Implement E731

README.md

@@ -122,6 +122,7 @@ lint rules that are obviated by Black (e.g., stylistic rules).
 | ---- | ----- | ------- |
 | E402 | ModuleImportNotAtTopOfFile | Module level import not at top of file |
 | E501 | LineTooLong | Line too long |
+| E731 | DoNotAssignLambda | Do not assign a lambda expression, use a def |
 | E902 | IOError | No such file or directory: `...` |
 | F401 | UnusedImport | `...` imported but unused |
 | F403 | ImportStarUsage | Unable to detect undefined names |

examples/generate_rules_table.rs

@@ -11,6 +11,7 @@ fn main() {
         CheckKind::IOError("...".to_string()),
         CheckKind::ImportStarUsage,
         CheckKind::LineTooLong,
+        CheckKind::DoNotAssignLambda,
         CheckKind::ModuleImportNotAtTopOfFile,
         CheckKind::NoAssertEquals,
         CheckKind::RaiseNotImplemented,

resources/test/fixtures/E731.py

@@ -0,0 +1,2 @@
+a = lambda x: x**2
+b = map(lambda x: 2 * x, range(3))

src/check_ast.rs

@@ -377,7 +377,20 @@ impl Visitor for Checker<'_> {
                     self.seen_non_import = true;
                 }
             }
-            StmtKind::Delete { .. } | StmtKind::Assign { .. } | StmtKind::AnnAssign { .. } => {
+            StmtKind::Assign { value, .. } => {
+                self.seen_non_import = true;
+                if self
+                    .settings
+                    .select
+                    .contains(CheckKind::DoNotAssignLambda.code())
+                {
+                    if let ExprKind::Lambda { .. } = &value.node {
+                        self.checks
+                            .push(Check::new(CheckKind::DoNotAssignLambda, stmt.location));
+                    }
+                }
+            }
+            StmtKind::Delete { .. } | StmtKind::AnnAssign { .. } => {
                 self.seen_non_import = true;
             }
             _ => {}

src/checks.rs

@@ -10,6 +10,7 @@ use serde::{Deserialize, Serialize};
 pub enum CheckCode {
     E402,
     E501,
+    E731,
     E902,
     F401,
     F403,
@@ -36,6 +37,7 @@ impl FromStr for CheckCode {
         match s {
             "E402" => Ok(CheckCode::E402),
             "E501" => Ok(CheckCode::E501),
+            "E731" => Ok(CheckCode::E731),
             "E902" => Ok(CheckCode::E902),
             "F401" => Ok(CheckCode::F401),
             "F403" => Ok(CheckCode::F403),
@@ -63,6 +65,7 @@ impl CheckCode {
         match self {
             CheckCode::E402 => "E402",
             CheckCode::E501 => "E501",
+            CheckCode::E731 => "E731",
             CheckCode::E902 => "E902",
             CheckCode::F401 => "F401",
             CheckCode::F403 => "F403",
@@ -88,6 +91,7 @@ impl CheckCode {
         match self {
             CheckCode::E402 => &LintSource::AST,
             CheckCode::E501 => &LintSource::Lines,
+            CheckCode::E731 => &LintSource::AST,
             CheckCode::E902 => &LintSource::FileSystem,
             CheckCode::F401 => &LintSource::AST,
             CheckCode::F403 => &LintSource::AST,
@@ -126,6 +130,7 @@ pub enum CheckKind {
     IfTuple,
     ImportStarUsage,
     LineTooLong,
+    DoNotAssignLambda,
     ModuleImportNotAtTopOfFile,
     NoAssertEquals,
     RaiseNotImplemented,
@@ -151,6 +156,7 @@ impl CheckKind {
             CheckKind::IfTuple => "IfTuple",
             CheckKind::ImportStarUsage => "ImportStarUsage",
             CheckKind::LineTooLong => "LineTooLong",
+            CheckKind::DoNotAssignLambda => "DoNotAssignLambda",
             CheckKind::ModuleImportNotAtTopOfFile => "ModuleImportNotAtTopOfFile",
             CheckKind::NoAssertEquals => "NoAssertEquals",
             CheckKind::RaiseNotImplemented => "RaiseNotImplemented",
@@ -176,6 +182,7 @@ impl CheckKind {
             CheckKind::IfTuple => &CheckCode::F634,
             CheckKind::ImportStarUsage => &CheckCode::F403,
             CheckKind::LineTooLong => &CheckCode::E501,
+            CheckKind::DoNotAssignLambda => &CheckCode::E731,
             CheckKind::ModuleImportNotAtTopOfFile => &CheckCode::E402,
             CheckKind::NoAssertEquals => &CheckCode::R002,
             CheckKind::RaiseNotImplemented => &CheckCode::F901,
@@ -211,6 +218,9 @@ impl CheckKind {
             CheckKind::IfTuple => "If test is a tuple, which is always `True`".to_string(),
             CheckKind::ImportStarUsage => "Unable to detect undefined names".to_string(),
             CheckKind::LineTooLong => "Line too long".to_string(),
+            CheckKind::DoNotAssignLambda => {
+                "Do not assign a lambda expression, use a def".to_string()
+            }
             CheckKind::ModuleImportNotAtTopOfFile => {
                 "Module level import not at top of file".to_string()
             }
@@ -255,6 +265,7 @@ impl CheckKind {
             CheckKind::IfTuple => false,
             CheckKind::IOError(_) => false,
             CheckKind::ImportStarUsage => false,
+            CheckKind::DoNotAssignLambda => false,
             CheckKind::LineTooLong => false,
             CheckKind::ModuleImportNotAtTopOfFile => false,
             CheckKind::NoAssertEquals => true,

src/linter.rs

@@ -136,6 +136,30 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn e731() -> Result<()> {
+        let actual = check_path(
+            Path::new("./resources/test/fixtures/E731.py"),
+            &settings::Settings {
+                line_length: 88,
+                exclude: vec![],
+                select: BTreeSet::from([CheckCode::E731]),
+            },
+            &autofix::Mode::Generate,
+        )?;
+        let expected = vec![Check {
+            kind: CheckKind::DoNotAssignLambda,
+            location: Location::new(1, 1),
+            fix: None,
+        }];
+        assert_eq!(actual.len(), expected.len());
+        for i in 0..actual.len() {
+            assert_eq!(actual[i], expected[i]);
+        }
+
+        Ok(())
+    }
+
     #[test]
     fn f401() -> Result<()> {
         let actual = check_path(

src/pyproject.rs

@@ -239,6 +239,7 @@ other-attribute = 1
                 select: Some(BTreeSet::from([
                     CheckCode::E402,
                     CheckCode::E501,
+                    CheckCode::E731,
                     CheckCode::E902,
                     CheckCode::F401,
                     CheckCode::F403,

src/settings.rs

@@ -46,6 +46,7 @@ impl Settings {
                 BTreeSet::from([
                     CheckCode::E402,
                     CheckCode::E501,
+                    CheckCode::E731,
                     CheckCode::E902,
                     CheckCode::F401,
                     CheckCode::F403,