decimal.Decimal(0.0)  # Should error
+   81 |+val = decimal.Decimal("0.0")  # Should error
+82 82 | 
+83 83 | val = decimal.Decimal("0.0")
+84 84 | 
+
+RUF032.py:85:23: RUF032 [*] `Decimal()` called with float literal argument
+   |
+83 | val = decimal.Decimal("0.0")
+84 | 
+85 | val = decimal.Decimal(10.0)  # Should error
+   |                       ^^^^ RUF032
+86 | 
+87 | val = decimal.Decimal("10.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+82 82 | 
+83 83 | val = decimal.Decimal("0.0")
+84 84 | 
+85    |-val = decimal.Decimal(10.0)  # Should error
+   85 |+val = decimal.Decimal("10.0")  # Should error
+86 86 | 
+87 87 | val = decimal.Decimal("10.0")
+88 88 | 
+
+RUF032.py:89:23: RUF032 [*] `Decimal()` called with float literal argument
+   |
+87 | val = decimal.Decimal("10.0")
+88 | 
+89 | val = decimal.Decimal(-10.0)  # Should error
+   |                       ^^^^^ RUF032
+90 | 
+91 | val = decimal.Decimal("-10.0")
+   |
+   = help: Use a string literal instead
+
+ℹ Unsafe fix
+86 86 | 
+87 87 | val = decimal.Decimal("10.0")
+88 88 | 
+89    |-val = decimal.Decimal(-10.0)  # Should error
+   89 |+val = decimal.Decimal("-10.0")  # Should error
+90 90 | 
+91 91 | val = decimal.Decimal("-10.0")
+92 92 |

ruff.schema.json

@@ -3738,6 +3738,7 @@
         "RUF03",
         "RUF030",
         "RUF031",
+        "RUF032",
         "RUF1",
         "RUF10",
         "RUF100",