[ruff] Needless else clause (RUF047) (#15051)

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_linter/resources/test/fixtures/ruff/RUF047_for.py

@@ -0,0 +1,49 @@
+### Errors
+
+for _ in range(0):
+    loop_body_is_not_checked()
+    break
+else:
+    pass
+
+
+for this in comment:
+    belongs_to()  # `for`
+else:
+    ...
+
+
+for of in course():
+    this()
+else:
+    ...
+# this comment does not belong to the else
+
+
+### No errors
+
+for this in second_comment:
+    belongs()  # to
+# `else`
+else:
+    pass
+
+
+for _and in so:
+    does()
+# this
+else:
+    pass
+
+
+for of in course():
+    this()
+else:
+    ...  # too
+
+
+for of in course():
+    this()
+else:
+    ...
+    # too

crates/ruff_linter/resources/test/fixtures/ruff/RUF047_if.py

@@ -0,0 +1,84 @@
+### Errors
+
+if False:
+    condition_is_not_evaluated()
+else:
+    pass
+
+
+if this_comment():
+    belongs_to()  # `if`
+else:
+    ...
+
+
+if elif_is():
+    treated()
+elif the_same():
+    as_if()
+else:
+    pass
+
+
+if this_second_comment():
+    belongs()  # to
+      # `if`
+else:
+    pass
+
+if this_second_comment():
+    belongs()  # to
+    # `if`
+else:
+    pass
+
+
+if of_course():
+    this()
+else:
+    ...
+# this comment doesn't belong to the if
+
+
+if of_course: this()
+else: ...
+
+
+if of_course:
+    this() # comment
+else: ...
+
+
+def nested():
+    if a:
+        b()
+    else:
+        ...
+
+
+### No errors
+
+
+if this_second_comment():
+    belongs()  # to
+# `else`
+else:
+    pass
+
+
+if of_course():
+    this()
+else:
+    ...  # too
+
+
+if of_course():
+    this()
+else:
+    ...
+    # comment
+
+
+if of_course:
+    this() # comment
+else: ...  # trailing

crates/ruff_linter/resources/test/fixtures/ruff/RUF047_try.py

@@ -0,0 +1,76 @@
+### Errors
+
+try:
+    raise try_body_is_not_checked()
+except:
+    pass
+else:
+    pass
+
+
+try:
+    this()
+except comment:
+    belongs()
+except:
+    to()  # `except`
+else:
+    ...
+
+
+try:
+    of_course()
+except:
+    this()
+else:
+    ...
+# This comment belongs to finally
+finally:
+    pass
+
+
+try:
+    of_course()
+except:
+    this()
+else:
+    ...
+# This comment belongs to the statement coming after the else
+
+
+### No errors
+
+try:
+    this()
+except (second, comment):
+    belongs()  # to
+# `else`
+else:
+    pass
+
+
+try:
+    and_so()
+except:
+    does()
+# this
+else:
+    ...
+
+
+try:
+    of_course()
+except:
+    this()
+else:
+    ...  # too
+
+try:
+    of_course()
+except:
+    this()
+else:
+    ...
+    # This comment belongs to else
+finally:
+    pass

crates/ruff_linter/resources/test/fixtures/ruff/RUF047_while.py

@@ -0,0 +1,49 @@
+### No errors
+
+while True:
+    loop_body_is_not_checked()
+    break
+else:
+    pass
+
+
+while this_comment:
+    belongs_to()  # `for`
+else:
+    ...
+
+
+while of_course():
+    this()
+else:
+    ...
+# this comment belongs to the statement coming after the else
+
+
+### No errors
+
+while this_second_comment:
+    belongs()  # to
+# `else`
+else:
+    pass
+
+
+while and_so:
+    does()
+# this
+else:
+    ...
+
+
+while of_course():
+    this()
+else:
+    ...  # too
+
+while of_course():
+    this()
+else:
+    ...
+    # this comment belongs to the else
+

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

@@ -1240,6 +1240,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::IfKeyInDictDel) {
                 ruff::rules::if_key_in_dict_del(checker, if_);
             }
+            if checker.enabled(Rule::NeedlessElse) {
+                ruff::rules::needless_else(checker, if_.into());
+            }
         }
         Stmt::Assert(
             assert_stmt @ ast::StmtAssert {
@@ -1349,6 +1352,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::AsyncBusyWait) {
                 flake8_async::rules::async_busy_wait(checker, while_stmt);
             }
+            if checker.enabled(Rule::NeedlessElse) {
+                ruff::rules::needless_else(checker, while_stmt.into());
+            }
         }
         Stmt::For(
             for_stmt @ ast::StmtFor {
@@ -1438,14 +1444,19 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                     refurb::rules::for_loop_writes(checker, for_stmt);
                 }
             }
+            if checker.enabled(Rule::NeedlessElse) {
+                ruff::rules::needless_else(checker, for_stmt.into());
+            }
         }
-        Stmt::Try(ast::StmtTry {
-            body,
-            handlers,
-            orelse,
-            finalbody,
-            ..
-        }) => {
+        Stmt::Try(
+            try_stmt @ ast::StmtTry {
+                body,
+                handlers,
+                orelse,
+                finalbody,
+                ..
+            },
+        ) => {
             if checker.enabled(Rule::TooManyNestedBlocks) {
                 pylint::rules::too_many_nested_blocks(checker, stmt);
             }
@@ -1512,6 +1523,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::ErrorInsteadOfException) {
                 tryceratops::rules::error_instead_of_exception(checker, handlers);
             }
+            if checker.enabled(Rule::NeedlessElse) {
+                ruff::rules::needless_else(checker, try_stmt.into());
+            }
         }
         Stmt::Assign(assign @ ast::StmtAssign { targets, value, .. }) => {
             if checker.enabled(Rule::SelfOrClsAssignment) {

crates/ruff_linter/src/codes.rs

@@ -997,6 +997,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "041") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryNestedLiteral),
         (Ruff, "043") => (RuleGroup::Preview, rules::ruff::rules::PytestRaisesAmbiguousPattern),
         (Ruff, "046") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryCastToInt),
+        (Ruff, "047") => (RuleGroup::Preview, rules::ruff::rules::NeedlessElse),
         (Ruff, "048") => (RuleGroup::Preview, rules::ruff::rules::MapIntVersionParsing),
         (Ruff, "049") => (RuleGroup::Preview, rules::ruff::rules::DataclassEnum),
         (Ruff, "051") => (RuleGroup::Preview, rules::ruff::rules::IfKeyInDictDel),

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

@@ -79,6 +79,10 @@ mod tests {
     #[test_case(Rule::InvalidAssertMessageLiteralArgument, Path::new("RUF040.py"))]
     #[test_case(Rule::UnnecessaryNestedLiteral, Path::new("RUF041.py"))]
     #[test_case(Rule::UnnecessaryNestedLiteral, Path::new("RUF041.pyi"))]
+    #[test_case(Rule::NeedlessElse, Path::new("RUF047_if.py"))]
+    #[test_case(Rule::NeedlessElse, Path::new("RUF047_for.py"))]
+    #[test_case(Rule::NeedlessElse, Path::new("RUF047_while.py"))]
+    #[test_case(Rule::NeedlessElse, Path::new("RUF047_try.py"))]
     #[test_case(Rule::IfKeyInDictDel, Path::new("RUF051.py"))]
     #[test_case(Rule::UsedDummyVariable, Path::new("RUF052.py"))]
     #[test_case(Rule::FalsyDictGetFallback, Path::new("RUF056.py"))]

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

@@ -21,6 +21,7 @@ pub(crate) use missing_fstring_syntax::*;
 pub(crate) use mutable_class_default::*;
 pub(crate) use mutable_dataclass_default::*;
 pub(crate) use mutable_fromkeys_value::*;
+pub(crate) use needless_else::*;
 pub(crate) use never_union::*;
 pub(crate) use none_not_at_end_of_union::*;
 pub(crate) use parenthesize_chained_operators::*;
@@ -75,6 +76,7 @@ mod missing_fstring_syntax;
 mod mutable_class_default;
 mod mutable_dataclass_default;
 mod mutable_fromkeys_value;
+mod needless_else;
 mod never_union;
 mod none_not_at_end_of_union;
 mod parenthesize_chained_operators;