[pylint] Implement nan-comparison (PLW0117)

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

@@ -0,0 +1,76 @@
+import math
+from math import nan as bad_val
+import numpy as np
+from numpy import nan as npy_nan
+
+
+x = float("nan")
+y = np.NaN
+
+# PLW0117
+if x == float("nan"):
+    pass
+
+# PLW0117
+if x == float("NaN"):
+    pass
+
+# PLW0117
+if x == float("NAN"):
+    pass
+
+# PLW0117
+if x == float("Nan"):
+    pass
+
+# PLW0117
+if x == math.nan:
+    pass
+
+# PLW0117
+if x == bad_val:
+    pass
+
+# PLW0117
+if y == np.NaN:
+    pass
+
+# PLW0117
+if y == np.NAN:
+    pass
+
+# PLW0117
+if y == np.nan:
+    pass
+
+# PLW0117
+if y == npy_nan:
+    pass
+
+# OK
+if math.isnan(x):
+    pass
+
+# OK
+if np.isnan(y):
+    pass
+
+# OK
+if x == 0:
+    pass
+
+# OK
+if x == float("32"):
+    pass
+
+# OK
+if x == float(42):
+    pass
+
+# OK
+if y == np.inf:
+    pass
+
+# OK
+if x == "nan":
+    pass

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

@@ -1283,6 +1283,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::MagicValueComparison) {
                 pylint::rules::magic_value_comparison(checker, left, comparators);
             }
+            if checker.enabled(Rule::NanComparison) {
+                pylint::rules::nan_comparison(checker, left, comparators);
+            }
             if checker.enabled(Rule::InDictKeys) {
                 flake8_simplify::rules::key_in_dict_compare(checker, compare);
             }

crates/ruff_linter/src/codes.rs

@@ -294,6 +294,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         #[allow(deprecated)]
         (Pylint, "R6301") => (RuleGroup::Nursery, rules::pylint::rules::NoSelfUse),
         (Pylint, "W0108") => (RuleGroup::Preview, rules::pylint::rules::UnnecessaryLambda),
+        (Pylint, "W0117") => (RuleGroup::Preview, rules::pylint::rules::NanComparison),
         (Pylint, "W0120") => (RuleGroup::Stable, rules::pylint::rules::UselessElseOnLoop),
         (Pylint, "W0127") => (RuleGroup::Stable, rules::pylint::rules::SelfAssigningVariable),
         (Pylint, "W0128") => (RuleGroup::Preview, rules::pylint::rules::RedeclaredAssignedName),

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

@@ -188,6 +188,7 @@ mod tests {
         Rule::UselessExceptionStatement,
         Path::new("useless_exception_statement.py")
     )]
+    #[test_case(Rule::NanComparison, Path::new("nan_comparison.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

@@ -38,6 +38,7 @@ pub(crate) use magic_value_comparison::*;
 pub(crate) use manual_import_from::*;
 pub(crate) use misplaced_bare_raise::*;
 pub(crate) use named_expr_without_context::*;
+pub(crate) use nan_comparison::*;
 pub(crate) use nested_min_max::*;
 pub(crate) use no_method_decorator::*;
 pub(crate) use no_self_use::*;
@@ -130,6 +131,7 @@ mod magic_value_comparison;
 mod manual_import_from;
 mod misplaced_bare_raise;
 mod named_expr_without_context;
+mod nan_comparison;
 mod nested_min_max;
 mod no_method_decorator;
 mod no_self_use;

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

@@ -0,0 +1,135 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::{self as ast, Expr};
+use ruff_python_semantic::SemanticModel;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for comparisons against NaN values.
+///
+/// ## Why is this bad?
+/// Comparing against a NaN value can lead to unexpected results. For example,
+/// `float("NaN") == float("NaN")` will return `False` and, in general,
+/// `x == float("NaN")` will always return `False`, even if `x` is `NaN`.
+///
+/// To determine whether a value is `NaN`, use `math.isnan` or `np.isnan`
+/// instead of comparing against `NaN` directly.
+///
+/// ## Example
+/// ```python
+/// if x == float("NaN"):
+///     pass
+/// ```
+///
+/// Use instead:
+/// ```python
+/// import math
+///
+/// if math.isnan(x):
+///     pass
+/// ```
+///
+#[violation]
+pub struct NanComparison {
+    nan: Nan,
+}
+
+impl Violation for NanComparison {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let NanComparison { nan } = self;
+        match nan {
+            Nan::Math => format!("Comparing against a NaN value; use `math.isnan` instead"),
+            Nan::NumPy => format!("Comparing against a NaN value; use `np.isnan` instead"),
+        }
+    }
+}
+
+/// PLW0117
+pub(crate) fn nan_comparison(checker: &mut Checker, left: &Expr, comparators: &[Expr]) {
+    for expr in std::iter::once(left).chain(comparators.iter()) {
+        if let Some(qualified_name) = checker.semantic().resolve_qualified_name(expr) {
+            match qualified_name.segments() {
+                ["numpy", "nan" | "NAN" | "NaN"] => {
+                    checker.diagnostics.push(Diagnostic::new(
+                        NanComparison { nan: Nan::NumPy },
+                        expr.range(),
+                    ));
+                }
+                ["math", "nan"] => {
+                    checker.diagnostics.push(Diagnostic::new(
+                        NanComparison { nan: Nan::Math },
+                        expr.range(),
+                    ));
+                }
+                _ => continue,
+            }
+        }
+
+        if is_nan_float(expr, checker.semantic()) {
+            checker.diagnostics.push(Diagnostic::new(
+                NanComparison { nan: Nan::Math },
+                expr.range(),
+            ));
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq)]
+enum Nan {
+    /// `math.isnan`
+    Math,
+    /// `np.isnan`
+    NumPy,
+}
+
+impl std::fmt::Display for Nan {
+    fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            Nan::Math => fmt.write_str("math"),
+            Nan::NumPy => fmt.write_str("numpy"),
+        }
+    }
+}
+
+/// Returns `true` if the expression is a call to `float("NaN")`.
+fn is_nan_float(expr: &Expr, semantic: &SemanticModel) -> bool {
+    let Expr::Call(call) = expr else {
+        return false;
+    };
+
+    let Expr::Name(ast::ExprName { id, .. }) = call.func.as_ref() else {
+        return false;
+    };
+
+    if id.as_str() != "float" {
+        return false;
+    }
+
+    if !call.arguments.keywords.is_empty() {
+        return false;
+    }
+
+    let [arg] = call.arguments.args.as_ref() else {
+        return false;
+    };
+
+    let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = arg else {
+        return false;
+    };
+
+    if !matches!(
+        value.to_str(),
+        "nan" | "NaN" | "NAN" | "Nan" | "nAn" | "naN" | "nAN" | "NAn"
+    ) {
+        return false;
+    }
+
+    if !semantic.is_builtin("float") {
+        return false;
+    }
+
+    true
+}

crates/ruff_linter/src/rules/pylint/snapshots/ruff_linter__rules__pylint__tests__PLW0117_nan_comparison.py.snap

@@ -0,0 +1,82 @@
+---
+source: crates/ruff_linter/src/rules/pylint/mod.rs
+---
+nan_comparison.py:11:9: PLW0117 Comparing against a NaN value; use `math.isnan` instead
+   |
+10 | # PLW0117
+11 | if x == float("nan"):
+   |         ^^^^^^^^^^^^ PLW0117
+12 |     pass
+   |
+
+nan_comparison.py:15:9: PLW0117 Comparing against a NaN value; use `math.isnan` instead
+   |
+14 | # PLW0117
+15 | if x == float("NaN"):
+   |         ^^^^^^^^^^^^ PLW0117
+16 |     pass
+   |
+
+nan_comparison.py:19:9: PLW0117 Comparing against a NaN value; use `math.isnan` instead
+   |
+18 | # PLW0117
+19 | if x == float("NAN"):
+   |         ^^^^^^^^^^^^ PLW0117
+20 |     pass
+   |
+
+nan_comparison.py:23:9: PLW0117 Comparing against a NaN value; use `math.isnan` instead
+   |
+22 | # PLW0117
+23 | if x == float("Nan"):
+   |         ^^^^^^^^^^^^ PLW0117
+24 |     pass
+   |
+
+nan_comparison.py:27:9: PLW0117 Comparing against a NaN value; use `math.isnan` instead
+   |
+26 | # PLW0117
+27 | if x == math.nan:
+   |         ^^^^^^^^ PLW0117
+28 |     pass
+   |
+
+nan_comparison.py:31:9: PLW0117 Comparing against a NaN value; use `math.isnan` instead
+   |
+30 | # PLW0117
+31 | if x == bad_val:
+   |         ^^^^^^^ PLW0117
+32 |     pass
+   |
+
+nan_comparison.py:35:9: PLW0117 Comparing against a NaN value; use `np.isnan` instead
+   |
+34 | # PLW0117
+35 | if y == np.NaN:
+   |         ^^^^^^ PLW0117
+36 |     pass
+   |
+
+nan_comparison.py:39:9: PLW0117 Comparing against a NaN value; use `np.isnan` instead
+   |
+38 | # PLW0117
+39 | if y == np.NAN:
+   |         ^^^^^^ PLW0117
+40 |     pass
+   |
+
+nan_comparison.py:43:9: PLW0117 Comparing against a NaN value; use `np.isnan` instead
+   |
+42 | # PLW0117
+43 | if y == np.nan:
+   |         ^^^^^^ PLW0117
+44 |     pass
+   |
+
+nan_comparison.py:47:9: PLW0117 Comparing against a NaN value; use `np.isnan` instead
+   |
+46 | # PLW0117
+47 | if y == npy_nan:
+   |         ^^^^^^^ PLW0117
+48 |     pass
+   |