Enhance PEP 604 annotation handling by skipping dynamic Union creation in non-type definition contexts. Added support for implicit type aliases at the module level.

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP007.py

@@ -147,6 +147,18 @@ def f(types: tuple[type, ...]):
     return Union[types]
 
 
+# Don't emit lint for dynamic Union creation with function calls (e.g., Union[foo()])
+def get_types():
+    return (int, str, float)
+
+
+def g():
+    return Union[get_types()]
+
+
+# Implicit type alias at module level - should be flagged
+IntOrStr = Union[int, str]
+
 if __name__ == "__main__":
     u = f((int, str, float))
     print(u)  # typing.Union[int, str, float]

crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_annotation.rs

@@ -205,9 +205,15 @@ pub(crate) fn non_pep604_annotation(
                 return;
             }
 
-            // Skip dynamic Union creation (e.g., `Union[types]` where `types` is a variable).
-            // This cannot be converted to PEP 604 syntax because the types are determined at runtime.
-            if matches!(slice, Expr::Name(_)) {
+            // Only apply this rule in type annotation/definition positions, including implicit
+            // type aliases (e.g., `X = Union[int, str]` at module or class level).
+            // Skip dynamic Union creation (e.g., `Union[types]` or `Union[foo()]`) when not in
+            // type annotations, as these cannot be converted to PEP 604 syntax since the types
+            // are determined at runtime.
+            let in_type_definition = checker.semantic().in_type_definition();
+            let in_implicit_type_alias = is_implicit_type_alias(checker, expr);
+
+            if !in_type_definition && !in_implicit_type_alias {
                 return;
             }
 
@@ -327,3 +333,24 @@ fn is_named_tuple(checker: &Checker, expr: &Expr) -> bool {
 fn is_optional_none(operator: Pep604Operator, slice: &Expr) -> bool {
     matches!(operator, Pep604Operator::Optional) && matches!(slice, Expr::NoneLiteral(_))
 }
+
+/// Return `true` if the expression is part of an implicit type alias (e.g., `X = Union[int, str]`
+/// at module or class level).
+fn is_implicit_type_alias(checker: &Checker, expr: &Expr) -> bool {
+    let semantic = checker.semantic();
+    let scope = semantic.current_scope();
+
+    // Only consider module-level or class-level assignments as potential type aliases
+    if scope.kind.is_function() {
+        return false;
+    }
+
+    // Check if the current statement is a simple assignment
+    if let ast::Stmt::Assign(ast::StmtAssign { value, .. }) = semantic.current_statement() {
+        // Check if the Union expression is part of the assignment value
+        // We check if the expression's range is within the value's range
+        expr.range().start() >= value.range().start() && expr.range().end() <= value.range().end()
+    } else {
+        false
+    }
+}

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP007.py.snap

@@ -205,17 +205,6 @@ help: Convert to `X | Y`
 43 | 
 44 | 
 
-UP007 Use `X | Y` for type annotations
-  --> UP007.py:46:9
-   |
-45 | def f() -> None:
-46 |     x = Union[str, int]
-   |         ^^^^^^^^^^^^^^^
-47 |     x = Union["str", "int"]
-48 |     x: Union[str, int]
-   |
-help: Convert to `X | Y`
-
 UP007 [*] Use `X | Y` for type annotations
   --> UP007.py:48:8
    |
@@ -343,3 +332,14 @@ help: Convert to `X | Y`
 92 |     print(param)
 93 | 
 94 | 
+
+UP007 Use `X | Y` for type annotations
+   --> UP007.py:160:12
+    |
+159 | # Implicit type alias at module level - should be flagged
+160 | IntOrStr = Union[int, str]
+    |            ^^^^^^^^^^^^^^^
+161 |
+162 | if __name__ == "__main__":
+    |
+help: Convert to `X | Y`