[pyupgrade] Do not upgrade functional TypedDicts with private field…

… names to the class-based syntax (`UP013`) (#16219)

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP013.py

@@ -46,3 +46,9 @@
 X = TypedDict("X", {
     "some_config": int,  # important
 })
+
+# Private names should not be reported (OK)
+WithPrivate = TypedDict("WithPrivate", {"__x": int})
+
+# Dunder names should not be reported (OK)
+WithDunder = TypedDict("WithDunder", {"__x__": int})

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

@@ -1,6 +1,5 @@
 use ruff_diagnostics::{Applicability, Diagnostic, Edit, Fix, FixAvailability, Violation};
 use ruff_macros::{derive_message_formats, ViolationMetadata};
-use ruff_python_ast::helpers::is_dunder;
 use ruff_python_ast::{self as ast, Arguments, Expr, ExprContext, Identifier, Keyword, Stmt};
 use ruff_python_codegen::Generator;
 use ruff_python_semantic::SemanticModel;
@@ -15,12 +14,22 @@ use crate::checkers::ast::Checker;
 /// Checks for `TypedDict` declarations that use functional syntax.
 ///
 /// ## Why is this bad?
-/// `TypedDict` subclasses can be defined either through a functional syntax
+/// `TypedDict` types can be defined either through a functional syntax
 /// (`Foo = TypedDict(...)`) or a class syntax (`class Foo(TypedDict): ...`).
 ///
 /// The class syntax is more readable and generally preferred over the
 /// functional syntax.
 ///
+/// Nonetheless, there are some situations in which it is impossible to use
+/// the class-based syntax. This rule will not apply to those cases. Namely,
+/// it is impossible to use the class-based syntax if any `TypedDict` fields are:
+/// - Not valid [python identifiers] (for example, `@x`)
+/// - [Python keywords] such as `in`
+/// - [Private names] such as `__id` that would undergo [name mangling] at runtime
+///   if the class-based syntax was used
+/// - [Dunder names] such as `__int__` that can confuse type checkers if they're used
+///   with the class-based syntax.
+///
 /// ## Example
 /// ```python
 /// from typing import TypedDict
@@ -45,6 +54,12 @@ use crate::checkers::ast::Checker;
 ///
 /// ## References
 /// - [Python documentation: `typing.TypedDict`](https://docs.python.org/3/library/typing.html#typing.TypedDict)
+///
+/// [Private names]: https://docs.python.org/3/tutorial/classes.html#private-variables
+/// [name mangling]: https://docs.python.org/3/reference/expressions.html#private-name-mangling
+/// [python identifiers]: https://docs.python.org/3/reference/lexical_analysis.html#identifiers
+/// [Python keywords]: https://docs.python.org/3/reference/lexical_analysis.html#keywords
+/// [Dunder names]: https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers
 #[derive(ViolationMetadata)]
 pub(crate) struct ConvertTypedDictFunctionalToClass {
     name: String,
@@ -185,7 +200,10 @@ fn fields_from_dict_literal(items: &[ast::DictItem]) -> Option<Vec<Stmt>> {
                     if !is_identifier(field.to_str()) {
                         return None;
                     }
-                    if is_dunder(field.to_str()) {
+                    // Converting TypedDict to class-based syntax is not safe if fields contain
+                    // private or dunder names, because private names will be mangled and dunder
+                    // names can confuse type checkers.
+                    if field.to_str().starts_with("__") {
                         return None;
                     }
                     Some(create_field_assignment_stmt(field.to_str(), value))

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP013.py.snap

@@ -264,6 +264,8 @@ UP013.py:46:1: UP013 [*] Convert `X` from `TypedDict` functional to class syntax
 47 | |     "some_config": int,  # important
 48 | | })
    | |__^ UP013
+49 |
+50 |   # Private names should not be reported (OK)
    |
    = help: Convert `X` to class syntax
 
@@ -276,3 +278,6 @@ UP013.py:46:1: UP013 [*] Convert `X` from `TypedDict` functional to class syntax
 48    |-})
    46 |+class X(TypedDict):
    47 |+    some_config: int
+49 48 | 
+50 49 | # Private names should not be reported (OK)
+51 50 | WithPrivate = TypedDict("WithPrivate", {"__x": int})